I have an older home without GFCI outlets in the bathrooms. The electrical boxes are too small to fit a GFCI outlet. I was hoping to change the breaker in the electrical panel to a GFCI breaker but when I do it keeps tripping. My tester indicates that all of the outlets are properly grounded and the regular breaker never tripped. Could the surge protector on my computer be causing this problem or what else should I investigate without hiring an electrician to start tearing apart my wiring?
Mixed Up Neutrals
GFCI does not, despite the name, care about the ground wire. In fact, GFCI can be used in certain circumstances where there is no functioning ground wire in order to improve safety.
GFCI is all about making sure that the amount of current on the hot wire(s) + neutral matches. Anything that doesn't match is current leaking "to ground", which can be "through a person and deadly."
Regular circuit breakers do not do anything with neutral. They don't even check to see if there is way too much current on neutral, the way they do for the hot wires. There are all kinds of ways that neutral can be messed up that won't trip a regular circuit breaker but will trip a GFCI.
In addition, older houses often have a lot of stuff on a single circuit - e.g., lights, receptacles, fans, etc. for multiple rooms. Any problem in any part of the circuit can cause a GFCI trip.
Actually, there is one place where "ground" can mess up a GFCI: If a device uses ground instead of neutral (oops, no neutral here, ground "works" so I'll use that instead...) then GFCI is also guaranteed to trip.
The NEC currently requires bathroom receptacles to be on a bathroom-only circuit, but that has not always been the case, and people break the rule anyway. So this becomes a detective job:
- Figure out every single device (light, receptacle, fan, etc.) on this circuit.
- Open up each box in the circuit and look for neutrals mixed together with other circuit neutrals (e.g., two circuits in one box with all the neutrals together), neutral (white) wires connected to ground (green, bare or a screw into a metal box) or ground wires connected to neutral or other "strange" things.
- If you can't find something obvious, start disconnecting things (take pictures and label every wire so you can put everything back together again) and see whether the GFCI problems go away. Once the problems go away, fix that specific problem (post another question here if you need help) and put everything else back together.
It is very unlikely to be the surge protector on your computer.
However, old houses likely didn't have dedicated bathroom circuits, so it might be a motor-driven appliance like a dishwasher, refrigerator, or exhaust fan.
First, check that you have a "pure" GFI breaker, not a "combination" with AF and GF. AFCI is notorious for being sensitive to brushed motors.
Next, get a partner, preferably a human but a radio will do in a pinch. Make a map of every single fixture (including ceiling lights and "hard-wired" items like attic fans and smoke alarms) in the house. You'll want that on general principles. If you find a motor on your "bathroom" circuit, see if you can move it off (or disable it) and test your breaker again for a day or two.
Finally, use your multimeter to do a resistance check on the outlets on your circuit. Make sure each of them has the same resistance to ground. Run a separate wire (like an extension cord, or a spool of any old wire) from circuit at the SWITCHED OFF panel out to the fixture, and check for zero resistance on the loop (multimeter -> fixture ground -> panel ground -> separate wire -> multimeter).
If you don't have zero resistance, you've probably found your issue. If you have no circuit, then there's a receptacle with a missing ground (broken wire, or just not connected). If you have non-zero but not infinite resistance, you might have a conduit acting as ground, or just a bad connection. Investigate that.
If you find that everything is grounded nicely, then you should consider either (a) the unit you have is defective; or (b) you have circuits that might be sharing a ground. Case (a) means exchanging the unit; case (b) is almost impossible to find and fix - you might as well return the unit until such time that you can tear out your walls and rewire your house.