How do I connect a 4-wire oven to a 3-wire outlet?

Question

I have a new whirlpool dbl oven with a 4 wire wiring harness-white, black, red and green. My old single oven compartments junction box (house was built in '79), has 3 wires-black, white, and copper ground. 50amp, 240v circuit. How do I correctly connect these?

Answer 1

You'd have to look at the old receptacle to be sure. If it's a NEMA 10-50, then the following is true: it is being fed by common 6/2 grounded cable.

NEMA 10 is an obsolete and somewhat dangerous receptacle family, used in the old days for ranges and dryers. It provides hot, hot and neutral - 240V hot-to-hot, and 120V hot-to-neutral. It does not provide ground.

Often, this was installed using the common "/2" cable. There's no choice to color; it's always black, white and bare. In this usage, the white is not a neutral; it's the other hot. The bare wire is the neutral. Really.

Today, a white wire used as a "hot" must be marked with tape. Back in the old days, that was not required if the usage was obvious.

I would make the argument that the old circuit is "grandfathered", which it is. If the 10-50 receptacle broke, you could change it without breaking the grandfathering; in fact the stores sell 10-50R's for only that purpose. I would argue the same is true for changing it to a modern 4-prong NEMA 14-50. I would then use the NEC 2014 rules which allow retrofitting a true ground. Can't promise you the inspector would agree.

Answer 2

It appears that you might be trying to add a 240V appliance to a 120V supply. A 240V supply will contain Red, White and Black wires typically. The Red and Black are the 120V and the -120V lines, with white being common and bare copper being ground. The green on the appliance will match to the bare copper for Ground. This 3 way wire would have to be connected to a 2 pole circuit breaker, with red on one pole and black on the other.

Unfortunately, I do not think this appliance will work here, it appears you have only 120V supplied.