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To make my garage more usable as a workshop, I'm planning to add a subpanel which can feed a number of additional outlets. 60 amps would be plenty for my current needs, but I'd like to go with 100 so I have the option to add an electric heater, or possibly a welder.

My garage is attached and I've found a path for the feeders that is about 40' long. What I've found so far leads me to believe I need #3 copper THHN wires, plus a #8 ground. Does that sound right?

I've also gleaned that these need to be enclosed in conduit, but I'm having a harder time nailing down the specifics for that. I'm thinking plastic conduit would be easiest for me to work with, but do I need a specific type? Also, it will be routed out through the rim joist into the exterior garage wall built with 2x4s, so max hole size is 1.4".

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I love that you're putting a subpanel in your garage.

When you go to buy wire, check the price of aluminum. Last time I did this, I went looking for #8 copper, and got #2 aluminum at 1/2 the price. That was a bundle of 2-2-2-4 SER (service entrance) rated for direct burial, but I put it in conduit anyway.

Your finding that #3 copper will carry 100A looks right to me.

The conduit you want is schedule 40 gray. You'll also need a male adapter at the main panel + a lockwasher + a plastic bushing.

1.4" will allow a max 1" nominal conduit. Big wires are stiff, so bigger conduit is better. You might be able to run one size larger conduit + conduit bodies, and then drop the size just at the garage wall.

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