I'm converting garage space into workshop space. I want to add a 100A subpanel in the workshop.

The existing 200A main panel is in the same detached garage as the workshop-to-be; the subpanel will be ~25' away from the main panel on the same wall. The wiring will run through residential attic space.

From my research, it appears that my choices are #3THHN/THWN with either a #8 copper or #6 aluminum ground in 1-1/4 inch PVC conduit, or I could use #2/3 Romex without ground.

Is my research correct?

 

A 100 amp subpanel does not have to be fed with a 100 amp feed. Usually a 60 amp feed is enough for a workshop. What loads will be in the workshop?

 

No matter which way you do it (PVC conduit or NM B cable) or how many amps you feed it with, the subpanel must have an appropriately sized grounding conductor.

 

Ray, I tend to do the whole overkill thing. Loads will include a separate air conditioning system (the primary reason for the workshop conversion), standard garage power tools, a welder, a 20A laser cutter, occasionally an oven (not for food), a computer, and who knows what else.

Joe, something went wrong between my brain and my fingers when I started this thread... I will absolutely run a ground separate from the neutral between the main panel and the subpanel. I had intended to say "#2/3 Romex without CONDUIT."

Hopefully that makes this thread make more sense. So... am I correct that I can either use THHN in conduit or 2/3 Romex without conduit?

 

I doubt that you will find NM that large. Type SER cable will be available in the larger sizes.

 

You can get 2/3 w/g NM-b at a supply house. You will not find it at the big box.

At big orange you can get 2-2-2-4 SER AL for about $1.50 a foot. You would be limited to 80 amps but that would likely plenty for your shop.