GFCI tripping for exterior post lights controlled from a dimmer inside home

Robert B Zaccaro

New User
Location
New Jersey
Occupation
Handyman / Licensed Electrical Contractor
I have a friend who has post lights in back yard near pool. they are controlled by a dimmer switch inside the house. When the dimmer is at full brightness or the dimmer is replaced with a standard switch, the GFI trips. When the dimmer is at approximately 75% and showing 95 volts at the post lights, the GFI won't trip. The wiring installation is horrible and I have already corrected some issues but this remains. I am going to do more testing but I wanted to know anyones thoughts on the information I have so far. Thank you!
 

kwired

Electron manager
Location
NE Nebraska
Ground fault in the circuit, but does't "leak" enough current to trip the GFCI until effective voltage reaches a certain level?

Are the lights incandescent, LED or other electronically driven light sources? Those electronically driven types may not play so well with a dimmer and a GFCI and that could be the problem, if so try at least temporarily using incandescent lamps and see if that makes the issue go away.
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
post lights in back yard near pool. they are controlled by a dimmer switch inside the house
My opinion would be , it would be more practical having the dimmer switch that feeds the pole light being protected by a GFI. I've never considered or would attempt to put a GFI on a dimming function. But that's me,
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
My opinion would be , it would be more practical having the dimmer switch that feeds the pole light being protected by a GFI. I've never considered or would attempt to put a GFI on a dimming function. But that's me,
I don't think he is trying to "dim" the GFCI. The circuit is protected by a GFCI and is tripping when either the dimmer is at full brightness or a regular switch is used.
 

hillbilly1

Senior Member
Location
Atlanta,Ga
Since you say post lights, divide and conquer, probably a damaged cable underground. Disconnect at first pole, send full voltage to the line. If that holds, reconnect and disconnect at next pole, and so on. This will give you an idea what part of the circuit is the issue. A megger would be handy to test the integrity of the wire. Could also be moisture in one of the lights, so disconnecting them one at a time is another option.
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
I'd probably try putting a normal breaker on the circuit, put an amp probe on the hot then the neutral see if there is an unbalanced reading, if not then try a different GFI Breaker or try what was mentioned in post #2 or post #6
 

LarryFine

Master Electrician Electric Contractor Richmond VA
When the dimmer is at full brightness or the dimmer is replaced with a standard switch, the GFI trips. When the dimmer is at approximately 75% and showing 95 volts at the post lights, the GFI won't trip.
There is a fault, and the GFCI doesn't detect it until its internal electronics receive enough voltage.

A GFCI should not be on the load side of a dimmer.

Added: I read more, now we need to find out what order the devices are in.
 

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Electrician
There is a fault, and the GFCI doesn't detect it until its internal electronics receive enough voltage.

A GFCI should not be on the load side of a dimmer.
The GFCI is not on the load side of a dimmer, the GFCI is feeding the dimmer. The OP said that it will trip even with a regular switch.
 

winnie

Senior Member
Actually, from reading the OP, we don't know the order of the devices. The installation _should_ be GFCI protection then dimmer then lamps, but it could be dimmer first.

If it is dimmer first, then my guess is that there is a ground fault, but the GFCI doesn't function when dimmed.

If the installation is properly GFCI first, then my guess is that it is a high impedance fault which doesn't leak enough current until at full voltage.

-Jon
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Actually, from reading the OP, we don't know the order of the devices. The installation _should_ be GFCI protection then dimmer then lamps, but it could be dimmer first.

If it is dimmer first, then my guess is that there is a ground fault, but the GFCI doesn't function when dimmed.

If the installation is properly GFCI first, then my guess is that it is a high impedance fault which doesn't leak enough current until at full voltage.

-Jon
Goid point actually. Looking at a couple of on line spec sheets, it appears that GFCIs are designed around 115VAC +-10%. 95V is about -17.5%, so out of tolerance. If the GFCI is downstream of the dimmer, that might explain it. The OPs description is a little ambiguous. He never says it is a GFCI breaker (which would put it ahead of the dimmer), it might be a GFCI in an outdoor outlet controlled by an indoor switch and they tapped off down stream of the outlet to run the lights, putting the GFCI in series with the dimmer when the switch was replaced. So with the switch back in, the GFCI gets full voltage and trips on the down stream GF, but with the dimmer at 82.5. % voltage, the GFCI shuts down and fails to trip.

 
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