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As a part of renovating my garage this fall, much of the wiring is being re-done. Since doing the work before putting up walls will be much easier now than adding circuits in the future, I'd like to run two circuits for future "big" tools.

The first is a welder- I'm very interested in learning how to weld, and will likely buy a welder in the next 1-2 years. I have heard that most of the quality welders are 220-230 volt devices, and use anywhere from 30-60 amps. If you were renovating a garage, and wanted to put in a circuit for a future welder, what amperage branch circuit would you run?

Second- in the next few months I'll be investing in a good air compressor. Currently I have a little 1-gallon pancake style compressor. In the garage, I'd ideally like something that could drive a set of pneumatic tools, be used to inflate tires, etc. I know compressors come in size from tiny, to ginormous. What size compressor (and corresponding branch circuit) would you recommend planning for?

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up vote 3 down vote accepted

In addition to amperage, you should decide what voltage you want to be able to use, and if 220, which type of outlet you're likely to need.

A 220v welder will take about half the amperage of a similar 110v welder. 90-100 amps is fairly common, but you can find smaller (and larger) welders. An air compressor will probably take a lot less current, maybe 20-30 amps for 220v or 20-50 amps for 110v.

Look around at air compressors and welders you might eventually be interested in and you can get a good idea of the amperage you want to wire for.

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A quick check on Amazon shows that many welders are 115V / 20A devices, which is not much. If that's all you want, then any size subpanel you put in the garage will be enough. 60A is a common subpanel size.

When I went to run a 30A subpanel to my septic system, I found that the smallest allowable copper wire was twice as expensive as some enormous (#2) aluminum. That's enough for 90A, ignoring voltage drop over long runs. Point is, you may end up running feeders that are way bigger than you think you need, giving you plenty of headroom for your machines later.

If you run everything in big conduit (2"), you have the option of running a single 20A MWBC to the garage now, with the breakers in the main panel, and upgrading when you've picked out your welder.

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I'm sure this question is moot for the original writer but for anyone else with a similar interest, the important point is that the garage is going to be used as a shop and the electrical infrastructure should be flexible to accomodate future needs. Thus I would recommend you install a 100 AMP sub-panel (220V of course) and surface mount it so that you can run new romex to new surface mount receptacles as needed. Go ahead and install the receptacles you know you need now. When/if you want to rearrange your shop or get new heavy machinery, it will be easier to wire them up.

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