Quiz Question

Every time any quiz comes in any trade magazine, the first thing I do is cover the answer and try my skill. That’s the same thing I did recently when the “Fundamental’s” Ohm’s Law quiz was given in the March 12 issue.

I checkmarked all six answers, but unfortunately I got No. 5 wrong. The question was as follows: “What happens to the amperage of an electric heating element when there is a decrease in the voltage supplied to the circuit?” I marked my answer as “Amperage is increased by voltage changes,” but NATE’s Mr. Patrick Murphy, who presented this quiz, marked the answer “Amperage is decreased by voltage changes.” How? This is totally confusing.

I know the basic logic of amps and voltage. If any condensing unit of 110/115 volts gives 14 amps, the same unit made for 208/220 V will give you 7 A. The same thing here, if the voltage drops, then the amps will increase and if voltage is increased, certainly the amps will decrease, which either way is no good to any condensing unit or to a compressor unless it’s ±10% in the voltage range.

Doesn’t this logic apply to an electrical heating element? And why is it different? Can anyone explain this to me?

Miguel Barreto Mike’s Refrigeration Service Plus Union City, NJ

Publication date: 04/09/2001

Author’s Reply

Great to see that the quiz work gets everybody thinking. Your analysis in regard to motors is correct. The amperage of a motor will increase when the voltage is decreased. This has to do with a motor being an induction device.

However, an electric heating element is a purely resistive device. It behaves exactly as Ohm’s Law predicts. The Law [I (amps) = E (volts) divided by R (ohms)] is for dc circuits or ac single-phase circuits with purely resistive loads. If you enter values in the formula, then you will find that the amperage does drop when the resistance stays the same and the voltage drops.

I hope this clears up the information and I look forward to hearing from you in the future if you have any more questions.

Pat Murphy Director, Technical Development NATE

Publication date: 04/09/2001