best way to do workbench 120v/240v outlets w/ GFCI protection

[RustyShackleford]

I'm running a strip of workbench outlets (THHN in conduit) in a shipping container. I want two 20amp 120v circuits plus a 240v outlet. It should be GFCI protected, at least for the 120v outlets. I'm trying to figure out the best solution.

A multiwire branch circuit (MWBC) would be most elegant: neutral and 2 hots. Thanks to 210.4(c) exception 2. But, a Siemens 2-pole 20amp GFCI breaker is $80 or so. Worse, I've heard 2-pole GFCI can miss some faults, with ground fault currents on opposite legs of the MWBC possibly cancelling each other out.

Since it's not long runs and I'm using 3/4" conduit, maybe better to just run separate circuits: two neutral-hot pairs for the 120v circuits, and neutral-hot-hot for the 240v. Protect the 120v circuits with single-pole breakers, or GFCI outlets. But two single-pole GFCI breakers is just as expensive as the 2-pole one. And I end up with 7 wires instead of 3 - kinda ugly. And I need to acquire some odd wire colors, or mark the wires (the second neutral even if I run the 240v in another conduit).

Ideas ?

 

[JFletcher]

I'm running a strip of workbench outlets (THHN in conduit) in a shipping container. I want two 20amp 120v circuits plus a 240v outlet. It should be GFCI protected, at least for the 120v outlets. I'm trying to figure out the best solution.

A multiwire branch circuit (MWBC) would be most elegant: neutral and 2 hots. Thanks to 210.4(c) exception 2. But, a Siemens 2-pole 20amp GFCI breaker is $80 or so. Worse, I've heard 2-pole GFCI can miss some faults, with ground fault currents on opposite legs of the MWBC possibly cancelling each other out.

Since it's not long runs and I'm using 3/4" conduit, maybe better to just run separate circuits: two neutral-hot pairs for the 120v circuits, and neutral-hot-hot for the 240v. Protect the 120v circuits with single-pole breakers, or GFCI outlets. But two single-pole GFCI breakers is just as expensive as the 2-pole one. And I end up with 7 wires instead of 3 - kinda ugly. And I need to acquire some odd wire colors, or mark the wires (the second neutral even if I run the 240v in another conduit).

Ideas ?

I'd run separate neutrals and dedicated circuits (3 total) and use $13 GFCI receptacles. Having a 240V receptacle will be nearly useless imho if it isnt on its own circuit. and you might want to go with 1" EMT in case you decide in the future to get a big air compressor or what not that needs #6 (from your post I assume this is for you and not a customer).

eta: you dont need to mark the wires by color: tape associated conductors together or use crimp marks

 

[RustyShackleford]

Thanks. Yes, for me personally - so since I don't even have a need for the 240v now, I can add it at leisure if needed. I'm using PVC conduit, so maybe should bump to 1" if I need to pull 3 more wires (adding the 240v) through with the two neutral-hot pairs and EGC already there.

 

[JFletcher]

Thanks. Yes, for me personally - so since I don't even have a need for the 240v now, I can add it at leisure if needed. I'm using PVC conduit, so maybe should bump to 1" if I need to pull 3 more wires (adding the 240v) through with the two neutral-hot pairs and EGC already there.

You're welcome.

If it were me I'd probably pull a #10 ground just in case I were to install something like a large air compressor - it would need only 2 new wires instead of 3.

 

[RustyShackleford]

you dont need to mark the wires by color: tape associated conductors together or use crimp marks

Is it ok to twist pairs of THHN together ? The EM fields cancelling out shouldn't be a problem , but maybe heat, or stressing the jacket ?

Easy to do by chucking in a drill, and should give a really nice clean look.

 

[JFletcher]

Is it ok to twist pairs of THHN together ? The EM fields cancelling out shouldn't be a problem , but maybe heat, or stressing the jacket ?

Easy to do by chucking in a drill, and should give a really nice clean look.

It isnt cat6a lol.

I wouldnt take the time to twist them at all. Set up your wire spools in a 2 x 2 configuration, or a left pair/right pair*. Tape a black and white THHN pair together, then the second pair 6" back. or 2' back. That's how I keep track of what neutral goes with which hot... that and/or crimp marks on the wire (none on pair 1, one each on pair 2, two each on pair 3, and so on). The wires are in conduit and not visible, so no need for looks.

When pulling multiple wires thru a conduit, fill is based on them laying on top of one another w/o any pre-twisting.

*I've pulled as many as 20 catx cables at once using staggered heads and markings on the boxes/position of the spools in the rack to know which ones are which. With those, pulls can be 200'+ and the staggers are 10-15'; it's the best way I found to cable hotels, whose rooms are ~15' across and there's ~10' vertical with the risers.

 

[RustyShackleford]

I've got a spool of red, thinking I'd do the MWBC. So I don't even need to identify the pairs (one will have black hot and the other red) - just make sure one neutral stays paired with each color hot.

But ... "When pulling multiple wires thru a conduit, fill is based on them laying on top of one another w/o any pre-twisting" ... are you saying twisting would somehow affect things, either temperature de-rating or allowable number of wires in for a given conduit size ?

 

[ptonsparky]

I've got a spool of red, thinking I'd do the MWBC. So I don't even need to identify the pairs (one will have black hot and the other red) - just make sure one neutral stays paired with each color hot.

But ... "When pulling multiple wires thru a conduit, fill is based on them laying on top of one another w/o any pre-twisting" ... are you saying twisting would somehow affect things, either temperature de-rating or allowable number of wires in for a given conduit size ?

He is saying don't bother to twist them. It means nothing to the conduit fill or electrical operation of the circuit.