Electrical – How to hook up the wires for the double oven installation

You need to swap the cord on your range for a 3-pin range cord (NEMA 10-50) and configure the bonding jumper appropriately, then install a NEMA 10-50 receptacle for your stove. (This is allowed for existing branch circuits, just not new ones.)

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.