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I am installing an outlet in my garage for a 220v mig welder. The welder specs call for 30A breaker.

The outlet for the welder is only 2 feet from the garage sub panel. The wiring refernece charts I've seen say I can use 12 gauge copper wire for this short of a distance. Does this make sense? I would have expected it to say that 10 gauge copper wire was the smallest acceptable wire to use.

The reason I'm even asking is that I am also wiring the garage for an air compressor and an air conditioner. Both will be on their own 220V 20A breakers. They will be located about 30 wire feet (one way) from the sub panel. This will require enough wire that it makes financial sense to buy the smallest gauge that will work safely and meet code.

Accourding to this link http://www.elec-toolbox.com/calculators/voltdrop.htm I could even use 14 gauge copper wire.

Realistically, what wire gauge copper wire should I install in my garage for these tools?

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  • That calculator you linked to, doesn't appear to work correctly.
    – Brad Gilbert
    Oct 27, 2011 at 22:32
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    In what way does it not work? I'm not defending or endorsing the linked calculator, it is just the first one that came up when I went searching for online calculators.
    – Scott Bruns
    Oct 27, 2011 at 23:08
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    Put in 1000 amps for the current draw, and it only goes up to 10 gauge. At that amount of current, it would blow like a fuse.
    – Brad Gilbert
    Oct 27, 2011 at 23:23
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    What size wire is feeding the sub panel? A mig welder, air compressor, and AC unit are going to be quite a load. Make sure your sub panel feeders are up to the task first.
    – Tester101
    Oct 28, 2011 at 11:07
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    The sub panel is 100A. I had it installed so I expect the electrician used the correct wire size. I'll check anyway.
    – Scott Bruns
    Oct 28, 2011 at 19:29

3 Answers 3

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The calculator you referenced is only appropriate for voltage drop and makes the disclaimer that the results of the calculation do not account for current specs or NEC. Any circuit fused for 30 amps must use a minimum of 10 ga copper or 8 ga alu. Longer runs may require an upgrade of wire size. In your case, use at least 10 copper for your welder regardless how far it is from the breaker panel. I'm sure one of my buddies here will have a NEC ref for ya.

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  • Thanks for mentioning copper and aluminum. I will be using copper. I'll update my question.
    – Scott Bruns
    Oct 27, 2011 at 21:09
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    Shirlock is correct. For this short a run, I agree with your choice of copper. It's easier to work with, and for short runs the difference in price is minor. For longer runs at higher ampacities, especially if you upsize the wire for voltage drop, the wire cost is a bigger part of the total project. Suddenly aluminum becomes very attractive. Be sure to ask new questions about aluminum here at DIY.SE. Also, if your garage is very large, you may consider a new 50A subpanel to feed the A/C, compressor, and convenience receps. It may be cheaper, and you'll get less voltage drop.
    – Jay Bazuzi
    Oct 28, 2011 at 1:47
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    The garage is not large. Since I knew I would run a welder and A/C at some point I had a 100A sub panel installed. The upcost at the time was not that much since the distance to the main panel is not very far. The big cost was trenching and laying the conduit. It was a lot of work.
    – Scott Bruns
    Oct 28, 2011 at 19:38
  • Shirlock, I ended up installing a 40A breaker with 8 gauge copper wire. I reread the welder manual and it suggested 40A as the recommended breaker size. The funny thing is that the welder manual says the cord on the welder is only 12 gauge copper. Anyway, thanks for helping me set this up correctly.
    – Scott Bruns
    Oct 31, 2011 at 17:16
  • Good luck and enjoy using that welder.
    – shirlock homes
    Oct 31, 2011 at 18:42
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I am not a lawyer, electrician, or welder. I have a 1970s Miller 250 amp AC/DC welder with 12 ga power cord and a NEMA 6-50 plug on it. That made me curious and I found that NEC does or at least did allow you to up-rate wire for reduced duty cycle welders. The receptacles do have to be marked For Welder Use Only. So, I guess you could use 10 ga wire with a 60 amp breaker if it was only used for a welder with 50% or less duty cycle. That said, I use 6 ga wire with a 50 amp breaker and either a NEMA 6-50 or 14-50 receptacle for all of my single phase small to medium size welders and plasma cutters.

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We'll assume you're in a jurisdiction where your laws adopt the NFPA's national electric code (NEC) standard. As of NEC 2017:

Although 10 AWG copper wire can carry 35 A at 75° C per NEC 310.15(B)(16), NEC 240.4(D) specifies the maximum allowable overcurrent protection (i.e. circuit breaker) as 30 A.

For these small wire sizes (as specified in 240.4(D)), run length is not considered. Voltage is irrelevant to current capacity.

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  • Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here.
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