Residential Service Entrance Conductor Temperature Rating to Use for Capacity and 2017 NEC 310.15(B)(7)

[Rossn]

Hi,

I am going through a capacity review in advance of some significant changes to electrical demands in the dwelling, and needing to confirm my service capacity, based on design... after that I can get into load calcs. I have a ticket in with the POCO, but they may only discuss up through the service point.

Questions:
- Based on the below, can the the 90C rating be used for the service entrance conductors, or can I only consider the 75C ampacity?
- Based on the below, what would you consider my service capacity (assuming the

My service entrance - POCO Overhead (I want to say #2) to 500MCM/500MCM/400MCM aluminum conductors, which feed a 320A Milbank U4702-X-5T9-K3-K2-IL meter socket, and I confirmed with the mfg that the service entrance conductor lugs are 90C rated. There is a notable horizontal run of 3" RMC between the mast and meter socket. I have a picture from 2019 that shows the 500KCMIL conductors are XHHW-2. It is unclear if the 400 KCMIL is XHHW or XHHW-2.

I am seeing 500 KCMIL XHHW-2 Al listed at 75C: 310A, 90C: 350A. I assume the 400 KCMIL, even if XHHW, is not the limiting factor.

Since these conductors serve the entire load of the dwelling, it appears the 2017 NEC 310.15(B)(7) 83% rule will apply.

I see that 2023 NEC now includes a reference table in 310.12A has been included, which includes 500kcmil aluminum at 350A service, though it is not clear to me if the 75C must always be used, or if this is a generic table encompassing both XHHW and XHHW-2.

I know there are a few pretty knowledgable folks on here... can anyone help with this one?

Thanks!

 

[wwhitney]

The missing information is what happens on the load side of the meter. What conductors do you have, and what size OCPD(s) do they run to, in what configuration?

On the line side of the meter, your service is rated for 400A. That is because you have only 90C rated terminations, so you may utilize the full 90C ampacity of the 500 MCM Al conductors, which is 350A, and per 2017 NEC 310.15(B)(7) for a rating of 400A on a residential service, you only need to provide conductors with an ampacity of 400*83% = 332A, which you have exceeded. So if your load calc comes out to 400A or less, while including a 125% factor for the continuous portion of the load, that portion of the install is fine.

But the load side of the meter requires separate consideration, and any conductors that land on a breaker will be limited to their 75C ampacity per the termination rating of the breaker. Also, if you have two or more breakers each supplied directly from the load side meter lugs, 310.15(B)(7) will not apply to those conductors.

Cheers, Wayne

 

[Sponsor]

 

[Rossn]

The missing information is what happens on the load side of the meter. What conductors do you have, and what size OCPD(s) do they run to, in what configuration?

On the line side of the meter, your service is rated for 400A. That is because you have only 90C rated terminations, so you may utilize the full 90C ampacity of the 500 MCM Al conductors, which is 350A, and per 2017 NEC 310.15(B)(7) for a rating of 400A on a residential service, you only need to provide conductors with an ampacity of 400*83% = 332A, which you have exceeded. So if your load calc comes out to 400A or less, while including a 125% factor for the continuous portion of the load, that portion of the install is fine.

But the load side of the meter requires separate consideration, and any conductors that land on a breaker will be limited to their 75C ampacity per the termination rating of the breaker. Also, if you have two or more breakers each supplied directly from the load side meter lugs, 310.15(B)(7) will not apply to those conductors.

Cheers, Wayne

Hi Wayne - hope you are doing well! Thanks so much for weighing in on this one! Secretly I was hoping you would, as not only have you been a great help here (especially with my drains), but also educated me through other posts of yours I've read on various electrician and EV sites. So, thank you!

It's great to hear that on the upstream side of the meter, we should qualify for the full 400A intermittent capacity. It's how my untrained eyes had interpreted the text and what I had hoped for, but I had no experience to lean on.

I'm also assuming that given my 120v loads low relative to the 240V and because you made no mention of it, you wouldn't have any concern with that 400 MCM neutral affecting capacity, even if it was only XHHW/75C.

I had the master electrician, who did the service upgrade 4 years ago, out yesterday and in his head it is still 320A service... he says 'no, a 400A meter socket is huge (true)', but he doesn't seem to get that the 320 class gets referred to at it's continuous 80% rating and the 200 class gets referred to at it's intermittent 125% rating.

Ah yes.... well didn't want to make the topic too big initially!

Here is what the service equipment and feeders look like (-, --, --- represent different layers, if that makes sense):

Milbank U4702-X-5T9-K3-K2-IL Ring Less Meter Socket, 600 VAC, 230 A, 1 Phase, NEMA 3R Enclosure [Exterior]
- Homeline HOM816M200PFTRB 200A Main Breaker 8-Space w/ Feed-through Lugs
Fed by ~4' 4/0 Al [Exterior; main disconnect 1]
-- Homeline HOM4284M200PC 200A Convertible Main Breaker 42-space
Fed by ~14' 4/0 Al SER off feed-through lugs [Interior; main panel for main house]
--- Homeline HOM2448M100PC 100A Convertible Main Breaker 24-space
Fed by ~8' 1/0 Al SER [Interior; generator 'backup loads' panel]
--- (Tentative) Homeline HOM2040M100PC 100A Convertible Main Breaker 20-space
Fed by ~35-40' 1/0 Al SER [Interior; ran out of spaces for main house, so likely will need to add this one]
- Homeline HOM816M200PFTRB 200A Main Breaker 8-Space w/ Feed through Lugs
Fed by 4' 4/0 Al [Exterior; main disconnect 2]
-- Homeline HOM2040M200PC 200A Convertible Main Breaker 20-Space
Fed by ~92' (long run) 250 Al SER [Interior; Garage end of house primary panel in heated shop]
--- Homeline HOM2040M100PC 100A Convertible Main Breaker 20-Space
Fed by ~8' 1/0 Al SER [Interior; In-Law/Rental quarters above garage with dedicated panel in living space]
--- (Tentative) Homeline HOM2448M125PC 125A Convertible Main Breaker 24-Space
Fed by ~29' 2/0 Al SER [Interior; Garage proper - spaces for shop equipment, welder, EVSEs (x3)

Yep, that's a crazy number of panels... I'd prefer to have less. But, there is almost nothing that can use a tandem breaker these days (AFCI, GFCI req's), it is a large dwelling (4750'), original service equipment had branch circuits going both interior and exterior, new large kitchen, theater with dedicated circuits, hot tub, steam generator, mini-split, the in-law quarters with tentative kitchen and own mini-split, and then the garage shop equipment and EVSE's for both us and a renter. And, every panel has 2 spaces occupied for a MOV surge suppression module... add sub-panel breakers, that's 24 spaces alone!

4 years ago I objected to the electrician about using the 4/0 between the meter base and two 200A panels, but he could not be convinced otherwise, insisting this is how they are done. The inspector wasn't very electrical knowledgable, and did not even check those conductors. The sparky hadn't done many 320A services at the time, but they are more popular now (electrification movement has arisen). I suppose the 4/0 SER between the main and the 42-space panel is also a concern.

IIRC, the code concern around the 83% rule for feeders not feeding the entire dwelling is around possible reduction of load diversity.

The whole reason for trying to establish my overall capacity is to determine how to layout circuits, and if I can support a 12kW makeup air heater for the kitchen range vent hood, or if I need to drive that off the boiler (which will affect sizing of the replacement boiler). But, before going there... what level of concern do you think I should have around the load side feeders?

 

[wwhitney]

On the 4/0 Al from the meter enclosure to the each of the (2) 200A exterior main disconnect, 310.15(B)(7) does not apply, and the breaker termination rating of 75C limits those SECs to an ampacity of 180A. That's fine to protect at 200A as you have, but the calculated load on each of the two external service disconnects must be 180A or less. To bump that up to 200A, replace the 4/0 Al with 250 kcmil Al or 3/0 Cu.

In all other cases the panels are being supplied by conductors whose ampacity is at least the nominal rating you listed, so there are no other such bottlenecks in what you've described.

Cheers, Wayne

 

[Rossn]

On the 4/0 Al from the meter enclosure to the each of the (2) 200A exterior main disconnect, 310.15(B)(7) does not apply, and the breaker termination rating of 75C limits those SECs to an ampacity of 180A. That's fine to protect at 200A as you have, but the calculated load on each of the two external service disconnects must be 180A or less. To bump that up to 200A, replace the 4/0 Al with 250 kcmil Al or 3/0 Cu.

In all other cases the panels are being supplied by conductors whose ampacity is at least the nominal rating you listed, so there are no other such bottlenecks in what you've described.

Cheers, Wayne

Thanks for sharing your knowledge on that. Very, very helpful! Prior to this confirmation, I was thinking at 320 or 350 total amps, my load calcs could exceed the service. The 180A limitation may actually be an issue in my case. It actually disgusts me a little, but I felt a little less silly after my architect friend mentioned he had just put in some 600A service for a home where they fully electrified.

I'm not entirely understanding why you say the feed-through to the HOM4284M200PC over 4/0 SER is not also a code infraction, as I had interpreted 2017 NEC 310.15(B)(7)(2) to mean that the 83% rule is only applicable if the entire load of the dwelling passes through the conductors?

My immediate goal is to understand if I can (with minimal changes) comfortably support the future needs (including 60-80A EVSE draw) while using an electric makeup air heater, such that I do not upsize my boiler during the pending replacement.

High-level is that with the NEC optional (220.82) calculation I see (without 12kW makeup air heater)
Leg 1 (main house) - 146A
Leg 2 (garage/in-law quarters) - 143A

These numbers assume 60A available for 3 EVSEs, though I would prefer 80A.

If I do a 'common sense' calculations, I get 135A and 147A respectively.

Now add 50A makeup air duct heater to the mix (SCR modulated), and we're definitely beyond the 180A threshold, so it seems those conductors will need to be upgraded to proceed.

With the above numbers, do you foresee any code concerns with adding the 50A heater to either of the 200A legs? I was originally thinking there was some 125% factor to consider with the EVSEs, but now see some comments you made on mikeholt, where that does not seem to be a requirement of NEC... though factoring at 100% makes good sense to me.

Note: There are a few gray-areas in my calculations, where reductions may be possible with clarification.

 

[wwhitney]

I'm not entirely understanding why you say the feed-through to the HOM4284M200PC over 4/0 SER is not also a code infraction, as I had interpreted 2017 NEC 310.15(B)(7)(2) to mean that the 83% rule is only applicable if the entire load of the dwelling passes through the conductors?

It is correct that the 83% rule does not apply to the 4/0 SER. However, in any context, 240.4(B) allows a 180A ampacity conductor to be protected at 200A, provided the calculated load does not exceed 180A.

High-level is that with the NEC optional (220.82) calculation I see (without 12kW makeup air heater)
Leg 1 (main house) - 146A
Leg 2 (garage/in-law quarters) - 143A

The NEC optional method only applies to the conductors that carry the entire load of the dwelling unit, i.e. the 500 kcmil XHHW-2. For a feeder that carries only part of the load of a dwelling unit, you must use the standard calculation.

You also have the option to use 220.87 to install a recording load meter on each of the two 4/0 Al feeders for 30 days and use the recorded data to determine an actual maximum demand, which is further factored by 125% to account for uncertainty.

I was originally thinking there was some 125% factor to consider with the EVSEs, but now see some comments you made on mikeholt, where that does not seem to be a requirement of NEC... though factoring at 100% makes good sense to me.

An EVSE is a continuous load, so it does require a 125% factor for load calculations (unless you have special 100% rated breakers that basically don't exist in a residential context or at such low current ratings).

Cheers, Wayne

 

[Rossn]

It is correct that the 83% rule does not apply to the 4/0 SER. However, in any context, 240.4(B) allows a 180A ampacity conductor to be protected at 200A, provided the calculated load does not exceed 180A.


The NEC optional method only applies to the conductors that carry the entire load of the dwelling unit, i.e. the 500 kcmil XHHW-2. For a feeder that carries only part of the load of a dwelling unit, you must use the standard calculation.

You also have the option to use 220.87 to install a recording load meter on each of the two 4/0 Al feeders for 30 days and use the recorded data to determine an actual maximum demand, which is further factored by 125% to account for uncertainty.


An EVSE is a continuous load, so it does require a 125% factor for load calculations (unless you have special 100% rated breakers that basically don't exist in a residential context or at such low current ratings).

Cheers, Wayne

Ok, I think I get what you're saying on the 4/0 SER - in my case as long as some of the load is coming off at the main load center's breakers (20A) or calculated load is < 180, no need to make a change there; the feed-through on 4/0 should be fine.

Big thanks for bringing the optional method limitation to my attention. It's a little frustrating that I paid a master electrician to come out yesterday, and he told me we should only look at the optional method numbers, and not the standard method numbers.

Given a lot of these loads are a year or two out, and that my house has been in construction mode for a few years now, unfortunately I can't use the method with recording the load. That aside, I think that there is some value in having that data once I'm getting things operational, so if you know of any products you think are effective and cost effective, I'm interested to hear.

My next step is to closely review standard method 220.40 and revise my calculations. I'll probably work from 2023, just to know I'm on the latest. After that, it is probably worth looking at if energy management is an option to keep the electric heater in-scope. I don't think I want to sacrifice capacity to charge EV's below some threshold - maybe 50-60A combined.

The electrician also told me that large loads should always be on the main panel, especially EVSEs, which are considered continuous large loads. What are your thoughts about running the EVSEs off one of the sub panels? Running additional 150' SER from the service entrance is the alternative, but that does not sound that much better to me (and I may not have the space in those ranch/trailer panels, once I have some required loads in them (hot tub for disconnect accessibility, surge suppressors on each panel, free spaces for solar backfeed, etc).

 

[wwhitney]

Ok, I think I get what you're saying on the 4/0 SER - in my case as long as some of the load is coming off at the main load center's breakers (20A) or calculated load is < 180, no need to make a change there; the feed-through on 4/0 should be fine.

I don't know what you mean by the first part of the "or". All that matters is that the calculated load is < 180A, and the fact that they don't make 180A breakers, just 175A and 200A. Then you get to use 200A to protect your 180A conductors.

The electrician also told me that large loads should always be on the main panel, especially EVSEs, which are considered continuous large loads. What are your thoughts about running the EVSEs off one of the sub panels?

I haven't considered this question. As far as I can see the only issue that has bearing on this choice is voltage drop, assuming the near by panel has the same capacity as is available at the service disconnect.

If you have the same total load at some distant point, and the same conductor cross section going to that point, it doesn't matter whether all that load is carried by a single feeder, or you have multiple circuits with the load split up proportionally to the conductor cross section in each circuit. Conversely, if you add a heavy load at a point distant from the service, and you have a choice of running your branch circuit to a close panel vs back to the service, running it back to the service may give you less voltage drop (depending on the conductor sizes), attributable to having more conductor cross section between the service and that distant point.

Cheers, Wayne

 

[Rossn]

I don't know what you mean by the first part of the "or". All that matters is that the calculated load is < 180A, and the fact that they don't make 180A breakers, just 175A and 200A. Then you get to use 200A to protect your 180A conductors.


I haven't considered this question. As far as I can see the only issue that has bearing on this choice is voltage drop, assuming the near by panel has the same capacity as is available at the service disconnect.

If you have the same total load at some distant point, and the same conductor cross section going to that point, it doesn't matter whether all that load is carried by a single feeder, or you have multiple circuits with the load split up proportionally to the conductor cross section in each circuit. Conversely, if you add a heavy load at a point distant from the service, and you have a choice of running your branch circuit to a close panel vs back to the service, running it back to the service may give you less voltage drop (depending on the conductor sizes), attributable to having more conductor cross section between the service and that distant point.

Cheers, Wayne

Ok, next size up OCPD makes sense.

I believe he's approaching from the perspective of 1) heat as it relates to how how many different pieces of service equipment (and junctions/breakers/etc) you're jumping through, given continuous load and heat is a contributor to failures and 2) number of junctions 3) to be able to balance two EVSE's across the two 200A mains.

I don't think he's after voltage drop. For (2) 40A chargers, if considering just the first 92' (existing 250 SER) of the ~150' run to the EVSE's, then using the 250 SER in the middle of the night, assuming total load of 100A is 0.8% drop, whereas using 6 AWG Cu over the same 92' for a dedicated run would see a voltage drop of 1.4%. Assuming I don't have spaces on the first panel for 3 different EVSEs, I am thinking I may be better running those from the first (direct-feed from feed-through lugs) subpanel, versus running 2 or 3 different dedicated cables. But... definitely want to get this right.

I noticed in 2023 NEC 220.57, it seems like the 125% rule no longer applies for EVSE load calculations, but just sizing conductors/OCPD. Do you read it that way?

I spent much of the night reading through code and working on a fresh approach on the 2023 standard load calculation. There is a lot more than meets the eye.

I have a mini-split heat pump, which has rated loads:
Cooling - 3930w
Heating @17F - 6870w
Heating @5F 7910w

(Heads total 768w)

And the below tag. We bought this primarily for AC, as we have boiler for the primary radiant heat source. However, we may occasionally use the heat in the early fall and late spring, where it is only needed for the coldest part of the evening. I'm trying to find the right practical balance on this... do you think I should list as 6870w+768w @ 100%?

 

[wwhitney]

I noticed in 2023 NEC 220.57, it seems like the 125% rule no longer applies for EVSE load calculations, but just sizing conductors/OCPD. Do you read it that way?

No. With limited exceptions, article 220 does not deal with continuous versus non-continuous. Rather, you keep track as you go through Article 220 of the breakdown between continuous and non-continuous loads, so that your final answer is of the form "X amps, of which Y amps is continuous." [Note that this is not spelled out in the NEC, but Article 220 basically doesn't differentiate, whereas the sections of Articles 210 and 215 on OCPD and conductor sizing do differentiate, so you need to keep track.]

Anyway, EVSEs go into the calculation as continuous loads.

And the below tag.

OK, for the load calculation all the information you need is on the tag, ignore anything in the manual. If that is the only piece of equipment you have with such a tag, then the load contribution is MCA (minimum circuit ampacity) = 42A * Voltage = 240V for an answer of 10,080 VA.

If you have a feeder that supplies multiple such pieces of equipment, you can just add the above numbers, but you get a slight overestimate. The exact answer is a bit more complicated. If you're in this situation, post the name tags of all the other such pieces of equipment.

Cheers, Wayne

 

[Rossn]

(Forgot to use 'reply', so... @wwhitney ....)

Hmmm... I seem to have not clicked submit on my prior (not-posted) post... I'll try again.

If I'm hearing correctly, I don't need to factor continuous into my load calculation, but I need to be able to identify what portion of each of my load calcs is continuous.

I spent a good part of a day reading and calculating, and come back at 190A main house and 173A garage/studio/breezeway, without the 50A electric heater load.

Seems my options will be 1) reduce appliances 2) energy management at some level 3) use hydronic heat for the makeup air

I'm attaching a pdf of what I came up with. If you have the bandwidth, would I be able to talk you into glancing at it to see if there are any blatant mistakes with the approach? No worries or offense if you don't have the bandwidth or feel comfortable comfortable doing so.

 

[wwhitney]

If I'm hearing correctly, I don't need to factor continuous into my load calculation, but I need to be able to identify what portion of each of my load calcs is continuous.

Yes, where "factor" means multiply by a factor. Some quick comments on your load calc:

-Does one set of 4/0 Al SECs supply only the "main house" and all of the loads of the "main house," and the other likewise for the "garage/studio/breezeway"? If so, then if you can argue that one or both or those are complete dwelling units, then you could use the 83% factor and the optional calculation method for the dwelling unit(s). However, the 500 MCM SECs would lose the 83% factor.

- If you have a receptacle dedicated to a built-in microwave (versus just a countertop microwave that if you removed it, its receptacle would look like all the other countertop receptacles), use the microwave nameplate data, not the 1500 SABC allowance.
- The HVAC MCA already includes the extra 25% of its largest motor, so you don't need to add it in again. You do want to confirm that the 19A compressor RLA is the largest motor on the feeder; if it's not, subtract 25% of 19A from the HVAC MCA, and add 25% of the other largest motor.

[Also, as an aside, it's odd the HVAC MCA of 42A is so much larger than 125% * 19A (compressor RLA) + the fans. Are the indoor units powered from the outdoor unit? How many do you have? I'm guessing that since the outdoor unit can support up to 8 indoor units, the difference is 8 times an allowance for the indoor units. If you have much less than 8, there's an argument for reducing the 42A figure, although that would be supported by the code language.]

- The VA ratings on the kitchen refrigerators and freezers strike me as low--those are all from the nameplates? Maybe they've gotten more efficient that I realize.

- On the 3 pumps, how are you determining those values? If you don't already have a picture of the nameplate, how about a make/model number? Mostly just curious.

- If your dual fuel range has no electric burners, then I agree that you can treat it as an electric oven as you have. But I believe you get to use the row 2 demand factors for each of the two loads, since you have two total electric cooking appliances, albeit one from 220.55 Column A and one from 220.55 Column B. Not 100% sure.

- Since the steam shower is so rarely used, you could install an interlock system between it and another load, so that they can't both run simultaneously, and then you only need to count the larger of the loads.

- BTW, how do you like your heat pump dryer, and which model do you have? I have the WHD560CHW (no exhaust) in a louvered closet, and I find it leaks a fair amount of humid air, enough that I'm going to rejigger my HVAC a bit to address it.

Cheers, Wayne

 

[Rossn]

Thanks for your time in having a look, Wayne!

Yes, where "factor" means multiply by a factor. Some quick comments on your load calc:

-Does one set of 4/0 Al SECs supply only the "main house" and all of the loads of the "main house," and the other likewise for the "garage/studio/breezeway"? If so, then if you can argue that one or both or those are complete dwelling units, then you could use the 83% factor and the optional calculation method for the dwelling unit(s). However, the 500 MCM SECs would lose the 83% factor.

Basically, yes, but I think it may be a harder sell. That said, I do not think my inspector is knowledgable enough on electrical to call me on it, so I'm less concerned about that and more concerned about safety and liability if at some point the place is sold. And, on your point - I think losing the 83% factor would ultimately be more detrimental. I am think 'bang for buck', or $/amp, replacing those two short sections will be the best path forward.

- If you have a receptacle dedicated to a built-in microwave (versus just a countertop microwave that if you removed it, its receptacle would look like all the other countertop receptacles), use the microwave nameplate data, not the 1500 SABC allowance.

- The HVAC MCA already includes the extra 25% of its largest motor, so you don't need to add it in again. You do want to confirm that the 19A compressor RLA is the largest motor on the feeder; if it's not, subtract 25% of 19A from the HVAC MCA, and add 25% of the other largest motor.


[Also, as an aside, it's odd the HVAC MCA of 42A is so much larger than 125% * 19A (compressor RLA) + the fans. Are the indoor units powered from the outdoor unit? How many do you have? I'm guessing that since the outdoor unit can support up to 8 indoor units, the difference is 8 times an allowance for the indoor units. If you have much less than 8, there's an argument for reducing the 42A figure, although that would be supported by the code language.]

Good to know on the HVAC MCA and 25%. Does that mean I don't need to slot any other motor into that spot? Yes, 19A the largest motor on the home.

- The VA ratings on the kitchen refrigerators and freezers strike me as low--those are all from the nameplates? Maybe they've gotten more efficient that I realize.

Those ratings are either from name plates or a call to Sub Zero. I requested rated amps. We don't have the large refrigerator, as it will have to come later from a budgetary perspective, but I did get the freezer drawers value off the tag. This latest generation of refrigerators from Sub Zero did improve efficiency notably.

- On the 3 pumps, how are you determining those values? If you don't already have a picture of the nameplate, how about a make/model number? Mostly just curious.

The three pumps the home has are a) septic b) well c) sump

a) This one might run 1-2x/wk for a few mins. It was replaced a few years ago, and is pretty hard to access in the pit/tag (actually, probably not there, since they cut the cord end). The tag from the prior pump (installed by same septic company), which was a on the cord-end they cut off is still used in the alarm pedestal and reads Goulds Pumps, ITT PE51PA 0.5HP/115V/9.5A/1Ph. I have the Goulds manual that came from the new pump, but not the model number... I know the general form, and looked on their website... I believe it is 1/3 or 1/2 HP, and the only three they have that look like what I recall are below. Also, the alarm pedestal supports a pump of 1-12A@120V.

• GSP0311
  • 
• GEP
  • 
• GSP0511
  • 

b) Well pump... it's actually currently not working (have not fixed since we bought the house) and will likely be replaced at some point. We're also on city water. I used the pump off the quote from the well company, which was listed as:
"Grundfos Complete Pump 3/4 HP, 10 GPM, 2W, 230V, High Efficiency, Corrosion Resistant, All Stainless Steel Construction". I found a similar pump on their site for the current draw, but that was some time back and I don't recall which one.

c) Sump pump. This one runs maybe once or twice a year? Looks like I need to bump down my VA to 863

- If your dual fuel range has no electric burners, then I agree that you can treat it as an electric oven as you have. But I believe you get to use the row 2 demand factors for each of the two loads, since you have two total electric cooking appliances, albeit one from 220.55 Column A and one from 220.55 Column B. Not 100% sure.

It has an electric griddle. They do not have a tag on the range and I had to escalate through a few persons to get an answer on the rated power consumption on it. The reason I didn't go to the lower value is because one fell into column A and the other into Column B... I wasn't sure how to approach, either. Maybe there is a slight reduction there if I go down to the higher of the two values? I'm not clear, either. But the differential isn't massive, either.

- Since the steam shower is so rarely used, you could install an interlock system between it and another load, so that they can't both run simultaneously, and then you only need to count the larger of the loads.

I agree it's the best candidate for energy management, with little to no impact. Do you know of a cost-effective way to have that interlock? My service equipment is homeline. I think Eaton's smart breakers are marketed as fitting other mfg's panels, but I'm not sure on how they could leveraged, and if they have a breaker at that size.

- BTW, how do you like your heat pump dryer, and which model do you have? I have the WHD560CHW (no exhaust) in a louvered closet, and I find it leaks a fair amount of humid air, enough that I'm going to rejigger my HVAC a bit to address it.

Cheers, Wayne

My heat pump dryer is only theoretical at the moment I have an issue in that I have both opening windows and a HRV Air intake that will be in close proximity to the gas dryer vent, so that is where I started looking at electric and heat pump came up on my radar. My options to address the issue are either running the exhaust duct on the underside of an exterior deck for 10' or running it down an exterior wall and have it terminate lower than code allows (the inspector approved the plan with a roof above it). However, once I saw the heat pump dryer option, that seemed to check all the boxes (except possibly performance), and simplified the situation. They definitely are still going through some growing pains.

Mine would be in a dedicated laundry/mechanical room, about 750 ft^3. The climate is also very arid here. The window could remain cracked in there, though I had hoped to change that. Do you think I'd have an issue with that setup?

If you don't mind me asking - It sounds like you might be retired? If so, what was your line of work that you have such deep knowledge of electrical and plumbing code? It seems like you have it down very well, and can clearly interpret it. Don't feel like you have to answer that one if you don't prefer to. I fully respect that.

 

[Rossn]

On the line side of the meter, your service is rated for 400A. That is because you have only 90C rated terminations, so you may utilize the full 90C ampacity of the 500 MCM Al conductors, which is 350A...

One more thing... I was doing some reading up on the 90degree lugs, and notice a comment you made over on mikeholt:

"If someone says "lug rating" that typically refers to the rating as a standalone part. And that would be the meaning of any rating stamped on the lug itself. But once that lug is incorporated into a piece of equipment, you don't care about the lug rating anymore, you care about the overall equipment rating, as the lugs will conduct heat to the rest of the equipment."

Does it make sense for me to reach out to Milbank again to get clarification if the equipment is rated for 75C or 90C? I specifically asked about the lugs, and that was probably a mistake.

 

[wwhitney]

Good to know on the HVAC MCA and 25%. Does that mean I don't need to slot any other motor into that spot?

Yes.

The three pumps the home has are a) septic b) well c) sump

OK, so for the pumps or any other load that is basically just a motor, you need to use the nameplate HP rating and Table 430.248, not the nameplate amps. Then as usual you include the extra 25% of the largest motor on the entire feeder, which is presumably the 19A HVAC compressor, which is already included in its MCA.

It has an electric griddle. They do not have a tag on the range and I had to escalate through a few persons to get an answer on the rated power consumption on it. The reason I didn't go to the lower value is because one fell into column A and the other into Column B... I wasn't sure how to approach, either. Maybe there is a slight reduction there if I go down to the higher of the two values? I'm not clear, either. But the differential isn't massive, either.

So, on the one hand, if it has an electric griddle, maybe you should count it as a range, and then you are stuck with Column C? On the other hand, it's under 8.75 kW, so going with the row 2 Column A and Column B numbers is probably defensible. You're trying to get as low a number as is defensible, knowing that it will still be an overestimate as Article 220 is very conservative.

I agree it's the best candidate for energy management, with little to no impact. Do you know of a cost-effective way to have that interlock?

If you want the steam shower to have priority whenever it is used, and if the subordinate load has some contacts in its controls that would let you force it to stay off, then you can use a current sensing relay. It consists of a split CT that would go over one of the wires to the steam shower, and whenever the current in that wire is above a threshold level, that induces enough power in the relay itself to switch a pair of contacts. I have used these a couple times:

https://www.veris.com/126825/category/current-monitoring/current-switches

Mine would be in a dedicated laundry/mechanical room, about 750 ft^3. The climate is also very arid here. The window could remain cracked in there, though I had hoped to change that. Do you think I'd have an issue with that setup?

That would be less of an issue than in a closet, and maybe whatever model you get will leak less humidity. Just a possibility to keep in mind, something I didn't expect of heat pump dryers.

Does it make sense for me to reach out to Milbank again to get clarification if the equipment is rated for 75C or 90C? I specifically asked about the lugs, and that was probably a mistake.

I would think that any time you ask a manufacturer about a lug or termination in a piece of equipment, they would give you the rating as used in that piece of equipment, rather than in isolation on a shelf. But I wouldn't swear to it.

On the other hand, some other random guy on the Internet: https://forums.mikeholt.com/threads/mllbank-meter-base.146953/

Cheers, Wayne

 

[Rossn]

OK, so for the pumps or any other load that is basically just a motor, you need to use the nameplate HP rating and Table 430.248, not the nameplate amps. Then as usual you include the extra 25% of the largest motor on the entire feeder, which is presumably the 19A HVAC compressor, which is already included in its MCA.

Ok, got it. Thanks for cluing me into using the HP rating vs nameplate amps. I kind of get it and also seems to go against the grain of not using mfg amps I'll make those updates.

So, on the one hand, if it has an electric griddle, maybe you should count it as a range, and then you are stuck with Column C? On the other hand, it's under 8.75 kW, so going with the row 2 Column A and Column B numbers is probably defensible. You're trying to get as low a number as is defensible, knowing that it will still be an overestimate as Article 220 is very conservative.

I believe I can defend it on a gas range, at a minimum not negligent. Yep, using the most conservative NEC approach, and then having things like dryers way over-spec'd and counting that mini-split, when it would never be used in the dead of winter, unless the boiler has a failure, not to mention some high current appliances and pumps rarely used.

That would be less of an issue than in a closet, and maybe whatever model you get will leak less humidity. Just a possibility to keep in mind, something I didn't expect of heat pump dryers.

It's good to know. Outside of the humidity issue, how do you like yours? And were you able to tie in to the washer drain? I haven't started to noodle that yet.

Not sure you want to thrown money at the issue, or that you have duct access to do so, but perhaps a spot HRV like this could help manage that humidity issue.

I would think that any time you ask a manufacturer about a lug or termination in a piece of equipment, they would give you the rating as used in that piece of equipment, rather than in isolation on a shelf. But I wouldn't swear to it.

On the other hand, some other random guy on the Internet: https://forums.mikeholt.com/threads/mllbank-meter-base.146953/

Cheers, Wayne

You would think they'd understand where you're coming from, but think he literally gave me the lug rating. "Me: Are the lugs on meter socket xyz 75C or 90C? Matt@Milbank: Hold on, I have one right here on my desk... it's a CU 9 AL, so that means it is a 90C rated lug".


There were a few other questions I didn't respond to previously:

If you have a receptacle dedicated to a built-in microwave (versus just a countertop microwave that if you removed it, its receptacle would look like all the other countertop receptacles), use the microwave nameplate data, not the 1500 SABC allowance.

I was going that way, but it looked to my benefit to consider it an incremental SABC. 1500VA x 0.35 vs 1200VA x .75, if I did that right. It's a microwave that is rated for either countertop or install, but going to sit on a shelf in a cabinet, with required clearances.

[Also, as an aside, it's odd the HVAC MCA of 42A is so much larger than 125% * 19A (compressor RLA) + the fans. Are the indoor units powered from the outdoor unit? How many do you have? I'm guessing that since the outdoor unit can support up to 8 indoor units, the difference is 8 times an allowance for the indoor units. If you have much less than 8, there's an argument for reducing the 42A figure, although that would be supported by the code language.]

Yes, the indoor units are powered directly from the outdoor unit. I have 5 indoor units. There may be a branch box that allows more, but they can only total a given number of BTUs. The head units draw almost nothing... they're a few fans and motors to sweep the louvers up/down/left/right

I had looked at this last year, and think that is why I went into the manual and aiming to consider the rated loads for one of the following conditions and had this note in my initial calc:
"MXZ-8C48NAHZ2: 3930w cooling, 6870w heating@17f, 7910w heating@5f. Heads total 3.2A actual. This totals max: 36.2A, but install manual calls for min ampacity of 42A. Note, if not considering heating, should be at 19.6A
Spec sheet rec. 45A breaker. Install manual says max OCPD = 50A".

I was initially thinking my 'worst case' would be the 36.2A. The branch box probably has a small load, too.

If you want the steam shower to have priority whenever it is used, and if the subordinate load has some contacts in its controls that would let you force it to stay off, then you can use a current sensing relay. It consists of a split CT that would go over one of the wires to the steam shower, and whenever the current in that wire is above a threshold level, that induces enough power in the relay itself to switch a pair of contacts. I have used these a couple times:

https://www.veris.com/126825/category/current-monitoring/current-switches

I like this idea. I had not gotten too deep into considering energy management yet, but just starting to poke around. I wasn't clear how accepting AHJ would be of 'custom/homegrown solutions' like this. Now, if you were having a PE stamp on your prior installs, no doubt, no issue. The nice thing about this one is the simplicity of the setup... it is more of the lockout you mentioned than energy management based on some sort of decision making. So I can see a reasonable case made for this, provided it can fail safe. I read through briefly, but need to dig closer to see if it is normally open / closed and if it's a simple dry contact or if some low voltage (say 24VDC) can pass through the response side of the device - do you know?

Last night I did glance at the span panels and the eaton smart breaker solutions, and on one hand I feel like I might be missing the boat going with old/dumb service equipment, but also knowing this is a rapidly evolving area that is about to explode with options. I just hate to come back later at the expense and effort and re-work a bunch of stuff for battery/generator/solar/etc, though the current options may not even fit my physical constraints (e.g. I have to use a trailer/ranch panel outside, and I can only use shorter panels in the garage and rental, with limited stud bay space).

All that said, I feel like with this massive residential service I have, I should be able to make just about anything work with a few tricks out of the bag.

Specifically (on your proposal), my true preference would be to have the steam shower lowest priority (very infrequent use, and if it doesn't turn on, someone can go figure out what needs to go off, but I honestly doubt that will materialize). But, what you propose is simpler, and could likely tie into the 24v controls for the electric heater. They're somewhat similar sized loads, so it wouldn't be a bad match up, and with the heater already having low voltage controls, it is a good marriage.

I think the appliances I have that are worth considering energy management on (for service and panel current management, but not solar/battery detailed management) are:
- Steam Shower
- EVSEs
- Electric Duct heater for makeup air
- Hot tub heater (there's a little more risk with this one in cold climate, though I would need to keep the circulation pump enabled and have some alerting if the temp dropped below a threshold).
- Possibly well pump, as it would mainly be used for irrigation

Alas, before I dive into that too far, I need to first know my capacity (is it 400A, 380A or 373A) and calculated loads. I also need to go run the optional calc for the service entrance conductors, as that should lower my combined current demand.

 

[wwhitney]

It's good to know. Outside of the humidity issue, how do you like yours?

No complaints that would apply if it were in a dedicated laundry room. On the drain, it has a very small metal u bend at the end of the drain pipe, and the companion washer has a slightly smaller than normal drain house outlet, so together they fit in a normal 2" standpipe opening.

You would think they'd understand where you're coming from, but think he literally gave me the lug rating. "Me: Are the lugs on meter socket xyz 75C or 90C? Matt@Milbank: Hold on, I have one right here on my desk... it's a CU 9 AL, so that means it is a 90C rated lug".

OK, that answer doesn't fly. You need a better answer, or else you'll have to assume the meter terminals are rated 75C, with the various follow on effects.

I was going that way, but it looked to my benefit to consider it an incremental SABC. 1500VA x 0.35 vs 1200VA x .75, if I did that right. It's a microwave that is rated for either countertop or install, but going to sit on a shelf in a cabinet, with required clearances.

I don't think that works--the SABCs are for receptacles that serve the countertops or general purpose wall outlets. A receptacle in a cabinet isn't an SABC.

I wasn't clear how accepting AHJ would be of 'custom/homegrown solutions' like this.

No clue. If the steam shower and the other load are nearby, you could use a manual transfer switch near the steam shower, normally in the position to run the other load, but when you want the steam shower, flip it to the other position. Pretty sure any AHJ would accept that, it's not at all home brew. Would be much bigger and perhaps ugly.

Cheers, Wayne

 

[Rossn]

No complaints that would apply if it were in a dedicated laundry room. On the drain, it has a very small metal u bend at the end of the drain pipe, and the companion washer has a slightly smaller than normal drain house outlet, so together they fit in a normal 2" standpipe opening.

Interesting.

OK, that answer doesn't fly. You need a better answer, or else you'll have to assume the meter terminals are rated 75C, with the various follow on effects.

Agreed. I will be calling Monday.

I don't think that works--the SABCs are for receptacles that serve the countertops or general purpose wall outlets. A receptacle in a cabinet isn't an SABC.

I had a feeling you would say that. Technically you are right. But I did have the conversation with AHJ, who basically told me to have the doors off during inspection.

No clue. If the steam shower and the other load are nearby, you could use a manual transfer switch near the steam shower, normally in the position to run the other load, but when you want the steam shower, flip it to the other position. Pretty sure any AHJ would accept that, it's not at all home brew. Would be much bigger and perhaps ugly.

Cheers, Wayne

That is an interesting approach, though not sure it will pass the spouse test. Once I figure out my capacity and demand, I'll start working through some options on that front.

 

[wwhitney]

OK, that answer doesn't fly. You need a better answer, or else you'll have to assume the meter terminals are rated 75C

FWIW, UL 414 (Standard for Meter Sockets) says in section 27.10.5:

If a meter socket is intended for and has been tested for use with 90C ampacity-sized conductors on the line side of the meter socket in accordance with Exception No. 1 to 14.6, the meter socket shall be marked "Conductor sized for 90C may be used on the line side of the meter socket" or equivalent.

So you should be able to look for such a marking, although I don't know if it could be inside the enclosure, which you couldn't safely check with the line side energized. If you talk to the manufacturer, that's the marking you want to ask about.

With the interpretation that you have a service for just one dwelling unit, those 500 MCM conductors get to use the 83% factor, so if you are limited to the 75C ampacity of the 500 MCM Al, that gives you a service rating of 310/83% = 373A. That already exceeds twice the 180A ampacity of the 4/0 Al feeders you have, so the 500 MCM isn't currently a bottleneck.

But it does limit your options for benefiting from upsizing the 4/0 Al feeders to 250 MCM Al or 3/0 Cu Al--you'd need to upsize the 500 MCM Al to either 600 MCM Al or 400 MCM Cu to get a full 400A capacity. [Or two parallel sets of 4/0 Al would get you 395A, or two parallel sets of 250 MCM Al or 3/0 Cu would get you over 400A.]

Cheers, Wayne

 

[Rossn]

FWIW, UL 414 (Standard for Meter Sockets) says in section 27.10.5:

So you should be able to look for such a marking, although I don't know if it could be inside the enclosure, which you couldn't safely check with the line side energized. If you talk to the manufacturer, that's the marking you want to ask about.

Thanks - I will ask specifically about that. I went back and looked at pictures I had from before it was tagged shut, and from what I can see, I don't see that marking... I suspect it will support only 75C, but will check with Milbank tomorrow and report back.

With the interpretation that you have a service for just one dwelling unit, those 500 MCM conductors get to use the 83% factor, so if you are limited to the 75C ampacity of the 500 MCM Al, that gives you a service rating of 310/83% = 373A. That already exceeds twice the 180A ampacity of the 4/0 Al feeders you have, so the 500 MCM isn't currently a bottleneck.


But it does limit your options for benefiting from upsizing the 4/0 Al feeders to 250 MCM Al or 3/0 Cu Al--you'd need to upsize the 500 MCM Al to either 600 MCM Al or 400 MCM Cu to get a full 400A capacity. [Or two parallel sets of 4/0 Al would get you 395A, or two parallel sets of 250 MCM Al or 3/0 Cu would get you over 400A.]

Cheers, Wayne

My guess is that 373A is going to be my max service, and I'll need to work with that. I went back and ran the calcs for the service conductors, and ended up at 341A for the standard approach and 375A for the optional approach. I'm getting ready to check the neutral for ampacity, per 220.61, but practically speaking, can't think I have enough 120V loads to exceed a 400 MCM.

Is it right that when a mfg specs (for example) mini split with MCA of 42A @ 230, I should be using a value of 9660 VA, but then at the end I can divide the sum of all the VA for the calculation by 240? As silly as it may sound, I'm not clear when it's appropriate to use 230 vs 240... I had always considered those to be a nominal value.