What do you need to operate a 240V appliance from a 120V socket?

Answer 1

To operate a 240 volt small appliance from a 120 volt source, a step up transformer will be needed. The sizing of this transformer is dependent upon the load of the 240 volt appliance.

Once the load amperage is established on the secondary side of the transformer (240) using the transformers ratio the primary side amperage can be established. This being a two to one ratio the primary side of the transformer will be double the amperage of the secondary.

For example a 1 amp load at 240 volts would need to draw an input of 2 amps at 120 volts. Notice how the transformer wattage stays in balance. W = A x V, 1 x 240 = 240 watts and 2 x 120 = 240 watts.

If the amperage of the primary side of the transformer (120) is above what a normal 15 amp wall receptacle can output then a larger 120 volt circuit will be needed.

The next step up is 20 amps with #12 wire used as the feeder conductor. Next is 30 amps with #10 wire used as the feeder conductor. This is about as far as you can go on a single breaker output of 120 volts.

Answer 2

You need the other phase, and a new outlet. Answer Completely new wiring and a different type receptacle. 10/3 cable will be required which isn't exactly cheap. For a short run though....maybe $20 for a scrap piece. Answer Not to mention a new breaker is needed.

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Given that the question states "an" appliance - just one not a house-full - you don't have to go through all that rigmarole. You could use just a step-up transformer plugged into the wall socket - but ensure the earth continuity (unless using double-insulated appliances), power rating, and obviously connector types, are suitable.

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The heading photograph, incidentally, is of the UK pattern, 230V (used to be 240V) a.c. "mains" plug and switched-socket. The large, un-shrouded pin is the earth and also opens a simple mechanical safety-gate within the socket to give the live and neutral pins access to their receptacles. The gate was introduced to guard against the risk of children poking things into sockets!