Size of electric wire needed to serve overhead projector and screen off 20 amp breaker
Unless there's a problem with power quality, which there shouldn't be over a 100' run. 12 AWG copper conductors should be fine for a 20 ampere branch circuit.

If the equipment draws 20 amperes on a continuous basis, upgrading to 10 AWG copper can increase power quality. But I doubt your equipment will draw anywhere near that amount of current, so #12 should do fine.