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I'd like to add a 50A sub panel to my attached garage workshop.

The sub panel feeder line will run from my 200A main panel, through my basement ceiling space and enter through the garage wall. The sub panel will be 50 feet from the main panel.

Some questions for you all:

  1. does a 50A sub panel seem like an OK size for a one-man wood shop?
  2. is 6/3 NM-B romex the right wire for the 50A, 50-foot run?
  3. is it OK to run the new feeder wire through my ceiling space?
  4. should I use a second 2-pole 50A breaker for the sub panel disconnect switch?
  5. could I up it to a 60A sub panel with the same 6/3 wire or would I need 4/3?

Thank you in advance for your help, I will be doing the work myself but will get a permit and inspection so want to avoid any code violations!

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  • 1
    Have you looked on here for similar questions?
    – Solar Mike
    May 25, 2019 at 9:51
  • Is the run from the main panel to the new subpanel location going to be a "straight shot", or is it more like a snake?
    – ThreePhaseEel
    May 25, 2019 at 13:45
  • Your “future capacity” item list is power hungry, especially the electric car charger. You should likely consider upgrading this feed, the marginal cost of materials now is slight compared with time and materials to upgrade later.
    – Tyson
    May 25, 2019 at 14:15
  • Also, what make/model is your main panel, and how many spaces do you have free in it?
    – ThreePhaseEel
    May 25, 2019 at 14:54

1 Answer 1

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"Future capacity" is job one when installing a panel. Think big. Make sure you never run out of spaces. Small "main breaker" panels tend to have very few spaces, so I recommend aiming for spaces and disregarding main breaker or panel capacity. It doesn't matter anyway; read on.

6/3 NM-B is the legal minimum cable for that run. You are a long way from worrying about voltage drop for distance reasons (I don't even bother checking til 110'). You don't need to worry about "wet locations" so NM-B will suffice.

Cables in a ceiling space are usually not a problem, but there is some technical stuff about plenums (air handling spaces) that I'm not up to speed on because I just avoid them.

The subpanel does not need any disconnect switch since it's inside the same building. You might as well use a main-lug panel. Just FYI, if you did need a disconnect, and you used a main breaker, size doesn't matter. What matters is the feed breaker. There's no practical way to coordinate trips so the nearer breaker trips first, if that's what you're thinking. A 30A feed supplying a 225A sub is fine.

Copper #6 or aluminum #4 is officially rated for 55 amps. 50A is fine. However *you are allowed to "round up" to the next breaker size, so 60A is a free upgrade!

Going up, #4 copper is good for 70A only. #2 aluminum is good for 80A (75, round up) and #1 aluminum is good for 100A. (at 100A, you get a free "bump" because you're now allowed to use the 75C thermal column in the big temperature/ampacity chart. Aluminum is much cheaper and perfectly safe for large feeders like this; just use the goop and torque to spec.

Look at your excellent diagram. Note how you have the two subpanel breakers abreast. In normal panels, both of these are clipping onto the same panel bus bars, which for some reason are called "stabs". Now you have to start worrying about stab limits which are stated in your panel instructions. I recommend dodging this issue, by staggering these so your subpanel feeds are sharing their stabs with ordinary 15/20A branch circuit breakers.

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  • This is great info. I see on your linked "What is a tandem breaker" post that you say "Nowadays in new or remodel work, most circuits must be GFCI and/or AFCI". Does my 2-pole feeder breaker or any of my 2-pole circuit breakers in the new garage sub panel have to be GFCI/AFCI?
    – mitchf
    May 28, 2019 at 17:15
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    @mitchf yeah, quite a few, though I'm not exactly sure on the big 240V loads. If those must be, then one economical way to do that is use a GFCI as the feed breaker. However it's more walking when it trips. Also try real hard to have the lighting not on GFCI or not on the same GFCI as equipment, otherwise you could find yourself in the pitch black with fingers 3" away from a still-spinning saw blade - and even if you have a SawStop, I'm not sure its detection circuit will work with power off.
    – Harper - Reinstate Monica
    May 28, 2019 at 17:30

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