# How many #12 wires are needed for 240V and 120V circuits in a shed in metal conduit?

I need to run a 50A 240V and 12A 120V circuit from a panel in a shed to a metal box (outside rated) in the same shed. I know I don't need a ground wire because the conduit can serve as the ground. That reduces me to 3 #6 wires for the 240. My question is, since the 240 will be a balanced (welder), load, can I use the 240 #6 neutral as the neutral for the 120V 20A circuit, and only run one #12 wire? The hot side of the #12 will have a 20A breaker, so the 20A rated outlet should be properly protected.

Side issue: do the breakers have to be GFCI?

3 different issues:

• 240V vs 240V/120V

If your 240V circuit is truly a 240V circuit then it doesn't need a neutral. In which case you have two 6 AWG hot wires for that circuit, not 3. In which case, this whole question doesn't really apply.

On the other hand, if the 240V circuit is actually a 240V/120V circuit (like a clothes dryer) then it needs neutral in order to provide the 120V part of the circuit. In which case the neutral can't be shared with any other circuit.

It isn't a question of "balanced". A welder (or other pure 240V load) doesn't use the neutral twice - half for one hot, half for the other hot. It doesn't use the neutral at all. If you did use the neutral "balanced" - e.g., two identical loads, both 120V, on separate hots and a shared neutral then, in theory, the question would arise. But there is no such thing. Even if you had two pure resistive loads that start out exactly the same, at some point one will be turned on and not the other - and the circuit is no longer "balanced".

The concept of "balanced" is important on a larger scale. You generally want the loads for an entire building to be reasonably balanced overall. But that is not a factor at the individual circuit level - either a circuit uses the neutral (not balanced) or it doesn't (no concept of "balanced" applies).

• Shared Neutral

Shared neutrals are a bad idea for a bunch of reasons. So if you actually need a neutral for a 240V/120V circuit, you can't share it with the neutral of a separate 120V circuit. But even if you could, you can't do that if either circuit requires GFCI protection as that will cause major problems.

• GFCI

The 120V circuit definitely needs GFCI protection. That can be at point of use (GFCI/receptacle) or it can be at the breaker.

For the 240V circuit, it will depend on your local rules. If your jurisdiction has adopted NEC 2020 and not made an exception then 240V receptacles require GFCI protection. Since 240V GFCI/receptacles are not a thing (at least not yet, as far as I can tell), that means GFCI/breaker. Which may be hard to get, based on my electrician's recent experience - I didn't need any for my house because NEC 2017, but he needed for another customer. However, this only applies to receptacles. It does not, as I understand it, apply to hardwired loads. For EV charging that means hardwire instead of using a receptacle. I don't know if a hardwired connection is an option for a welder, and if it is whether it would be practical in your particular setup.

The welder circuit will not use neutral at all. There is no need to run a white wire.

There is never any sharing of neutral between circuits. This is not allowed, ever. (MWBCs are one circuit). If you want multiple circuits to share a neutral, the only option is to share the hots too, by doing a subpanel.

The 20A circuit will need its own neutral.

Given that you will have multiple cables in a conduit, you need to derate their ampacity by taking the 90C thermal rating of the wires and multiplying by 80%. Thus, #12 is down to 24A (it's 20A anyway), #6 THWN copper is now 60A... and #8 copper and #6 aluminum are 44A.

Metal conduit underground must be RMC or IMC type. Costly, but it only requires 6" of burial cover, except for 12" under vehicle pathways. When used as such it is a valid ground path and no ground wires are needed in the pipe. Note that aboveground you can switch to far cheaper EMT and still get the same benefits. Flexible metal conduit cannot reliably carry ground.

The 20A 120V circuit will need a GFCI somehow.

NEC 2020 adds a GFCI requirement for most 240V receptacles, and as of 1/1/2023, hardwired loads other than EVSEs. However states have been very slow to adopt NEC 2020 and most deleted this requirement as too costly. Still, a good idea for a welder just for YOU protection!

EVSE's provide their own internal smart GFCI which will self-reset a number of times, and adding a stupid GFCI will only impede that.

• The outbuilding already has a subpanel installed; this question is relating to a box holding a 240V welder outlet and a 120V 20A outlet inside the outbuilding. Original question was misleading in that regard. will try to correct. Sep 23, 2022 at 16:47
• @Gary Well a second subpanel isn't out of the question; being on the same building eliminates ground rods, so it's just a cost thing: cost of sub+cheap feeder vs cost of 2 sets of circuit wires. Of course you can use AL wire for the welder circuit; the NEMA 6 socket will surely be listed for AL wire + 75C thermal. Sep 23, 2022 at 17:59
• @Harper-ReinstateMonica Except in Montgomery County, MD - no branch circuits on AL :-( Sep 23, 2022 at 18:17