New wall oven installations require a four-wire circuit for 240 volts, and the circuit must have a separate ground wire, insulated neutral wire and two hot wires. The size of the wire depends on the oven being installed.
Most newer homes are wired to suit this required four-wire circuit configuration. Many older homes have the same 240-volt circuit, but it has been wired with a three-wire cable that has the ground wire and two hot wires, leaving out the insulated neutral wire. Most new electric ovens can work with either type of circuit, although it is important to check the specifications of the new oven to ensure that it can work either way.
If a new wall oven is going into a home with an older three-wire circuit, it is common to bond the oven's neutral and ground wires together and to connect those bonded wires to the ground wire on the three-wire circuit. If a homeowner is installing an older oven that just has three wires into a home featuring a four-wire circuit, it is common to cap off the neutral wire from the circuit and leave it unused. People with questions about their own ovens and wiring configuration should consult a professional electrician.