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I bought an electric convection oven. The manual says the NEMA 14-30 plug should be connected to:
120/240 Volt, 20 Amp, 60 Hz power supply or 120/208 Volt, 20 Amp, 60 Hz power supply.
I understand the amps and AC frequency, but I'm unsure about the volts. Does it mean the oven will work on either a regular 120 volt circuit or a 240 volt circuit? Or does this mean I need a tandem breaker to make a 240 volt circuit?
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.
