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I have a 200amp main panel running the house, I added a 100amp sub panel(#1)to power my finished basement,I installed a 100amp breaker in the main panel and ran aluminum feed to power the sub panel#1. I only installed eight breakers and have 12 spaces left in 100 amp sub panel#1 (both panels are in the basement). I installed a 60amp breaker in the 100amp sub panel#1 and ran ROMEX(TYPE NM-B)6/3 with bare ground Wire(to feed the future 2nd sub panel in attached garage) to my single car attached garage(6/3 wire is in garage awaiting 2nd sub panel).

I plan on installing a pool and expanding the garage to a two car in the future. What type of sub panel in the garage can I install to the 6/3 romex wire? Can I use a 100amp panel? The second sub panel will only power lights a few outlets and a pool.

Thanks, Garrett

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  • Is this garage attached or detached? – ThreePhaseEel Apr 21 '20 at 1:31
  • The garage is attached – Garrett Apr 21 '20 at 2:07
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Can't run Romex (NM-B) outdoors, need UF-B.

The "100 amp" or whatever on the subpanel is an absolute limit

OK so you live in Wyoming and the posted speed limit is 80 mph. The tire shop has tires rated for 85 mph, 112 mph, or 130 mph. Would you want the 85 mph tires? You'd be running them at redline all the time.

And so it is with subpanels. 100A or whatever is the redline, as far as bus rating.

The main breaker is irrelevant in a subpanel

As far as main breakers in the sub, that's only there to be a disconnect switch, so the amp rating on it doesn't matter.

So ultimately the answer is "feel free to use a 200A-breakered, 225A-bussed subpanel fed off a 60A breaker".

What would not be OK is powering a 100A-bussed panel off a 125A breaker.

Is your panel too small?

If your basement panel is getting close to full, one option now is to upgrade it to a larger panel, and then upcycle the old panel to the garage.

When planning breaker spaces, keep in mind NEC 2020 drops a bunch of new GFCI requirements that force even more (nearly all, at this point) breakers to be full-width breakers. Narrow half-breakers or 2-in-1 breakers (we call them double-stuff) can no longer be counted on.

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  • Thanks for the info...I only have six breakers being used in the 100amp panel for the basement and I will use underground wire when running to pool. – Garrett Apr 21 '20 at 1:35
  • BTW: the OP's garage is attached (which makes NM OK), but the fact they're wiring a pool from the subpanel means that they may not be able to use ordinary UF for the feeder to the pool. (It depends on what NEC edition the OP's working from -- 2017 reworded a bunch of stuff on that front.) – ThreePhaseEel Apr 21 '20 at 2:06
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Yes you can use a 100 or even larger sub panel. You can use 6/3 with ground. You may be limited on the total load on the 2nd sub because now you have 2 panels on the original 6/3. I would consider taking the 2nd sub all the way back to the main panel, if the wire cost is an issue I would actually go larger and go aluminum for the feeder. Looking at the cost of copper vs aluminum you could just about double the feeder size for less with aluminum. The big question will be can your main panel handle everything you want (even just the fully loaded first sub with a sub). By going aluminum back to the main panel if the main is not large enough and you having to upgrade the main you would not be limited by the sub of a sub. If the garage is attached no extra ground rod is needed guessing this because you said nmb.

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