I am replacing a double oven. 1974 model tappan. The new combo has 4 wires, red, white, black and green. The wall / supply is a Romex wire with white, black and bare. The new unit, Frigidaire FGMC2765PFK has the microwave plugged in to the top of the oven. How do I connect the wires for proper operation?
It took a bit of digging through the Frigidaire site, but I found the specifications page.
Voltage Rating 240V/208V/60Hz
Connected Load (kW Rating) @ 240/208 Volts 6.9/5.2
Minimum Circuit Required (Amps) 30
Single phase 3- or 4-wire cable, 120 / 240 or 120 / 208 Volt, 60 Hertz AC only electrical supply with ground required on separate circuit fused on both sides of line.
Circuit required - it says 30 Amps. This is confirmed by the installation manual which covers many models but says for an appliance rating between 4801W and 7200W (6.9kW = 6900W), circuit recommended = 30A. This is actually a good thing as it is quite likely that the existing circuit for your Tappan was 30A or larger. If so, the wires should be OK (except for the number of wires...), though you may actually need to downsize the breaker if it is currently a 40A breaker.
Wires required - it says 3 or 4. However, the installation manual (see bottom of page 3 and top of page 4) makes it quite clear that a 3-wire connection is only OK if (a) permitted by local code (it is typically grandfathered in, but not recommended, and possibly forbidden for a new appliance installation, and in any case not a good idea) and (b) the 3rd wire must be a neutral - i.e., a full size insulated current carrying wire, NOT a bare ground as you currently have.
Bottom line: You need at least one new wire. New oven needs 2 hots, neutral, ground. Old oven was either:
- 2 hots plus ground - need another wire for neutral
- 2 hots plus neutral combined with ground - not safe
If all you need to do is add one wire and the existing wires are in conduit then you can do that. If they are a cable then you will usually replace it with a new black /red /white /ground cable of the appropriate size.
If you add a wire to conduit, I believe (Harper or one of the other pros can confirm), you should actually use the existing white wire as neutral and add a different color (typically red, but anything not white, gray or green is OK) for the 2nd hot.