Hi. I just installed a 240volt 4000 watt 16.7 amp electric wall heater. I ran 10-2, as there are only 2 wires coming from heater plus ground. No neutral?? I hooked it up to a tandem 20amp breaker (2 20amp breakers connected). and ground wire to ground bar in panel. its working.. am i all set? just wasnt sure what breaker to use. also, its a 40 foot run.