Wiring a shop: How many circuits?

[Steve Meliza]

Properly laid out and thought about, all these things could still work with #10 everywhere. I don't know about the wire size rating you call out. I wired all my 5-20R with #10 and didn't have a problem but I'd have to check and see if they are rated for it or not--honestly do not know. Now if you go with conduit, then you can pull whatever you like whenever you like. With Romex, it takes a bit more planning.

Remember that Murphy rules the universe...you'll need/want it if you don't have it!

The Leviton outlets that I looked at were all rated for #12 max, but I just found a Cooper 5-20R that is rated for up to #10 so it is possible if one shops carefully.

The fill violation issue is that 1/2" EMT can only have 9 current carrying THWN wires of #12 and only 5 of #10 THWN. The larger 3/4" EMT allows 16 and 10 respectively. If using 1/2" EMT you're essentially now limited to two circuits (4 current carrying conductors) unless you start breaking out the multi-wire branch circuits which doesn't help as a conversion to 240V would still need 2 conductors per circuit.This also applies to device boxes, if the general 120V outlets are on two circuits with a pair of duplex outlets in each box (one on each circuit) then the box must be at least 30 cu. in. if using #10 wire and that requires a 4" x 2 1/8" square box and a raised device cover gives you maybe 36 cu. in. total which would allow the use of a fifth #10 conductor.

Let's say you bump to 3/4" EMT so you can throw a bunch of conductors in it and not have conduit fill issues. If you have 4-6 conductors you must de-rate the wire to 80%, if 7-9 conductors de-rate to 70%, 10-20 conductors drops to to 50%, etc. THWN 90C #12 is rated for 30A and #10 for 40A so you can jam 9 conductors of #12 into a conduit and still be used with a 20A breaker but only 6 of the #10 conductors are allowed to still have it be a 30A circuit. So where #12 could have 9 conductors in 1/2" EMT and allow a large degree of future expansion, going to #10 can limit you to as little as 5 conductors (1/2" EMT) or as much as 6 (3/4" EMT), not a lot of flexibility for that extra cost and hassle.

Another consideration is that if conduit is used you must pull in black wire for the hots and white wire for the neutrals. When converting to 240V you must replace the white wire with black wire. You can't use the black tape on white wire trick like you can with Romex.

So if using conduit you'll not convince me that a string of 20A 120V general purpose shop outlets should be wired #10 just in case some day you want to strip it all out and go 30A 240V instead of just pulling in new wires. Romex is of course a different consideration as you mentioned as the wires are not easily changed and fewer fill issues to dance around, but even then I don't think it is as realistic as changing a home run circuit from 20A 120V to 30A 240V.

Some of my NEC 2014 sources: 310.15(B)(3)(a), 310.15(B)(16), 314.16, and Table C.1

[John Sanford]

I didn't read all the responses, but one thing I would suggest is a SEPARATE 20a 120v circuit for ceiling outlets. Separate because it's simpler to wire. Ceiling because drop reels are very, very nice at allowing you to work in the middle of the space without cords on the floor. 2 120v 20a for the walls, a smidge over 4' up. Think about how you want to control your lights. All on at once, or do you want to be able to switch half off? If so, wire appropriately.

You'll likely end up wanting at least 2 240v circuits for tools. One for a tool, and the second for a dust collector running at the same time. Throw in any dedicated (i.e. "home run") 240v circuits for HVAC, compressor, to get your count of 240v. One thing to keep in mind is that you are not a business, so you may be able to put multiple machines on a circuit that would otherwise have to be solely dedicated, depending on your local code. Commercial installations aren't allowed to because multiple people can be running multiple machines, but you are unlikely to have the problem. So having a honkin' 40a circuit that services both your 250 class MIG welder and Wide Belt Sander is a possibility. (Note: if you ever find yourself in a situation where you're running both a welder and a WBS, let us know... ) (Yes, I know, you don't list either in your requirements, I'm just pointing out the flexibility you have using an extreme example. But you know you really do want a welder. Dontchya?)

[Andy McKenzie]

I didn't read all the responses, but one thing I would suggest is a SEPARATE 20a 120v circuit for ceiling outlets. Separate because it's simpler to wire. Ceiling because drop reels are very, very nice at allowing you to work in the middle of the space without cords on the floor. 2 120v 20a for the walls, a smidge over 4' up. Think about how you want to control your lights. All on at once, or do you want to be able to switch half off? If so, wire appropriately.

You'll likely end up wanting at least 2 240v circuits for tools. One for a tool, and the second for a dust collector running at the same time. Throw in any dedicated (i.e. "home run") 240v circuits for HVAC, compressor, to get your count of 240v. One thing to keep in mind is that you are not a business, so you may be able to put multiple machines on a circuit that would otherwise have to be solely dedicated, depending on your local code. Commercial installations aren't allowed to because multiple people can be running multiple machines, but you are unlikely to have the problem. So having a honkin' 40a circuit that services both your 250 class MIG welder and Wide Belt Sander is a possibility. (Note: if you ever find yourself in a situation where you're running both a welder and a WBS, let us know... ) (Yes, I know, you don't list either in your requirements, I'm just pointing out the flexibility you have using an extreme example. But you know you really do want a welder. Dontchya?)

Absolutely! I'm not sure what I'd do with the belt sander, but maybe I could get a Bridgeport milling machine, and machine one end of a piece while I weld the other one? Hmm....

[Jim Dwight]

I put a GFCI outlet as the first outlet in both my 120V circuits. That is required by code. You can also use a GFCI breaker but they are more expensive. GFCI breakers are not super reliable but mine are doing fine so far. When the GFCI is the first outlet it protects all the downstream outlets. They are also bulky and take up a lot of space in the box. Something to consider if you are trying to fit multiple outlets in a box.

[Zane Harris]

Dedicated circuit for lighting (I have two, but I think that was overkill)
Dedicated circuit for each major tool, or multiple tools can be combined on one circuit if they are NEVER run simultaneously
Dedicated circuit for dust collection (I have one circuit dedicated to a shop vac, and another dedicated to a dust collector)
Dedicated circuit for air compressor
At least two general use circuits
Dedicated circuits for any heating, cooling, etc.

[Marc Burt]

After overdoing it in my last shop I went more simple in this one. I have one common framed wall and 3 poured cement. The framed wall I just went romex and on the concrete I surface mounted 1/2" conduit roughly 4' off the ground. Previously I did quad outlets every 4' and that was way overkill, turns out I don't move my machines very much, so this time I'm mostly 6' and less in some places. I have two 20 amp 110v circuits sharing the quad box (one circuit on the left sides the other on the right) I did this in my last shop and liked it. I currently have just one 220v circuit. This is the only cutback I made that I regret. I wish I had another. Part of the reason (maybe most) was that I had a subpanel before and here I just made do with empty slots in the panel. I'm full right now so if I decide to expand I'll need to install a sub.

Of course I also have a dedicated lighting circuit.

I really don't understand why leaving 2 circuits in shared boxes behind is unfair to future homeowners. If they can't figure that out then they have no business whatsoever messing with their electrical anyway IMO.