Controller: small voltage drop under load

[jdudek]

I am doing some testing on my controller. Noticed that the AC voltage drops by about 5V under load. Voltmeter reads 120V at the box input when the element is off, but drops to 115 when the element is switched on. Element on current reads 18.5A or so (This is a 2250 W element, 120V/20A circuit).

is this normal or do I have an issue somewhere?

 

[RufusBrewer]

5V drop is to be expected.

Measuring a voltage source under load conditions is where you establish normal.

 

[doug293cz]

Using Ohm's Law (V = I*R) leads to R = V/I. The resistance of the power feed leading up to your controller is 5 / 18.5 = 0.27 ohms. A 20A circuit should be wired with 12AWG wire, which has a resistance of 1.6 ohm/1000 ft. Your feed resistance is equivalent to 1000 * 0.27 / 1.6 = ~170 ft of 12AWG wire. Since the circuit length contains both the hot wire length and neutral wire length, the one way wire distance works out to ~85 ft. The wires are probably somewhat shorter than this as there will also be some resistance in the connections at the service breaker panel, and the connections in the receptacle/plug.

Net: a 5V drop for an 18.5A load seems perfectly reasonable.

Brew on

 

[jdudek]

Thanks that’s makes a lot of sense.