An electrician must decide between 3 heaters. The wire that she is using is rated 15A maximum, anything beyond that is a fire hazard. Two of the heaters operate off a standard 120V circuit. One is rated 900W and the other 1800W. The third operates off a 240V circuit and can deliver 3000W of power. Which heater should be used and why?

Question

anonymous · Answer

I almost feel like its a trick question or I'm overthinking it. I know that  the 1st heater would use 7.5A, the 2nd would use 15A and the 3rd would use 12.5A. But is it always better to you a lower Amp.. or would it be better to use the one in the middle.

kropot72 · Answer

It is definitely unsafe to operate wire continuously at its maximum rated safe current. If wire is operated at its maximum rated safe current, there is no safety margin against supply over voltage or against restricted cooling of the wire.
Regarding the choice between the remaining two heaters, a 900 W heater would usually be adequate only for heating small areas. Therefore if just one heater is chosen the 3000 W unit should be selected, especially if it has user switchable power level.

anonymous · Answer

Thanks!

kropot72 · Answer

You're welcome :)

kropot72 · Answer

Another consideration is the circuit voltage. The question states that the 900 W and 
1800 W "operate off a standard 120V circuit". However the 3000W heater "operates off a 240V circuit". Therefore if a 240V circuit is not available, the economics of providing such a circuit could make choosing the 3000 W heater impractical.