GFCI protection for multiwire branch circuit in shipping container workshop

Question

Wiring my shipping container workshop, I'm running a 30- or 40-amp feeder from main breaker panel in house, to a main lug subpanel in the workshop. Wiring within the workshop using THHN in PVC conduit and metal boxes.

For the main row of outlets along my workbench, I'm adopting a cute plan (I may have heard here). Run a multiwire branch circuit (MWBC), so four 12awg wires (green, white, black, red). Each outlet box will have one outlet driven from the black ungrounded conductor and one outlet driven from the red ungrounded conductor (either two separate duplex outlets or a split duplex outlet). Neutral will be pigtailed, and circuit will be protected by two-pole 20amp breaker. One box will have a 240v outlet using both ungrounded conductors. So far so good.

But I realize GFCI protection is probably a good idea, at least for the workbench outlets, given it's a big steel box sitting on the ground (though the floor is wood). I don't have room in the main panel to have a GFCI breaker driving the feeder (need space-saver type), and I'm not using a main breaker subpanel. One scheme might be to simply put use a GFCI breaker for the two-pole breaker protecting the outlet MWBC. Or I could simply put two GFCI outlets at the box nearest the door, which will be where I plug in extension cords for outdoor use, and such.

I'd appreciate folks' thoughts on my best solution.

Answer 1

Since you have a metal structure and metal boxes, use metal conduit specifically EMT. Buy a bender and it's not that hard to work with. The EMT itself is a valid ground path, and you will not need to run a ground wire at all. Attaching the metal boxes and EMT clips to a metal building is icing on the cake. I have exactly this situation and I never run grounds, and there's never been the slightest doubt as to the reliability and ampacity of grounds.

Price 2-pole GFCI breakers for the panel you are using. You may be in for a shock. Your scheme of using MWBC will absolutely require a 2-pole GFCI. ThreePhaseEel's advice of "just run one more neutral wire and get out from under the exotic breakers" is very good advice.

Using the same circuit for both 120V receptacles and also a 240V receptacles, I'm not a fan. Generally if an American appliance wants 240V, it's because it needs that much power, and so it needs all of it. THHN is cheap and containers are not large, so just run an extra pair and be done with it.

Remember the 310.15 derate rules: you can have 9 active conductors in any conduit of any size (before having to upsize wire per the derate). Grounds never count, nor do neutrals in split-phase circuits (including MWBC)**. So if you run a 240V circuit, a 120+240V MWBC, and two plain 120V circuits, you are at 8. I often will lay two conduits next to each other if I expect to exceed 4 circuits.

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** Neutrals don't count in split-phase because they only carry differential current: Suppose a neutral is carrying 15 amps, and one hot is carrying 20, that means the other hot is carrying 5. The heat from 2 wires carrying 15+5 amps is less than the heat of one carrying 20A, so those 2 wires get counted as one. Heat is a function of current squared.

You ask, why 9? In a 3-phase "wye" configuration, neutral doesn't count for the same reason. So three "wye" circuits add up to 9 conductors that count (12 wires in 3/4" EMT conduit since conduit=ground). At 277V you can put 277*20*9=49860W - frickin 50KW, enough to light up a CostCo.

Answer 2

Bare the ground to save space

First off, I would suggest using a bare ground wire instead of green THHN. This saves you a bit of conduit space, which is something you will be thankful for later when you go to pull wires.

Put the 240V on its own breaker and homerun

I would put the 240V circuit on its own breaker and homerun. This gives you more flexibility with the GFCI protection for the 120V receptacles, and also allows you to upgrade it to a bigger 240V circuit provided conduit space allows (you can use 1" conduit and a 5" square box with a reducing ring for this).

Double the neutral for double the fun

On your 120V run, I would run two separate neutrals instead of a MWBC, as neither conduit fill nor the 310.15 derate are an issue for this application. You can still run the split receptacles by breaking both tabs (hot side and neutral side) off each receptacle, and it allows you to use two GFCI receptacles or two single pole GFCI breakers instead of a two pole GFCI breaker for the circuit. (Two pole GFCIs can be a bit..."blind" at times due to ground fault currents on opposite legs of the MWBC effectively cancelling each other out.)

As Harper indicated, you can use grey wire or a tape flag for the 2nd neutral. If you have a supplier for it, striped (white/red, white/black, etal) THHN is a thing, too.