[Lizardo]

I installed an external 15A GFI outlet controlled by a Lutron Caseta smart switch. In one of the receptacles our LED holiday lights are plugged in — it all works exactly as expected.

Next I plugged in a Hampton Bay landscape transformer into the other receptacle. Yard lights came on so, I thought I was home free. I turned the outlet off via HomeKit and then back on, and the GFI tripped. This happens every time — even when the transformer isn’t actually attached to any lights. (I get that the secondary shouldn’t affect the CFI, but I thought what the heck). If I turn the outlet on, unplug the transformer, then reset the CFI, then plug the transformer back in, it works. Switch it off and then on again and the CFI is tripped.

Then I plugged the transformer into another Caseta-switched outlet inside, and it works perfectly. The only difference I can see is the GFI.

This doesn’t seem like a complicated scenario, so I’m at a loss. Is the transformer drawing a lot of current when it starts? If so, why doesn’t it trip the GFI when I plug it in?

Thanks for any advice. Circa 1985 wiring, with neutral.

 

[rjniles]

GFCi receptacles don't trip due to overload , they trip due to ground fault. There is an imbalance of the current on the hot and neutral wire.

 

[viper]

Pretty common. A transformer acts similar to an induction motor on start up, with high inrush, which can trip up a GFI. There are a few work arounds but not that DIY friendly, sometimes bringing them up on a PTC to reduce inrush. Another work around is ditching the transformer for an SMPS, which will have a soft start, but they are not weather proof on their own.

If you have a 3 pin transformer, you can try a 2 to 3 pin adapter to ditch the ground. Yes, this defeats the purpose of GFI, but.... Many transformers don't have a ground pin anyway, but can still trip a GFI just due to the inrush.

 

[A Little Short]

Pretty common. A transformer acts similar to an induction motor on start up, with high inrush, which can trip up a GFI. There are a few work arounds but not that DIY friendly, sometimes bringing them up on a PTC to reduce inrush. Another work around is ditching the transformer for an SMPS, which will have a soft start, but they are not weather proof on their own.

If you have a 3 pin transformer, you can try a 2 to 3 pin adapter to ditch the ground. Yes, this defeats the purpose of GFI, but.... Many transformers don't have a ground pin anyway, but can still trip a GFI just due to the inrush.

In-rush, nor overload, will trip a GFCI. They trip on an imbalance between hot and neutral with somewhere between 5mA and 6mA of difference. The transformer may be causing some "noise" that the GFCI circuitry doesn't like, but in-rush wouldn't cause it.

 

[viper]

In-rush, nor overload, will trip a GFCI. They trip on an imbalance between hot and neutral with somewhere between 5mA and 6mA of difference. The transformer may be causing some "noise" that the GFCI circuitry doesn't like, but in-rush wouldn't cause it.

Beg to differ. Repeatedly replicated the results on the bench with countless devices and GFIs. A 2 week study. Learned a few things.

 

[A Little Short]

Beg to differ. Repeatedly replicated the results on the bench with countless devices and GFIs. A 2 week study. Learned a few things.

Unless it's a GFCI breaker, it won't trip on in-rush as the receptacles have no OCP built in.

 

[viper]

Unless it's a GFCI breaker, it won't trip on in-rush as the receptacles have no OCP built in.

I'm telling you they will, depending on the loads and the actual GFI device. We did a pretty serious study about 10y ago on them for a laboratory. It depends on the actual load against them. I cannot go into much detail because we were paid for the work but our recommendations had a few select brands and commercial spec, which had higher mA and more reliable parameters.

Something as simple as a refrigerator on a GFI can be an issue. 100 reliable starts, then 1 trip. That is just not acceptable. Basically we needed or desired hysteresis in circuit, but that would be counter productive with the intent of a GFI. There is a balance in there.

 

[A Little Short]

I'm telling you they will, depending on the loads and the actual GFI device. We did a pretty serious study about 10y ago on them for a laboratory. It depends on the actual load against them. I cannot go into much detail because we were paid for the work but our recommendations had a few select brands and commercial spec, which had higher mA and more reliable parameters.

Something as simple as a refrigerator on a GFI can be an issue. 100 reliable starts, then 1 trip. That is just not acceptable. Basically we needed or desired hysteresis in circuit, but that would be counter productive with the intent of a GFI. There is a balance in there.

I'm not saying they won't trip, I'm saying it's not from in-rush or over current unless its a GFCI breaker. Several things can cause GFCI receptacles to trip, not related to imbalance. They have circuitry in them that can be affected by "noise" in the line, lightning, surges, etc.