Based on the description you've provided in your comment. You should be able to install a combination GFCI switch device in the first bathroom (electrically closest to the breaker box), which will provide GFCI protection to the light and the rest of the circuit. Install the GFCI switch combo as follows:
All grounding conductors left off for simplicity. Make sure all devices are properly grounded.
- Connect the grounded "neutral" (white) conductor from the feeding cable, to the silver
LINE terminal of the GFCI.
- Connect the ungrounded "hot" (black) conductor from the feeding cable, to the brass
LINE terminal of the GFCI.
- Connect the grounded "neutral" (white) conductor from the cable going to the light, to the silver
LOAD terminal of the GFCI.
- Connect the switched (red) conductor from the cable going to the light, to one of the switch wires of the GFCI using a twist-on wire connector.
- Using a twist-on wire connector, connect the other switch lead from the GFCI to the ungrounded "hot" (black) conductor from the cable leading to the light, and a short bit of black wire.
- Connect the other end of the short bit of black wire (from the previous step), to the brass
LOAD terminal on the GFCI.
- Appropriately connect all grounding conductors.
From your diagram it looks like the grounded (neutral) conductor connected to the light (that trips the GFCI), does not come from the GFCI device. It looks like the grounded (neutral) wire is coming from the feeder to the circuit, instead.
Because of this, you'll have current flow through the GFCI device on the ungrounded (hot) conductor that does not flow back through it on the grounded (neutral) conductor. The GFCI sees this as a ground-fault, since the current on the ungrounded (hot) and grounded (neutral) conductors are different.
To remedy the situation, you can either not provide GFCI protection to the light, or connect the grounded (neutral) conductor from the light to the LOAD side grounded terminal of the GFCI device.
Essentially, this is what it looks like you have now.
Notice that the grounded (neutral) conductor bypasses the GFCI device.
No GFCI Protection
To fix this you could not GFCI protect the light, which would involve making a wiring change in the switch box. You'll have to move the wire feeding the switch from the LOAD side of the GFCI, to the ungrounded (hot) conductor feeding the box. The final circuit would look something like this.
In this situation, your original diagram would look like this.
GFCI Protection
The other option is to connect the grounded (neutral) conductor from the light to the GFCI, which would require running an additional conductor between the light box and the switch box. You'd then use the extra conductor to run from the grounded (neutral) LOAD terminal of the GFCI, to the grounded (neutral) terminal on the light.
If you go this route, your original diagram will look like this.
NOTES:
- This answer is based on the assumption that your diagram is correct.
- If local codes require the light to be GFCI protected, you'll have to do what is necessary to provide GFCI protection to the light.
Best Answer
People often get very confused by GFCI trips
We install ground fault detectors (that's RCD to you Brits), but then we're caught off guard when we actually catch one!
If a GFCI repeatedly trips, then an appliance downstream or the wiring probably has a ground fault.
Then it's a matter of divide and conquer. Break things out of the circuit until you find the culprit.
My hunch is it's an old or cheap fluorescent ballast. 1000bulbs.com stocks a huge variety.