I want to add a sub panel to my garage to run a plasma cutter, welder, 220v air compressor and a CNC table. Not at the same time, but the plasma and air compressor probably will at times. I currently have 120v in my garage with a 2 wire pair in a 1.125 RMC conduit.

My question is, I looked online and I'm not sure what I can do with that conduit? Should I even be trying to add a 100 amp sub panel or just settle for 60?

From what I saw online I can do up to 4 #4 wires in 1" conduit and it seems like mine is just a little bigger than 1" in exterior diameter.

Unfortunately the section between the panel and the garage is cement so I can't run a new conduit without a bigger cost so I want to do what I can with the conduit I have. I also considered running 2 #8/3 pair for the plasma and air compressor and 1 #8/2 for 110 devices instead of doing a sub panel.

The conduit exterior diameter is a little over 1 inch and it's a thicker metal conduit so I assumed it was an RMC conduit. The conduit is running into the concrete floor across my backyard to a detached garage, I don't think it is fully in concrete, it is probably a couple of feet under the concrete, but still under the concrete which I can't cut.
--From a comment by the OP

What are my options considering my existing infrastructure?

  • 2
    Is the garage attached or detached? What makes you say it is "1.125 rmc conduit", that is 1-1/8" but that is not a trade size. Does the conduit say "RMC" on it? Is it metal, in concrete, are you sure? Commented Sep 15, 2021 at 8:37
  • 3
    Is that 1.125" the actual OD of the conduit, or...? Commented Sep 15, 2021 at 11:45
  • Also, what's the HP rating on your air compressor, and what amp rating of plasma cutter/welder are you looking at for that matter? Commented Sep 18, 2021 at 15:05

1 Answer 1


A lot will depend on how serious of a plasma cutter we're talking. There are a number of cutters that work below 50 amps. The super serious ones push 80-90 amps. Since even 4-gauge copper THHN caps at 95 amps, that won't work.

Three 6-gauge copper THHN-2 wires should be able to easily fit inside your 1" conduit and that's rated for 75 amps (RMC is your ground, provided it's properly bonded). I'd stick a 70 on it to be safe (you'll have some voltage drop). That should cover up to 50 amps of cutter and a blower. A current Google search puts 6-gauge at $1.06/ft and 4-gauge at $1.63/ft, so you'd save a decent chunk going with 70 amps.

  • Thanks @Machavity, So I would basically do 3 6 gauge wires to a new sub panel on a 70 amp breaker then from there I can break it off to separate breakers for my plasma, compressor and 110 etc.. I may run a smaller gauge wire for ground in case I don't trust the conduit for ground though.
    – DruidV1
    Commented Sep 16, 2021 at 21:32
  • Correct. Just make sure it's THHN and not 6/3 NM. There's a difference.
    – Machavity
    Commented Sep 16, 2021 at 22:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.