Even at 8°F, I only switched on my block heater (mine has a switch) about 30 minutes before starting. Or 1 hour if I wasn't in a hurry. With my Cummins 5.9 it really didn't take long to heat it up and it would turn over smoothly. Mine is like 750 watts so that's only 6.25 amps and as long as you don't run an electric space heater, turn on the coffee maker, or the MW, a 15 amp circuit should handle it fine for that short time before starting.
So that's my recommendation. Don't plug it in until 1 hour before starting the engine.