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I bought a UL-listed electric fireplace a couple years ago and I'm still trying to confirm everything is normal and safe in the winter. This is a 1500 W (12.5 A) cord and plug appliance running on a 15 A circuit in the USA.

I've already replaced all the switches and receptacles to remedy grounding and backstab issues. Also replaced the breaker with an AFCI type.

I still get slightly different results with this appliance depending which receptacle I use. So today I decided to test the circuit voltage to see what's really happening. Circuit diagram

I want the fireplace to run off receptacle R6 for placement, and I'm taking measurements at R8. With appliance turned off, I'm getting almost exactly 120 V hot to neutral and 0 V neutral to ground.

With the appliance on, I'm losing 7 V. The meter shows 113 V hot to neutral, 116 V hot to ground, and 3 V neutral to ground.

When I run the appliance off receptacle R3, it improves to 117 V hot to ground and 2.3 V neutral to ground.

To make it a fair test, I also tried running the appliance off receptacle R9, and the numbers dropped to 114 V hot to ground and 4.5 V neutral to ground.

Is it typical to get that much extra resistance and voltage drop just from using different outlets in the same room?

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    Here's a quick back of the envelope estimation. In your first case, you're dropping 7 V over a two way distance. If you assume you're drawing 12.5 A, the two-way resistance works out to 0.56 ohms for #14 cable. That's equivalent to ~250 ft two way wire run, or about 125 ft from your circuit breaker panel. If this is your distance then your voltage drop is reasonable.
    – SteveSh
    Commented Dec 30, 2023 at 13:59
  • Each connection adds a tiny bit of resistance. Add a high power use on a long smaller circuit, maybe. 15 amp circuit should only go up to 12 amps, so a bit over, plus anything else on that circuit and close to an overload.
    – crip659
    Commented Dec 30, 2023 at 14:02
  • As for difference in voltage drops between outlets in the same room, it comes down to 1) how the cables are run between the outlets (and so the cable length) and 2) how tight the connections are.
    – SteveSh
    Commented Dec 30, 2023 at 14:02
  • The best choice and probably the most PITA is to run a new 20 amp circuit to R6.
    – crip659
    Commented Dec 30, 2023 at 14:10
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    There's also the contribution of more or less contact resistance on different outlets, though given measuring on an outlet other than the one you are using, that's going to be slightly counterintuitive - an outlet with more contact resistance will reduce the current drawn (due to it's resistance adding to the heater's resistance) and thus the observed voltage drop elsewhere will be less, while an outlet with low contact resistance will allow the heater to draw a bit more current and the observed voltage drop will be more.
    – Ecnerwal
    Commented Dec 30, 2023 at 14:32

2 Answers 2

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Yes, that looks perfectly normal.

In general, it's a good idea to not put heavy heating loads on general purpose 15A circuits, as they don't leave much for anything else on the circuit.

With 90 feet of 14 AWG copper between the supply and the first outlet, a 12.5A load is expected to pull 120V down to 113V (see link above.) Additional wire length to the additonal outlets adds additional resistance and voltage drop. It's possible that the appliance is drawing a bit less current than the rating given your numbers, actually.

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  • Yeah one of those chicken vs. egg math problems. If the heater is 9.6 ohm then 113V / 9.6 = 11.8 A. I haven't invested in a clamp meter yet, but your answer is excellent.
    – Robert Chapin
    Commented Dec 30, 2023 at 14:13
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Every splice will typically introduce resistance, but your result of 113v is within "Range A" of ANSI C84.1 American National Standard for Electric Power Systems and Equipment and should expect "full satisfactory performance".

enter image description here

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2
  • ANSI C84.1 has been updated a few times, but the range has not been modified.
    – NoSparksPlease
    Commented Dec 30, 2023 at 15:35
  • That's a nice chart! Lots of information in an easy to read format.
    – SteveSh
    Commented Dec 30, 2023 at 17:34

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