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

[Rossn]

Presumably you mean potential code violation--the violation doesn't exist until the load on the 4/0 Al feeders exceeds 180A.

Cheers, Wayne

I read you loud and clear and will keep that present in my mind during discussions. That said, if the electrician did not run load calculations at time of service upgrade and sized the conductors for 180A, is that permitted?

 

[wwhitney]

I read you loud and clear and will keep that present in my mind during discussions. That said, if the electrician did not run load calculations at time of service upgrade and sized the conductors for 180A, is that permitted?

It's allowed with the assertion that the calculated load is 180A or less. Whether the inspector/building department requires that statement to be supported with an actual load calculation is more of a local permitting/enforcement issue. Residential services typically just don't get that much attention, as even if the calculated load is 250A, it's unlikely to see over 150A.

Cheers, Wayne

 

[Sponsor]

 

[Rossn]

It's allowed with the assertion that the calculated load is 180A or less. Whether the inspector/building department requires that statement to be supported with an actual load calculation is more of a local permitting/enforcement issue. Residential services typically just don't get that much attention, as even if the calculated load is 250A, it's unlikely to see over 150A.

Cheers, Wayne

The county came back today with the below statement. Clearly they feel confident in their interpretation. If I had a paid account with NFPA, I'd open a technical case, but that will cost me $175 for the membership, so I'll have to think if that's something I want to invest in. Certainly isn't good if the county is not reading the code correctly, as that exasperates the issue going forward with electrical contractors getting used to working that way. Perhaps the state board I e-mailed earlier will come back with a confirmation or contradiction.

"I have talked this over with the other electrical inspectors and it is our belief that the 4/0 aluminum meets the minimum requirement of the code. If you would like to size them to what you think is appropriate you are free to do so but in our eyes you have met the minimum requirement of the code and its current state is code compliant."

 

[wwhitney]

The quoted statement is correct if the calculated load is 180A or less. Do you have a statement from them say the allowable load is 200A on each 4/0 Al feeder in your situation? That would be incorrect.

If your question to them was about that distinction, then I would respond that they haven't actually answered the question.

Cheers, Wayne

 

[Rossn]

The quoted statement is correct if the calculated load is 180A or less. Do you have a statement from them say the allowable load is 200A on each 4/0 Al feeder in your situation? That would be incorrect.

If your question to them was about that distinction, then I would respond that they haven't actually answered the question.

Cheers, Wayne

It's a good point.

Here is the scenario I presented:
"Single family dwelling with 320A service. Meter enclosure (no disconnect) feeds two 200A main breaker panels. Load calculations show the 4/0 Al feeders between meter socket and the (2) 200A disconnects will see greater than 180A."

So, I think I can ask for clarity that they are allowing a load calc up to 200A on each of the feeders, but not allowable ampacity per-se. Personally, I don't know how they could justify their stance without use of the 83% rule.

I have multiple permits open with the county currently, so I have to be thoughtful about not aggravating the inspector and becoming a target. I think he did do his due diligence by taking it to discussion with the other electrical inspectors, and probably could have chosen to blow me off and tell me that was his ruling, so I have to give him credit for that - even if they didn't come to the right conclusion. I think my next point of contact can be clarifying and presenting information that is not meant to press them further, but be more information sharing.

Do I understand correctly that as AHJ, they can effectively override the code? Of course, he is indicating they believe it is meeting NEC, and not that they are overriding.

I think my only real shot at getting the county aligned with the code is if I get guidance from a more superior authority that it is not to code. In my head, that means either something directly from NEC or from the state electrical board. Does that sound right, or is there someone else who would influence them?

My intent would be to get confirmation if that authority that it does or does not meet code, and if not, then notify the inspector I intend to upgrade my conductors, and by the way, here is a letter from 'authority' that says I'm not to code, FYI.

In the bigger picture, do I understand correctly that if I use EMS to limit the feeder loads (between meter and each of the two disconnects), that I cannot have a set point > 160A?

Is there a section of code that deals with lockouts of particular equipment that works in that manner? i.e. 'if you have Appliance A on, Appliance B cannot run, so consider the load the greater of Appliance A or B'. e.g. the scenario where I use the steam shower generator to lockout the makeup air heater. I'm just making sure I can justify to an inspector using only the larger of the two loads from my calculation, though I am not finding NEC references this directly.

Edit to add: In my county, their documentation indicates they follow state electrical codes (which are simply 2023 NEC)

 

[wwhitney]

"Single family dwelling with 320A service. Meter enclosure (no disconnect) feeds two 200A main breaker panels. Load calculations show the 4/0 Al feeders between meter socket and the (2) 200A disconnects will see greater than 180A."

OK, so the answer you got was wrong.

Do I understand correctly that as AHJ, they can effectively override the code? Of course, he is indicating they believe it is meeting NEC, and not that they are overriding.

Not as a legal matter. You still have a duty to comply with the codes even if the AHJ gets it wrong. [Although there are typically provisions for alternate methods to comply with the goals of the code with AHJ approval; I would consider that to require explicit AHJ permission in reference to that section of the code.] Conversely, if the AHJ tries to impose a requirement that is clearly not in the adopted code, you could blow them off, but if you need to get your permit finaled, you would have to sue them.

Also, the actual AHJ is usually the building official or something like that, and the inspectors are just their agents, FWIW.

I think my only real shot at getting the county aligned with the code is if I get guidance from a more superior authority that it is not to code. In my head, that means either something directly from NEC or from the state electrical board. Does that sound right, or is there someone else who would influence them?

Maybe. If you PM me their email, I'll gladly email them, although I have no credentials. I will simply provide a more technically detailed argument.

In the bigger picture, do I understand correctly that if I use EMS to limit the feeder loads (between meter and each of the two disconnects), that I cannot have a set point > 160A?

This issue of continuous vs non-continuous loads requires a bit more thought as to how it would apply to an EMS. But my first answer would be 180A, as that is the ampacity of your conductors.

Ideally the EMS would understand both the conductor limit and the breaker limit, and you could tell it you have a 200A breaker and 180A conductors, and it would impose a short term limit of 180A for the sake of the conductors, and a long term limit of 160A for the sake of the breaker. [Which, BTW, is not a safety issue as a one off, just a nuisance issue of the breaker tripping. It becomes a safety issue if nuisance tripping is frequent enough for the user to take countermeasures that would reduce safety.]

Is there a section of code that deals with lockouts of particular equipment that works in that manner?

NEC 220.60.

Cheers, Wayne

 

[Rossn]

OK, so the answer you got was wrong.

It sure seems like it.

Not as a legal matter. You still have a duty to comply with the codes even if the AHJ gets it wrong. [Although there are typically provisions for alternate methods to comply with the goals of the code with AHJ approval; I would consider that to require explicit AHJ permission in reference to that section of the code.] Conversely, if the AHJ tries to impose a requirement that is clearly not in the adopted code, you could blow them off, but if you need to get your permit finaled, you would have to sue them.

Based on that, in the event of a fire due to the conductor being undersized, do you believe I'd be at risk for insurance coverage, even if the county has informed me that the existing conductor is sufficient? You're probably not a legal expert, but curious how you are viewing it.

Also, the actual AHJ is usually the building official or something like that, and the inspectors are just their agents, FWIW.

Thanks for that education.

Maybe. If you PM me their email, I'll gladly email them, although I have no credentials. I will simply provide a more technically detailed argument.

That is a beyond-generous offer, and I'm very appreciative of your gesture. I think you can argue it 10 times better than me, and will keep this as an option in my back pocket. In the moment, I feel like the period to argue my case has passed, and continuing to do so would not do good things for my other permits. Any argument at this point would require a team of inspectors to eat their words, in order to agree with what is being argued -- unlikely unless the case is made by another authority.

I want them to be straightened out, and not just for me (I can fix the situation, even if they think it's overkill), but for all the other residents of my county who may be having non-compliant installs, unknowingly.

Unless I hear back from an authority, I think I have to do what I think is best in my situation and come back and help straighten out the county after my permits are closed out.

This issue of continuous vs non-continuous loads requires a bit more thought as to how it would apply to an EMS. But my first answer would be 180A, as that is the ampacity of your conductors.

Ideally the EMS would understand both the conductor limit and the breaker limit, and you could tell it you have a 200A breaker and 180A conductors, and it would impose a short term limit of 180A for the sake of the conductors, and a long term limit of 160A for the sake of the breaker. [Which, BTW, is not a safety issue as a one off, just a nuisance issue of the breaker tripping. It becomes a safety issue if nuisance tripping is frequent enough for the user to take countermeasures that would reduce safety.]


NEC 220.60.

On 220.60 - perfect. I have read that previously, but forgot about it.

The reason EMS was on my mind this morning was as a potential argument the inspector might make (other thing aside, of course) -- that a residence with standard 200A using 4/0 feeders/SECs carrying full load of the dwelling could be in a situation similar to mine (from the perspective of disabling the concept of load diversity on a feeder/SECs) if they employed the use of an energy management system.... that is, with an EMS, they could be expected to exceed 180A, which is not different than my situation (with or without EMS).

So... I looked through the relevant EMS sections I could find, and found that 220.70 seems to poke a hole in that argument, since it calls for the set point of the EMS to be considered as continuous in the load calculation. And, that calculation gets used to size feeders, SECs, and OCPDs, right? So, I was then interpreting the max set point I could have on an EMS is 160A? Thoughts?

 

[wwhitney]

Based on that, in the event of a fire due to the conductor being undersized, do you believe I'd be at risk for insurance coverage, even if the county has informed me that the existing conductor is sufficient? You're probably not a legal expert, but curious how you are viewing it.

- If a fire occurred because the load on the 180A ampacity 4/0 Al was over 180A, which would be very hard to pinpoint as the reason (an undertorqued and thus high resistance connection at either end is much more of an issue), then that might absolve an insurance company of liability (unclear to me), but I would think the liability would primarily be on the contractor who did the work (IANAL).

- Your 4/0 Al will basically never see over 180A unless you supply (3) 40A continuous EVSE with it and are charging 3 EVs at 40A all at once, and your other loads add up to more than 60A in practice.

So... I looked through the relevant EMS sections I could find, and found that 220.70 seems to poke a hole in that argument, since it calls for the set point of the EMS to be considered as continuous in the load calculation. And, that calculation gets used to size feeders, SECs, and OCPDs, right?

Well, that's the question, does "for the purposes of load calculations" mean for Article 220? Or is it meant to apply to any calculation anywhere in the NEC that uses the word load?

In the latter case, with a hypothetical EMS and your 180A ampacity conductors, you'd need to set the EMS at 180A / 125% = 144A. However, that's due to what I consider an overly conservative flaw in the NEC (a peeve of mine); the lesser of (conductor ampacity, OCPD / 125%) is a more rational limit, and I submitted a Public Input for the 2026 NEC to in effect make that change.

Cheers, Wayne

 

[Rossn]

- If a fire occurred because the load on the 180A ampacity 4/0 Al was over 180A, which would be very hard to pinpoint as the reason (an undertorqued and thus high resistance connection at either end is much more of an issue), then that might absolve an insurance company of liability (unclear to me), but I would think the liability would primarily be on the contractor who did the work (IANAL).

That's a good point, and interesting to know that under torquing is a more common culprit of fire issues. I'm not sure all these guys use a torque wrench, though they're supposed to.

- Your 4/0 Al will basically never see over 180A unless you supply (3) 40A continuous EVSE with it and are charging 3 EVs at 40A all at once, and your other loads add up to more than 60A in practice.

For me, the concern is that without EMS, I could easily see 150A (64A EV charging + 50A duct heater + 40-50A hot tub heater) on any given winter evening without any typical house loads considered. Run one range on top of that, and we're starting to push the limits.

Well, that's the question, does "for the purposes of load calculations" mean for Article 220? Or is it meant to apply to any calculation anywhere in the NEC that uses the word load?

Yes, that is a question.

In the latter case, with a hypothetical EMS and your 180A ampacity conductors, you'd need to set the EMS at 180A / 125% = 144A. However, that's due to what I consider an overly conservative flaw in the NEC (a peeve of mine); the lesser of (conductor ampacity, OCPD / 125%) is a more rational limit, and I submitted a Public Input for the 2026 NEC to in effect make that change.

For my case, I'm pretty much always considering conductors that will support 200A (so 160A), even though they are not yet installed. That would be a nice enhancement, thanks for submitting.

After submitting 3 e-mails to the state electrical board without response, I called today and got the voicemail for the director overseeing that area. This evening, I received a phone call from a State electrical inspector who also teaches this section of the code. Super nice guy. He tells me that 310.12 is intended to allow up to 200A (not just calculated, but actual) over a 4/0 Al conductor in my situation, and that it is entirely acceptable per code. He says he knows the intention because he knows someone on the board that deals with that section of the code.

So, now I have the electrician, county electrical inspectors (multiple), and a state electrical inspector who is teaching electricians each telling me they are entirely confident NEC allows calculated 200A load over my 4/0 feeders. Assuming they are wrong, this shows NEC has some major issues to address, because publicly, this seems to be how it is accepted in the electrical community.