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I am putting a 125amp sub panel beside my main breaker and am figuring out the correct wire sizes.
For the 2 hots to the sub-panel, I will use 1/0 gauge.
For the ground, I will use 4gauge aluminum. For the neutral, I will use 1 gauge. I would like to use a cable as to avoid conduit to run the 2 hots neutral and ground to the sub panel from the main breaker.
I have also got a 125amp circuit breaker. What cable would I get? Does 1/0-4 make sense, since that is the largest I have? Is there a cable that exists that houses each of these sizes?\

Why am I doing this? I am putting a 60gallon air compressor in the garage, and need a 220 plug, but I do not have any 2 pole spaces available on the existing breaker.
I will be putting a 60amp circuit breaker then a 20amp on the subpanel (20 is moved from the main breaker to free up space for a 2pole). I suppose I would not use a 125amp breaker since it is not possible to draw that with only those 2 circuits in place? Would I use an 80 amp circuit breaker on the main panel for the sub panel since that is 20+60? After that, could I still use 1/0-3 since that is only rated for 120amp as you said or should I go ahead and move to 2/0-3 since that will not require any modifications if I decide to add circuits in the future?

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  • Have you actually looked at a cable supplier to see what's available? I certainly haven't seen 1/0-1/0-1-4 as a stock SE cable configuration... Commented May 18 at 3:25
  • You have a 125A breaker, is it to put in existing or part of the new panel Commented May 18 at 19:20
  • that was going to be the breaker for the subpanel (it would be in the main obviously), but I am rethinking that now, especially since ill never use 125amps and anything past 60amps seems to be quite expensive.
    – Noah
    Commented May 18 at 22:57
  • 2
    I think the dislike of conduit is rather silly. Conduit is a real lifesaver here, because it lets you pass circuits through between the panels quite easily without having to unhook cables and move the cables. I gather you're just not real familiar/comfort-zone with conduit? It's better. Commented May 19 at 6:09
  • @Harper I am not familiar with any of it LOL
    – Noah
    Commented May 19 at 15:35

2 Answers 2

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1/0-1/0-1/0-3 SER is probably your best standard configuration option for a 125A panel.

Response to Is it normal..?: Your new 125A panel just can't be protected by a feeder rated at higher than 125A. Your wire needs to be capable of handling the current allowed by the overcurrent protection.

To calculate the minimum wire size for 125A we look at table 310.16, your terminations are rated at 75°C column, we look down that column and find that 125 doesn't line up with wire size or standard breaker size in 240.6. But NEC 240.4(B) gives us a little break and allows you to round up to the next larger breaker when the rating of the wire doesn't line up with a standard breaker size. So 1/0 Al rated at 120A can use a 125A breaker. No reason to upsize to 2/0.

The minimum load you describe is 80A, give yourself capacity for one future 20A circuit you're up to 100A. You could reduce wire size one size, not a lot of savings there. If you're pressed to save a few bucks find another 2p20 or 2p30 to move out of main panel to make space for the compressor in main panel then even allowing a 20A future circuit you would be down to a 70A load, so #4 Al nominally rated at 65A would get you there.

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  • is it normal to go with the biggest size wire needed in these cases?
    – Noah
    Commented May 18 at 3:53
  • @Noah There is the minimum size allowed and sizes that sell/available. Bigger gauge is usually better and sometimes cheaper, if that sells a lot.
    – crip659
    Commented May 18 at 12:15
  • @NoSparksPlease see the updated question for why I am doing this/use case.
    – Noah
    Commented May 18 at 15:28
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I am putting a 125amp sub panel beside my main breaker and am figuring out the correct wire sizes.

Key is "beside". If the new panel is close to the old panel, use metal conduit and you can skip the ground wire. More importantly, if you use large conduit less than 24" long then you can use it for more wires, as needed, without the usual derate for multiple circuits in one conduit. That matters because you may want to rearrange circuits by moving some from the main panel to the subpanel for a variety of reasons, and that will provide the flexibility.

As will be discussed more, a 125A panel means that is the maximum you can feed it, but you can certainly feed it less than 125A. If you put your feed wires in sufficiently large conduit then you have the option to upsize later if necessary.

For the 2 hots to the sub-panel, I will use 1/0 gauge. For the ground, I will use 4gauge aluminum. For the neutral, I will use 1 gauge.

As you can see in the Southwire ampacity table, you can use 1/0 aluminum for up to 120A and, as already noted in another answer, if your panel does not have 120A breakers then you can use the next larger size (125A) instead.

However, I would be very cautious about downsizing the neutral. While you "know" 1 AWG aluminum at 100A maximum is enough, code doesn't always work that way. Except for an actual service entrance, you generally have to use the same size for neutral as for the hot conductors.

I would like to use a cable as to avoid conduit to run the 2 hots neutral and ground to the sub panel from the main breaker.

Why? If the subpanel is a long distance and/or a complicated path (e.g., through walls, around corners, etc.) then cable makes sense. But if the panels are truly beside each other then conduit is easy.

I have also got a 125amp circuit breaker.

Is that "I already have it and opened it up or got it used" or is it "I bought one thinking I need it but it can be easily returned"? If the former, use it, but you must then use large enough wire (e.g., 1/0 as discussed). If the latter then I would seriously consider returning it and getting a smaller breaker that is the right size for your use, and then you can use wire to match.

? What cable would I get? Does 1/0-4 make sense, since that is the largest I have? Is there a cable that exists that houses each of these sizes?

Yes, there are cables with the "typical" combinations. Including some smaller ones such as 2/2/2/4 which is a common size that allows for up to 90A for a subpanel feed.

Why am I doing this? I am putting a 60gallon air compressor in the garage, and need a 220 plug, but I do not have any 2 pole spaces available on the existing breaker. I will be putting a 60amp circuit breaker then a 20amp on the subpanel (20 is moved from the main breaker to free up space for a 2pole).

Keep in mind that with certain exceptions you can't move breakers from one brand of panel to another, even if they seem to fit. Unless the new panel is the same brand/product line as the main panel or the successor to that brand/product line, you need to double-check to make sure. But a standard 20A double-breaker is inexpensive for most panels unless you need AFCI and/or GFCI protection.

I suppose I would not use a 125amp breaker since it is not possible to draw that with only those 2 circuits in place? Would I use an 80 amp circuit breaker on the main panel for the sub panel since that is 20+60? After that, could I still use 1/0-3 since that is only rated for 120amp as you said or should I go ahead and move to 2/0-3 since that will not require any modifications if I decide to add circuits in the future?

This is where a Load Calculation becomes important. A Load Calculation can be on the entire service or any individual breaker panel.

A Load Calculation is not:

  • Add up breaker handles. That is way too high in most cases.
  • Electric bill usage divided by hours. That is way too low.
  • Electric bill maximum usage interval (or better, peak demand). That is conceptually correct, but not practically because usage patterns change over time. Maybe you never had a cold snap requiring auxiliary heat on Thanksgiving while cooking everything.
  • Random guesses based on what you think matters. You could be very high or very low.

It is a formula that takes into account the size of the house, fixed loads such as water heaters, EV charging, etc. Special formulas for cooking appliances. Largest of heating and air conditioning (since you won't run both at the same time). And a bunch of other things.

Before adding any large loads, you should:

  • Make sure you have capacity (Load Calculation for the entire service) to add the new loads.
  • Make sure the panel you are using has capacity (this determines your feed wire and breaker size for a subpanel feed) for the new loads.

Once you have that figured out, then you can start adding a subpanel and know what loads you can safely move between panels.

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