Looking for any insight on my situation:

I have an A/C unit that is rated 15A, 1240W, 115V. Since all of the bedroom and living room outlets in my apartment are all on the same 15A breaker, I decided to run a heavy duty extension cord to plug this A/C unit into one of the few outlets that is on its own breaker. It's a GFI outlet in the kitchen and the breaker is rated at 20A. In order to reach the kitchen from the bedroom where the A/C unit is, I ran a 10-gauge, 30A, 50-foot extension cord.

After the A/C unit runs for about thirty seconds, the GFI on the outlet trips and the unit shuts off. I did some research before purchasing the extension cord and felt confident that it was a high enough gauge to not cause an electrical hazard; I also figured the 20A breaker would be sufficient since the A/C unit rated at 15A. However, I never considered the GFI on the outlet. Is the outlet possibly faulty or 'extra sensitive', thus the break after I turn the unit on?

Any help would be much appreciated!

  • Can you try plugging it into a different GFCI, such as the other kitchen recep circuit, or the bathroom circuit? Commented May 5, 2021 at 23:32
  • I tried: 1) Plugging it into a different GFCI outlet in the kitchen -- same result. 2) Plugging it into an outlet without a GFCI that is on the same breaker as the original GFCI outlet I tried (those are the only two outlets on the breaker, nothing else is plugged in to them) -- the unit stayed on for longer, but eventually the GFCI tripped on the neighboring outlet and the unit turned off (I guess this non-GFCI outlet is a subsidiary of the GFCI one?) Unfortunately, I already secured the extension cord to the walls and can't reach the bathroom unless I take it all down... Commented May 6, 2021 at 1:48
  • Do you still have (or can you download) the installation or user instructions for the AC unit? What does it say about extension cords? Commented May 6, 2021 at 3:38
  • @SoundGuySays Yeah, it's not a useful test if it just trips the same GFCI. Use a different extension cord. We need to exclude that anyway to see if it's the extension cord is causing it. #16 is fine for testing for an A/C of that size. Now, the A/C has a GFCI of its own right on its own cord. Does that trip? Commented May 6, 2021 at 5:21
  • @Harper-ReinstateMonica -- 1) Yes, the A/C has its own GFCI and that does not trip; it's been stable the whole time. 2) I tried testing the A/C by using a 16-gauge extension cord (I think it's also 50 feet, at least 40 feet) and that worked without tripping the GFCI; I was able to run the A/C for several minutes. Perhaps the 10-gauge I'm using is too 'big' and causing the GFCI to think its detecting a leakage? Commented May 6, 2021 at 12:34

3 Answers 3


AC units and GFCI’s don’t always plat well together. I would bet you could plug this directly ac directly into the GFCI and about the same cycle would occur.

This happens all the time with Refrigerators and freezers , just search the site for proof. The GFCI receptacle should be unaffected by a 50’ extension cord of that large gauge. not much difference than using the load terminals on a GFCI circuit.

The manufacturer says not to use extension cords to limit their liability but 1200w would work fine on a 14 awg cord.

Why is the GFCI tripping out after a few seconds? The electronics first turn on then the fans then the compressor itself. The compressor is likely causing enough of a phase shift at startup to trip the GFCI. The reason is motors are inductive devices and when they first start their ac resistance is near zero as the high current incoming is almost pure inductive there is a 90 degree phase shift in voltage and current. Most use a capacitor to cancel the shift and provide true power for starting. This phase shift is seen on inductive devices and most likely the cause for the tripping.

You can prove it’s the AC and not the cord by plugging directly in to the outlet. There are many such questions on this site.

The phantom voltages mentioned by others has voltage but no current it takes current at 5ma to trip a GFCI. The instantaneous load of a 1200w system will be close 4000w and the large phase shift due to inductance causes problems.

  • A/C does not trip when direct connected
  • Same extension cord trips all GFCIs tested
  • Different extension cord does not trip any GFCIs

It's the extension cord

Since the problem clears when you use a different extension cord to the same GFCI.

The problem is not the wires being too heavy. That's not a thing.

I suspect more a case that the cable, being so heavy, is rather expensive when bought at the conventional stores. And the "online flea markets" (AliExpress, eBay, Amazon Marketplace) had much better priced ones. Such sales "do an end run" around the mechanisms which enforce consumer-safety laws in the retail channel. As such, the builders cut corners very hard, to the point the product is not safe.

You shouldn't have permanently installed it

I'm sorry that you now have to de-affix this extension cord from the wall, but you never should have done that in the first place. It is not legal to use extension cords "as a substitute for the permanent wiring of a building".

The thing normally used for that purpose is surface conduit such as Legrand Wiremold. Because it has a liftable lid it's more of a gutter than a conduit, so it would take #12 NM-B cable (stiff but simple) or three individual #12 THHN wires: green, white and any other color that isn't gray) It's not necessary to go the whole way, just get across whichever paths or thresholds make it impossible to use a loose cable.

  • I have to amend a previous comment of mine. This morning when I tried the #16 cord, the temperature in my apartment had dropped since yesterday. Thus, when I tried the A/C on the #16 this morning, and didn't lower the temperature setting on the A/C, it ran without a problem. I tried both cords again today with the lowest possible setting on the A/C and both the #10 and the #16 caused the GFCI on the outlet to trip. So, now I'm thinking the problem doesn't lie with the #10 cord. I apologize for communicating on a false positive! Commented May 6, 2021 at 18:47
  • @soundguysays the problem with refridgeration equipment is the compressor. These take ~90% of the power being utilized by the unit. When the compressor is turned on there is a huge phase shift and this is probably causing the issue, I probably would not run a 16 gauge cord. You might find the plug and sockets are the weak point.
    – Ed Beal
    Commented May 6, 2021 at 19:31
  • @EdBeal -- Yup, I'm tracking with everything now. Overall, it seems like this very new A/C just isn't playing nice with the GFCI outlet I'm trying to, even though it's dedicated. I was hoping to use the extension cord to circumvent having to pay for an electrician this season, but, alas! I'm going to have to anyway. Harper and Ed, thanks so much for the help! Cheers! Commented May 7, 2021 at 13:32

Most GFCIs will tell you that there is a maximum distance you can use with them, because at some point there becomes a capacitive effect in the wiring that will be misinterpreted by the GFCI as non-returning current, which is how it detects a ground fault. So it will give you nuisance trips. That distance varies between mfrs so I can't say for sure if that's your issue here, but it might be.

It can also be a general incompatibility with your A/C unit, especially if it is a newer one with variable speeds. The technology to make them variable speed is often going to also cause nuisance tripping of GFCIs.

Try plugging your A/C unit into the outlet without the extension cord. If it still trips, it's always going to.

  • That makes sense, thank you for the insight. Since I didn't expect this run-in with the GFCI, I had already secured the extension cord to the wall and installed insulation around the A/C unit. In order to try plugging it directly into the GFCI without the extension cord, I would have to undo my work on insulating it. At this point, I'm thinking I'll call an electrician because whether the distance (length of extension cord) is causing the problem, or if the GFCI is outdated relative to this new A/C unit, I'll either need to have GFCI outlet replaced or a new one installed in the bedroom. Commented May 6, 2021 at 1:52

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