I am looking at a 1650 watt water heater installed approximately 100 feet away from the breaker. In using some on-line voltage calculators available, it appears as though this branch circuit will require #10 AWG copper conductor.

Assuming these calculators are correct, the voltage drop is 4.4 % over #12 copper. I have some electricians advising that this is not correct, referring to some rather loose rules of thumb about up-sizing for runs in excess of 200 feet. "#12 AWG is fine"

I have also had it inferred that it makes a difference whether or not the load is resistive or inductive. I don't see anything about this in NEC 210-19 (using 1999). In fact FPN No. 4 explicitly includes heating loads.

Can anyone chime in here and authoratively settle the bet.

On the calculator I use 12 AWG is truly borderline with 5% drop, 10 AWG will keep you less then 3% drop.

The most recent job specs we get spec 10 AWG for 20 amp circuits longer than 75'.

{Don’t get your panties in a bunch if this explanation is not immediately understandable but} for heaters, power factor is close the 1.0, or “unity,” so only the resistive component [er, copper size] of a circuit matters. For motors, where PF<1, the reactive component of circuit impedance figures in, so conductor spacing and ferrous versus non-ferrous versus non-metallic raceway or cable jacket has an effect on voltage drop.

Offhand, I’d be tempted to save some copper by using a 240V (over 120V) heater, but looks like 240V elements start at >2kW. Then it’s a matter of what’s cheaper—a 2-pole breaker or fatter wire. If line voltage is normal, use 12AWG for 1650 watts at 100 feet. Change in performance will be barely measurable with larger cable.





[This message has been edited by Bjarney (edited 08-04-2003).]

On a resistive load, the effect of voltage drop will be less voltage applied to the heater load, which will reduce the total wattage of the heater.
In your case, the 4.4% voltage reduction would simply reduce your heater output to 1512 watts. On a motor load, low voltage will likely cause motor starting problems, and premature failure.
Remember that circuit components for electric heat often have to be sized at 125%, per 424.3.