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.
Best Answer
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.
** 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.