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Thought I might get a better response here than in chit chat for wiring tech help.
I want to wire my 240V 60 gal compressor into my garage. I have a 40 amp breaker on 8/3 wire coming from the house panel to my garage which is about 25 feet. I need to add another 30 feet from the existing 220 box in the garage to the other side of the garage for a total of 50 ft from the panel. I already have enough 10/3 to wire this but was unsure if it would be enough for what I want to do. It's going to be a beeotch to fish it that 30 feet also. Would like to use the 10/3 if possible.
Should I use more 8/3 ga. or will the 10/3 be suffiecient. This compressor will only run intermittently but I would like to use a Miller 175 Mig on there as well. ( not at the same time)

bennett
 

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You didn't specify the Amp requirements of your compressor and welder. It's hard to help you without that data.

Electrical code is different in every area, you need to call your local building code people and verify everything. Note that perminent installs often require an electrical permit and an inspection to verify code compliance. If you don't do this, and your house burns down because of a fire that originates from your new circuit, your insurance may be able to get out of paying. The permit in my area is a whole $20.

In general, 10AWG is good to ~30A and 8AWG is good to ~50A. That is somewhat conservative, but you get the idea. Note that what matters is the rating of the breaker protecting that circuit, not what you connect to it. If you have a panel in the garage, I would use it as a junction box and swap that 40A on the feed to a 30A and run 10/3 wire from the box. I just used romex (the white jacketed flat wire).

I run a Lincoln SP-175T and an Ingersol-Rand 2 stage compressor on my 10/3 feedline, one at a time. The welder is speced for 22A and the compressor for 15A. It's proected by a 30A breaker.

For fishing wire, you can do "surface runs". The wire must be protected and higher than 3 feet from the floor. Just run it in metal conduit and you're set.
 

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You should never add smaller wire to a larger breaker, in your case the load requirment would probably fine for now but if you sell the house or forget yourself you could overtax the rating of the wire. You would be better of to just do it right the first time and buy 30 ft of #8.
 
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