I have two existing 12/3 NM-B cables that are used to connect two different circuits, which are distributed from a junction box close to the location I want to add a washing machine. The washing machine requires 12/2 wire, and a 20A fuse. Each of these existing 12/3 cables has a spare red wire. Can I use one of these red wires as neutral, and the other as load to the 20A fuse? Should I pull a new cable instead?
You should definitely pull a new cable for the washing machine circuit.
Problems with your solution
- You can't use a red wire as a neutral (See NEC 200.6(A)).
- Circuit conductors (wires) must be contained in the same "raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or cord" (See NEC 300.3(B)).
- You have not clearly described where all the wires in the junction box come from/go to, or what else is on those circuits. So there's no way to know for sure if it's possible to use them to power the washing machine.
- Laundry branch circuits are not allowed to have any other outlets (See NEC 210.11(C)(2)).
- Laundry outlets must be GFCI protected (See NEC 210.8(A)(10)).
This would be called a multi wire branch circuit. The circuit breaker for the red and black wires must be on adjacent circuit breakers for it to be legal. The red black also need to be identified so the wiring doesn’t get separated at a later date as that would undersize your neutral. This is legal because 1 wire is on L1 and 1 wire is on L2 the return current on the white is 180 out of phase so if there were a big load on both wires the total would always be below the 20A rating in this case. (When I say adjacent breakers I mean full size not “split” or double breakers in 1 space). Let me be more specific you have to use the neutral and ground with the 12-3 with the black and red on adjacent breakers. I thought I had explained this but with minus it must not have been clear now it should be.
Maybe you CAN do this.
You cannot send current up one cable and down another. Wires are grouped for a reason, so magnetic fields cancel out. Anything inside a loop of wire becomes the core of a toroid, which is much more powerful than you think, especially if there is any metal inside.
Are you saying both 12/3’s power a circuit on their black wire, and the red wires are used for nothing? Hmm, there might be a way to do this, depending on what else they go to. Specifically, there must be a site near the washing machine where a crossover can be run, and one of them must serve no loads before that site - let's call that cable B.
First, bone up on what a multi-wire branch circuit is (MWBC). Critics, bear with me!
Cable A becomes a MWBC. Its black wire serves the loads already on it. Its red wire serves cable B past the washer.
Cable B is cut at a carefully chosen place that leaves enough wire to do this: The upstream side (the homerun, let's call it cable BB now) goes from the service panel to the washing machine site. The downstream side crosses over to cable A, and its black wire is now fed by cable A's red wire. Its neutral and ground tie to cable A's.
Circuit A is now an MWBC feeding cables A and B. At the panel, Cable A's red and black must go into a 2-pole breaker, or 2 singles tied with an approved handle tie. This will force them to be on opposite poles. If you use GCFI it must be a 2-pole GFCI.
Circuit BB now feeds the washer receptacle exclusively, as required by code.
You will almost certainly need to extend one or both cables to make this work. This is allowed, as long as the splices are inside a junction box and a sensible amount of excess lead is available.
Send your house's electrician a gift basket for spending the extra $5 to put in extra wire.