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I have a new 5,500 BTU portable A/C for my backyard shed. The shed has a 14/2 feed to a single GFCI socket. The A/C runs fine for some cycles but frequently trips the GFCI at the end of a cycle when the cooling is shutting down. It never trips the circuit when starting. This is true with nothing else is on the circuit.

Any advice to how I can get it to work? Is this an A/C issue or a GFCI issue? Would an RV soft start smooth out the shutdown surge? (It says it is for startup) https://camperreport.com/softstartrv-questions/ Could it be back EMF from the motor shutdown? (I am a novice on this issue.) Can I add a capacitor or reverse-biased fast diode to solve it? (Are they for noise only)

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  • It could be the induction. How long is the wire and where is the G FCI located
    – Traveler
    Jul 10, 2023 at 19:36
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    Could be an actual ground fault! Imagine that - a ground-fault circuit interrupter interrupting a circuit with a ground fault. Specifically, an air conditioner produces a lot of condensate. If the condensate goes someplace it shouldn't, it can cause a ground fault. When an air conditioner stops, the condensate may collect in unwanted places, and it only takes a tiny bit of hot-to-ground to trip the GFCI. Jul 10, 2023 at 20:05
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    Just to be clear, the GFCI is in the shed and the A/C is plugged directly into that? Precisely when does the GFCI trip - the same instant when the compressor and fans shut down, or some moments afterward?
    – Greg Hill
    Jul 10, 2023 at 21:19

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The trip on shutdown is probably due to an inductive voltage spike when current is interrupted to the motor. (and the exact timing of when in the AC sinewave this happens, is a major influencer, so it can seem random).

A capacitor resists changes in voltage by flowing potentially unlimited current. An inductor does the complement: it resists changes in current by flowing potentially unlimited voltage. A motor winding is an inductor. So when current is interrupted to the motor, voltage on hot (or possibly neutral) spikes upward to infinity, or until insulation breaks down somewhere, whichever comes first :) If the insulation breaks down between hot and neutral, that is fine. However if it breaks down between hot and ground; or neutral and ground; now we have a ground fault.

First, make sure your outlet is wired correctly with neutral on the taller slot and neutral being quite close in voltage to ground. If you had a hot-neutral reverse, that would make that spike problem worse.

Second, have a surge suppressor on there that includes MOVs connecting hot and neutral. Perfect world, that MOV shorts out the spike between hot and neutral, and the GFCI is fine with that.

I would think better appliances would include that to suppress the inductive kick/spike from the motor.

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  • The MOVs sound like a good idea. I would appreciate a suggestion as to the joule rating to use. Specs show cooling: 8.4A, Rated current 10.4A, LRA 45.5A and Input cooling power 920Watts. 110/120 volt portable 5,500 BTU A.C
    – Strider22
    Jul 14, 2023 at 17:56
  • @Strider22 I would either do a like-for-like replacement of MOVs already installed on the refrigerator (that would qualify as a repair), or use a consumer product containing movs, that is UL listed per NEC 110.2 and use it according to instructions per 110.3(B). There isn't really a safe or legal way to directly fit electronic components into AC mains wiring. Jul 14, 2023 at 18:11
  • End result: Bought another A/C with a zero crossing circuit for $100 less. It works on a 15 amp protected power bar with MOVs. No trips. The suggestion by the company to use a 30 amp slow blow fuse resulted the product being returned.
    – Strider22
    Sep 6, 2023 at 0:34

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