How can you convert a 3 phase 208 volt pizza oven to 240 volt 1 phase?

Eliminate one wire from the 3 phase 208 volt and install a 240/250 volt 2 pole breaker . but the oven has to be rated for 240 volts check the nameplate on the oven or it will not work.

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Some three phase resistive load devices will allow the device to be used on single phase. The wiring diagrams will be on the nameplate of the device. It usually involves the repositioning of jumpers to create new connections within the device to keep the resistive coil currents equal. This is to provide equal heating throughout the oven. Other wise there would be cool spots where the third phase leg resistive coils had been taken out of the circuit.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS