circuit breakerovenwiring
I bought a new single electric wall oven to replace an old one in an old house. It says it requires 20 amp and 240 volts. The old one was running on a 10 amp breaker. The wire from the breaker to oven is 10/3. Can I swap out a 20 amp breaker, and will I need to run new wire?
The old oven is so old I can't find any info on it to see if the wiring was even adequate for that. Previous owner was very "handy", so I'm not sure what has been done properly or not.
It's referring to the electrical service supplying the house. Most homes in the US are supplied by a single phase service, which is often described as a 120/240 Volt system. If your house is supplied by a three phase system, it could be a 120/208 Volt system.
In both these cases you'll be able to install the equipment you have.
NEMA 14 devices are four wire grounding devices. Which means they'll require two ungrounded "hot" conductors (X,Y), one grounded "neutral" conductor (W), and one grounding conductor (G). When installed on a single phase 120/240 volt circuit, NEMA 14 devices can supply 120 and/or 240 volt loads. A NEMA 14-30 device is designed to supply 30 ampere equipment.
The circuit will require a 30 ampere double pole breaker, and four at least 10 AWG copper conductors. You'll connect one ungrounded "hot" conductor to each of the X and Y terminals. The grounded "neutral" conductor goes to the W terminal. And the grounding conductor connects to the G terminal.
In this case, you would simply not hook up the neutral wire. Instead you can just put a wirenut on it and tuck it neatly into the electrical box.
Typically 240V appliances require the neutral wire so that they can run the electronics at 120v or provide a plug on the appliance. In this case, these devices are hooked up to one leg of the hot and the neutral, giving 120V.
If no neutral is required, then the device can only operate on 240V/208V, though at 208V you should expect the oven to take longer to heat up.
What it means by "Connect only to a 3-wire, 120/240-volt power supply; the neutral conductor is not required for the operation of the appliance," is that the 240V needs to come from two legs of 120V service; this device won't function on 240V mains like you'd find in Europe.
Best Answer
10/3 copper NM is rated for 30A under the NEC, so the old oven was more than adequately wired, and it will support the new oven without difficulty as well.
If you are of the unlucky type to have aluminum wire in your walls -- 10/3 aluminum NM will still handle 25A, so you will be fine.
However, the load must be considered as the nameplate load (no demand factors) per NEC Table 220.55 Note 4:
The adequacy of a 20A breaker, though, depends on whether your oven is treated as a continuous or noncontinuous load as per NEC 210.20(A):
and the definition of "continuous load" in Art. 100:
If your AHJ agrees with Tester and I that a residential oven is a noncontinuous load, then a 20A breaker is fine; otherwise, you'll have to stick with a 30A unit.