I'm not a licensed electrician, but I have years of experience with code-compliant (permitted and inspected) electrical work for RESIDENTIAL properties that I own. I'm currently preparing to make some electrical modifications in a COMMERCIAL space that I lease, and I'd like to do the work myself.

My project will be limited to extending existing circuits and replacing older wiring, conduit, receptacles, switches, and fixtures. I'm familiar with the basic concept of single-phase residential wiring vs three-phase commercial wiring, but, other than running wire through conduit, do any of the differences come into play downstream from the main panel?


Same basics apply, but there are going to be differences in wire cladding depending on property usage. Any idea yet what the space will be used for, legally-speaking? Assembly, retail, storage of rabid weasels?

  • Primarily office use with a touch of retail, but I'd like to allow for the possibility of rabid weasel storage down the road. – JoshR Sep 4 '17 at 1:46
  • Stick to fire-rated boxes & MC clad cabling, and you'll be fine. – NPM Sep 4 '17 at 1:48
  • Thanks. But MC-clad running through conduit would be redundant, wouldn't it? Is this where THHN comes in? – JoshR Sep 4 '17 at 2:35
  • Correct. MC is a stand-alone application. – NPM Sep 4 '17 at 10:53

You use conduit pretty much universally unless your AHJ says you don't have to. Metal conduit is the ground. AC is not.

You also work in THHN/THWN wire, and you use stranded wire unless you're a masochist. Shucking Romex is right out, except for pigtails.

We tend to use 12AWG because being stranded, it is cake to work with, and AHJs like 12AWG, and that means no backstabs, ever.

And you use a lot of colors. A residential romex-flinger only ever sees 3 colors: black white red, and you know that guy's been around when all the THHN in the conduit are Nazi flag colors. You definitely should buy every color under the sun, especially gray, which is the alternate color for neutral. Taste the rainbow!

Choose colors for clarity!

Romex people are habituated to using different colors for things that are actually the same. If your 3way circuit is supply-3w=3w-lamp, red and black for messengers? Use yellow and yellow. The two messengers are the same thing. (And maybe red or orange for the lamp run since it is a switched hot). Same with 2 hots in a 240 circuit - black and black, you don't need to tell them apart.

Here's a great example of what not to do: Two parallel conduits, 8 wires in each. One conduit has 10AWG wires: 4 red and 4 black, for 240V circuits. The other conduit has 12AWG wires: 2 red, 2 black, 4 white. The marks have all fallen off. By cut lengths it's obvious he meant each 240V circuit to have 1 red and 1 black, but you can't tell them apart!! UGH! I'm pulling it all out, and with minimum wire replacement, rearranging it thusly:

10 AWG: Each conduit gets 2 red and 2 black. 12AWG: each conduit gets one each of white, gray, black, and red or blue. (That's how I tell the conduits apart.) Now it's stupid-easy to distinguish all wires. How do I distinguish the two 240V circuits in each conduit? Easy, The two blacks are circuit 1, the two reds are circuit 2.

It's a mind-blower, isn't it?

Tape is also your friend. Get multicolor tape and don't spare it. It's good for grouping and good for marking. You're not allowed to remark hots to neutral or ground, so white, gray, or green on hots is just a marker.

The realistic max for conduits is 4 circuits, and there are only 2 neutral colors. So tape your gray neutrals white, and your white neutrals gray, for 2 more colors.

  • Did you mean "MC" when you said "AC" in "AC is not ground"? – ThreePhaseEel Sep 4 '17 at 2:17
  • Also, striped THHN is a thing :) – ThreePhaseEel Sep 4 '17 at 2:19
  • @ThreePhaseEel I meant armored cable, yes. Very cool about striped THHN. – Harper - Reinstate Monica Sep 4 '17 at 3:10
  • I mentioned the thing about MC because "armored cable" = AC, which still is a ground path, while ordinary MCI isn't (vs MCC which shows up practically speaking as MC-HL or that newfangled MCI-A stuff) – ThreePhaseEel Sep 4 '17 at 3:15
  • That's a ton of great info and far more than I'll likely ever use at DIY level. Thanks! – JoshR Sep 6 '17 at 4:22

One obvious difference is that the NEC has multiple articles for "required branches" and receptacle spacings for various rooms in residential installations but nearly none in commercial.

  • Interesting distinction. Thanks for pointing that out. – JoshR Sep 6 '17 at 4:20

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