150amp sub panel

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Joshua5438

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I am looking to install a 150amp sub panel in my shop, once built. It is 120ft from my main panel. Main panel is 200amp in a two car garage with little amp being used. My question is what size wire is required? I am thinking 4/0-4/0-4/0-2/0 AL MHF? Also thinking 3" conduit?
 

wwhitney

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Southwire Voltage Drop Calculator says 3/0 AWG Aluminum would give you 2.03% voltage drop with a full 150A load, and that is the NEC minimum size for a full 150A feeder. You could use 2/0 AWG Aluminum for a 135A feeder protected at 150A, with a voltage drop of 2.2% at 135A (this assumes your load calculation does not exceed 135A). So 4/0 AWG Aluminum should certainly be fine.

As for size, looks like 2" would be the minimum for Schedule 40 PVC for individual conductors. If you are pulling a plexed cable into the conduit, you'll have to find the effective diameter and confirm that the area of the bounding circle for the plexed cable is no more than 53% of the internal area. But 3" PVC should be fine.

Cheers, Wayne
 

Joshua5438

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Southwire Voltage Drop Calculator says 3/0 AWG Aluminum would give you 2.03% voltage drop with a full 150A load, and that is the NEC minimum size for a full 150A feeder. You could use 2/0 AWG Aluminum for a 135A feeder protected at 150A, with a voltage drop of 2.2% at 135A (this assumes your load calculation does not exceed 135A). So 4/0 AWG Aluminum should certainly be fine.

As for size, looks like 2" would be the minimum for Schedule 40 PVC for individual conductors. If you are pulling a plexed cable into the conduit, you'll have to find the effective diameter and confirm that the area of the bounding circle for the plexed cable is no more than 53% of the internal area. But 3" PVC should be fine.

Cheers, Wayne
Thanks for all the info! I was planning to do a 100A feed until I noticed how many amps an electric water heater requires. I plan to do a load calc before I purchase any wire but I may be able to get away with the 2/0 wire after doing the calcs, if I come in around 125A. Also it appears I may be good going with 3/0 as you have called out above if my load is just below 150A. Not trying to under do it but also dont want to way over spend for something that will not benefit me. Also, what size ground would I need for the 3/0 and/or 2/0 power feeds?
 

wwhitney

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I expect that for aluminum wire, the cost difference between 2/0, 3/0 and 4/0 will be low. I hear PVC conduit is expensive these days, not sure how big a difference 2" vs 3" would be (there's also 2-1/2"). You can price out the various options. Also check on the equipment specs on both ends, whether the lugs provided can accept 4/0 wire (if not, you can splice to a smaller size at each end, but that's some trouble). How would you provide a 150A supply for this feeder? In the few panels I'm familiar with, 2 position double pole breakers max out at 125A, so 150A would require a special 4 position double pole breaker.

As to sizing the EGC, it works like this: in aluminum, a #6 EGC is good for up to 100A, and a #4 EGC is good for up to 200A. However, this assumes that your circuit conductors are the minimum size allowed for the feeder. I.e. for a 100A breaker, #1 Al; for a 125A breaker, 1/0 Al for a load up to 120A, or 2/0 Al for a load of 120.5A to 125A; for a 150A breaker, 2/0 Al for a load of up to 135A, or 3/0 Al for a load of 135.5A to 150A. [That's all assuming 75C wires and no derating required for temperature or number of conductors.]

When your circuit conductors are upsized, the EGC should be upsized correspondingly. So if you put 1/0 Al on a 100A breaker, you should use a #5 Al EGC (which isn't commonly available, so use #4). Or if you put 4/0 Al on a 125A breaker, you need to use a #2 Al EGC. [There is an argument that for a calculated load of 120A or less, you would need a #1 Al EGC, but perhaps that's taking things too far, I'm not clear on this subtlety.]

This rule is found in the NEC as 250.122(B). The wording on that section has changed some in the past decade, so depending on which version of the NEC you are under (2014, 2017, or 2020), it may be that the EGC upsizing only applies when the circuit conductors are upsized for voltage drop, rather than in all cases. So if you have a compelling reason not to upsize the EGC (e.g. it would require bumping up one conduit size and you don't want to), and if you are under a version of the NEC that only requires it for voltage drop, and if the minimum size checks out for voltage drop and you are upsizing the circuit conductors for some other reason, then you could skip upsizing the EGC and just use #6 or #4 per the OCPD size.

Cheers, Wayne
 

Joshua5438

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I expect that for aluminum wire, the cost difference between 2/0, 3/0 and 4/0 will be low. I hear PVC conduit is expensive these days, not sure how big a difference 2" vs 3" would be (there's also 2-1/2"). You can price out the various options. Also check on the equipment specs on both ends, whether the lugs provided can accept 4/0 wire (if not, you can splice to a smaller size at each end, but that's some trouble). How would you provide a 150A supply for this feeder? In the few panels I'm familiar with, 2 position double pole breakers max out at 125A, so 150A would require a special 4 position double pole breaker.

I decided on 2.5" conduit.
I was looking at the special 4 position double pole breaker. I also read that you can install lugs to make it a pass through. I need to search that more and see if that's actually allowed.

As to sizing the EGC, it works like this: in aluminum, a #6 EGC is good for up to 100A, and a #4 EGC is good for up to 200A. However, this assumes that your circuit conductors are the minimum size allowed for the feeder. I.e. for a 100A breaker, #1 Al; for a 125A breaker, 1/0 Al for a load up to 120A, or 2/0 Al for a load of 120.5A to 125A; for a 150A breaker, 2/0 Al for a load of up to 135A, or 3/0 Al for a load of 135.5A to 150A. [That's all assuming 75C wires and no derating required for temperature or number of conductors.]

When your circuit conductors are upsized, the EGC should be upsized correspondingly. So if you put 1/0 Al on a 100A breaker, you should use a #5 Al EGC (which isn't commonly available, so use #4). Or if you put 4/0 Al on a 125A breaker, you need to use a #2 Al EGC. [There is an argument that for a calculated load of 120A or less, you would need a #1 Al EGC, but perhaps that's taking things too far, I'm not clear on this subtlety.]

This rule is found in the NEC as 250.122(B). The wording on that section has changed some in the past decade, so depending on which version of the NEC you are under (2014, 2017, or 2020), it may be that the EGC upsizing only applies when the circuit conductors are upsized for voltage drop, rather than in all cases. So if you have a compelling reason not to upsize the EGC (e.g. it would require bumping up one conduit size and you don't want to), and if you are under a version of the NEC that only requires it for voltage drop, and if the minimum size checks out for voltage drop and you are upsizing the circuit conductors for some other reason, then you could skip upsizing the EGC and just use #6 or #4 per the OCPD size.
Cheers, Wayne

I will have to read that a few more times to figure it out. That's a lot of info!
 

wwhitney

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I decided on 2.5" conduit.
I also read that you can install lugs to make it a pass through. I need to search that more and see if that's actually allowed.
You can use feed through lugs on the bus if you can get the appropriate part listed for your particular panelboard. Then you'll either need to run 200A conductors (the main breaker size) to a disconnect with a smaller OCPD, or else use one of the tap rules that would allow you to use conductors sized for the smaller OCPD.

Cheers, Wayne
 

Joshua5438

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You can use feed through lugs on the bus if you can get the appropriate part listed for your particular panelboard. Then you'll either need to run 200A conductors (the main breaker size) to a disconnect with a smaller OCPD, or else use one of the tap rules that would allow you to use conductors sized for the smaller OCPD.

Cheers, Wayne

I will look in to that. It's a Square D panel.
20210623_193807.jpg
 

wwhitney

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HOMC30UC is the part number for the cover. You'll need to keep looking for the part number for the panel.

Cheers, Wayne
 

wwhitney

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Those last two look like you took the dead front off? If so, one of those labels should have a panelboard model number on it.

But the second photo shows that HOML2125 as a 125A subfeed lug assembly, and it doesn't show HOML2200 (or HOML2225). So at the time the panelboard was made, it wasn't listed for HOML2200. That may well be because HOML2200 was not yet a product. In which case you could only use HOML2200 if Square D went back and tested HOML2200 in your panel. Which you could check if you find the model number.

But 125A would likely be fine, you can do the load calc. For an electric water heater, you could use a heat pump model. You'd still want to check the panel labels for the text about "maximum branch circuit breaker 125A" or whatever to confirm that HOM2125 would be OK to use.

Cheers, Wayne
 

wwhitney

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No, that one says "see panelboard interior for catalog number". So there should be another label with the model number.

I'm not familiar with the history of the Homeline series, it may be that it's a new enough product line that all of them are now listed for the 4 position breakers. For 150A, that would be HOM2150BB, and the instruction sheet is here: https://download.schneider-electric..._Name=48840-229-01.pdf&p_Doc_Ref=48840-229-01

125A with HOM2125 would be simpler, but that only accepts up to 2/0 wire.

Cheers, Wayne
 

Joshua5438

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No, that one says "see panelboard interior for catalog number". So there should be another label with the model number.

I'm not familiar with the history of the Homeline series, it may be that it's a new enough product line that all of them are now listed for the 4 position breakers. For 150A, that would be HOM2150BB, and the instruction sheet is here: https://download.schneider-electric..._Name=48840-229-01.pdf&p_Doc_Ref=48840-229-01

125A with HOM2125 would be simpler, but that only accepts up to 2/0 wire.

Cheers, Wayne

I will search for more labels tomorrow. I was about to ask about the HOM2150BB. Will let you know what I find.
 

Joshua5438

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It looks like my panel is compatible with the 125, 150 and 200 amp breaker. Unfortunately the 150A breaker is $300. Not sure if that is typical or if that due to all the price increases. If I do 150, I will do the 4/0 MHF wire. If I do the 125A I will do 2/0?
 

Joshua5438

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That makes sense to me. Time to do the load calc. I bet 125A will be plenty.

Cheers, Wayne
Researching the amp draw now. I have some big items going in so I am a little concerned. I have a welder but it will not be used much (50A). I also plan to have a 2 post car lift (40A) and air compressor (20A). The big item right now is the tankless electric water heater. It requires 80A breaker. I may go with a traditional water heater as that is just a lot of draw. I hate to keep a tank of water heated in an unheated garage that will only be used once a week.
 

Joshua5438

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I may look into having another meter installed. Not sure how much the power company will charge to install? It may be my best bet at this point.
 

wwhitney

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I may look into having another meter installed. Not sure how much the power company will charge to install? It may be my best bet at this point.
A common thing to do is to get a 400A service with (2) 200A service disconnects all in one box, one can service your existing main, and the other can serve your outbuilding.

Cheers, Wayne
 
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