What size gauge wire is required on a 20 amp circuit?

Answer 1

Answer for USA, Canada and countries running a 60 Hz power supply service.

A # 12 copper wire insulation 90 degrees C is rated at 20 amps.

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But be very careful: before you can get proper advice about the right size wire to use you have to include information about the length of run and the intended installation method!

Is it to be hung between buildings in the open air or fixed to a wall, etc. ... ?

Is it to be in a conduit or clipped to a cable tray, or is it running underground, etc. ... ?

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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.

Answer 2

The General rule is 1 square millimeter of cable per 10 amps so in this case you would use the available 2.5 sq mm cable

12 AWG wire for typical residential use.

Answer 3

12 gauge.