If the 1500 watt electric heater is a permanent installation and wired heater you should use 12 AWG wire and a 20 AMP breaker.
But if it's a 1500 watt electric space heater it can work fine on a 15 AMP breaker with 14 AWG wire as long as you don't use more than the 14 AMPS.
A 1500 Watt electric heater uses 12.5 AMPS
So it's better to have the electric space heater or electric heater on 12 AWG wire and a 20 AMP breaker.