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I have a 125amp main panel in my house. I need to add a subpanel directly below it. My plan is to use a 100amp feed with a breaker in main, run through 1-1/2” conduit to subpanel.

Wire will be no more then 3’. I will use 3/7 black copper for feeds 3/7 white copper for neutral. And 6 gauge copper for ground.

Does this sound like a good plan?

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  • The lugs you'll be landing on are aluminum, FYI. Commented Mar 22, 2023 at 21:06
  • @Harper-ReinstateMonica I believe it's OK for copper to land on AL, but if it's AL wire, it needs to have the NOALOX 'goop' on it to prevent oxidation. While many here have expressed the much lower cost of AL for this short a distance, I'd go CU, easier to work with and no goop required. Also have to agree with your advice to the OP go go big, like you said, breaker spaces are cheap. I'd even recommend a 200 amp "sub", that way they could upgrade to 200 amps later if needed, IE: EV charging, converting to an electric range, heat pump, etc. Commented Mar 22, 2023 at 22:07
  • @George the goop isn't required unless the terminal instructions say it is, per UL. Nobody ever got fired for using it, though. But I would be more inclined to use it where the metals are not alike. Commented Mar 22, 2023 at 22:25
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    Is there a reason you're using copper for this? Commented Mar 23, 2023 at 4:16
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    Please clarify what "3/7" means. Also, larger wire sizes often only come in black and you mark with white tape/paint/other for the neutral.
    – FreeMan
    Commented Mar 23, 2023 at 13:25

2 Answers 2

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If you're adding it because you're out of breaker spaces, put the panel right next to the main, and connect it with as many pass-through conduits as you can. One large enough for the feeder, and the others 3/4" (well, 1/2" will do, but most of the knockouts are 3/4").

The multiple conduits will give you redundant grounds (presuming they are EMT metal, and I would advise skilling up on metal rather than go with PVC merely because of your familiarity with it).

However, the purpose of the multiple conduits is free movement of circuits from panel to panel. A circuit whose Romex is routed into the original panel, can be placed in the new subpanel simply by extending its hot wire(s) and neutral through the conduit into the new sub, and landing them on the breaker and neutral bar there.

I would put it "next to" rather than "below" so you don't have height limitations that would limit the number of available breaker spaces. Spaces are CHEAP. Do not scrimp on the smallest panel that will meet today's needs; go BIG and cover all future needs also.

Also, hypothetically, bigger panels tend to be 200A rated. That's a GOOD thing. Think where you'll be, then, if you find you need a 200A service upgrade to support your additional loads. With a 200A rated subpanel, you simply "flip it" and make the 200A subpanel you already installed (today) into your main panel... and the 125A panel the subpanel. That was easy, and just saved you a metric ton of money.

And while we're thinking about flexibility, I recommend a generator interlock on the subpanel. That will make it easy to put loads in the subpanel on generator should there be an urgent need in the future. The new subpanel, especially with the conduit pass-throughs I mentioned, is a "natural" to be a Critical Loads subpanel. I can't tell you how many people don't think about generators at all, and then suddenly need one badly (and do very dangerous stuff to hook it up).

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Not sure what "3/7" means. 3 AWG copper is fine (75 C rating) for 100A.

If you use metal conduit for the connection (highly recommended - you are connecting two big metal boxes anyway) then you actually don't need to put in a ground wire. If you don't use metal conduit then you need an 8 AWG ground wire for up to 100A, according to NEC 250.122.

1.5" conduit is larger than you actually need. Which is good, because it will give you flexibility to move circuits from the old panel to the new panel. But you should make sure that the actual conduit is < 2' long or else it is subject to derating rules based on the number of circuits. My guess is you already planned that based on (a) one panel above the other and (b) 3' total wires, but just making sure for those who are not aware of these concerns.

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    7 strands, I suppose.
    – isherwood
    Commented Mar 22, 2023 at 20:31

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