1

I wired a bath renovation with all power routed through a GFCI outlet. With power on, and all switches off, when I touch the white wire and the bare copper wire in a fixture box, the GFCI trips (did it accidentally at first).

Here is what I have:

  1. Dedicated circuit from the breaker panel to the bath GFCI
  2. Load out of GCFI to 3 gang switch box
  3. In switch box, COPPER WIRE from the LINE in from GFCI and COPPER WIRES from 3 fixtures connected together with a pigtail out to ground the box
  4. In the switch box, WHITE WIRE from the LINE in from GFCI and WHITE WIRES from 3 fixtures connected together
  5. In switch box, BLACK WIRE from Line in from GFCI connected to 3 BLACK WIRES pigtails, one of each then connected to each switch
  6. In switch box, BLACK WIRE from each fixture connected to one pole of each of the switches

All fixtures work fine without tripping the GFCI. If I hook up a non-GFCI outlet to a fixture box, and plug in a 3-prong appliance, it works fine. I checked to ensure the Line/Load and polarity on the GFCI is correct, no visible bare wires anywhere, all connections are tight. Tried a new GFCI and the same occurs.

So, with all that, with all switches off, why would the GFCI trip if I touch white to copper in one of the fixture boxes? Seems to me there shouldn’t be ANY current in the white wire, let alone bleeding in the ground. Why would the GFCI trip if I touch the wire ends, but not trip when I hook up an outlet and appliance?

Maybe it's a predictable anomaly. But I thought GFCI's worked on change in resistance in the ground, and would think a direct connection of the white and copper to have low resistance versus an appliance. Ideas or explanations?

  • Hi Paul! It looks like you’ve accidentally created a couple of accounts. If you’d like to merge them (there are some advantages to doing so), the instructions are here. Welcome to the site! – Niall C. Dec 3 '14 at 18:13
2

GFCI devices work by comparing the current incoming on the hot wire to the current leaving on the neutral wire. If there's a difference, it means electricity is leaving the circuit somehow (a "ground fault") and the GFCI cuts power*. The GFCI doesn't do anything with the ground conductor—in fact, you don't even need to have one at all. One way of upgrading a 2-prong outlet (without ground) to a 3-prong outlet is by inserting a GFCI device upstream or using a GFCI outlet.

So I'm guessing there is a small amount of current in your scenario, and when you touch the white and ground together, some of that current goes into the ground and doesn't return through the GFCI neutral. Do any of you lights have timers, nightlight, etc.?

(BTW I'm not sure it matters for this question but your description of how the wires up is a little confusing to me. LINE and LOAD are opposite sides of the GFCI device, but you use them interchangeably. If you want a downstream device to be protected by the GFCI it must be connected to the LOAD side. Anything on the LINE side is considered upstream and is unaffected by the GFCI.)

  • THanks for the notes. I thikn I have it answered. You're right, I used line/load interchangably...the load out of the GFCI became the "line in" to the switch box – Paul Dec 3 '14 at 20:11
2

The GFCI is tripping on its own power draw.

GFCI works by measuring the current on the hot and neutral wires entering the outlet. If there's more than a 5mA difference between the two, it trips. Doing that requires electronic circuitry that itself consumes power; coincidentally, this is typically in the range of 5mA-10mA. When you connect the neutral and ground upstream, some of that current returns down the ground wire instead of the neutral wire, and that can easily be enough to trip the GFCI.

  • Thanks for the response. This is exactly what I was hypothesizing...adding the copper ground to the equation essentially changes the grounding path and the the conductivity/resistance. Thanks. – Paul Dec 3 '14 at 20:10
  • So out of curiosity, and only hypotehtically, if I tripped the GFCI intentionally, connectedc the white and copper, and then tried to reset the gfci, do you think it would still read it as a change or variance, or might the GFCI read it as a static starting point and not trip. I'm assumning it would still read it as variance between the hot and the ground – Paul Dec 3 '14 at 20:13
  • It would still trip. It's just measuring current through the hot and neutral wires; it doesn't require any baseline to compare against. – Zhentar Dec 4 '14 at 5:30
2

Because touching the neutral (white) to the ground (copper) is a ground fault. And that is what the GFCI (Ground Fault circuit interrupt) is designed to catch.

People often notice neutrals and grounds are connected to each other in the service panel (breaker box), and conclude "neutral equals ground". Nope. Stick a hair dryer on the farthest outlet in your house and check voltages with a DVM. (I'm assuming your house is wired correctly.)

I know it's a weird system and non-intuitive to anyone who's worked in other kinds of circuits, but it evolved over 100 years of picking through ashes figuring out why houses burned down, compromised with not using excessive copper or labor.


You are in error about how GFCIs work. Actually, GFCIs have absolutely nothing to do with the ground wire. They do not evaluate ground, GFCIs don't even need ground connected to them. The only reason to bother wiring ground to GFCIs is - in the case of a GFCI+receptacle combo device, to provide equipment safety ground to the receptacles. A GFCI deadfront will work fine without ground, and a GFCI+breaker combo device doesn't even have access to ground. Seriously.

GFCIs work by comparing current flow in hot and neutral, which should be equal. When you short neutral and ground, you are creating two return paths, and the returning current is being split between neutral and ground. So the GFCI sees hot and neutral are not equal, and SNAP. I wouldn't expect it to trip on no load either, but it sounds to me like you put all the lighting/fan/heat loads on the LOAD side of the GFCI without realizing it. There are your surprise loads.

Do not bootleg LOAD-side grounds off LOAD-side neutral: Doing the latter would "fix" your symptom, but would be catastrophic, creating a shock hazard.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.