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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?

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    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
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    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

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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.

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  • 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

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