We have an old wall oven and cook top, circa 1950's in our home. I want to know, is it okay to wire my new freestanding oven/range using 8/3 wire to the existing 10/3 wiring in the wall on the existing 40 amp breaker. Will it trip the breaker if I cook using the oven and the range or will it melt the wires and cause a fire? How safe is it and is it recommended. :confused:

A 40 amp circuit needs a minimum of #8 wire, or the risk is overheating of the #10 wire, and risk of fire.

Usually a cooktop/oven combination uses a #8 feeder cable that junctions to two #10 cables for each of the appliances at one of the appliances.

Are you sure the wire leaving the breaker is #10?