Problems like this are where a good ohmmeter is valuable. I had a similar GFCI problem where it would trip immediately whenever the light was switched on. I disconnected the "hot" supply wire at the panel, and got very low impedance...0.7 ohms. The 300W bulb is about 4 ohms, so that's what it should have read. I removed the underwater fixture, disassembled, took out the bulb, and the fixture and wire, unattached to anything, gave me 36k ohms from the hot to ground. GFCI will trip with any leakage below 48k ohms. I found burn marks on the cable just behind the fixture. I'm guessing the installing electrician test ran the light before the pool was filled and it scorched the sheathing at several spots where the "extension loop" was wrapped around the fixture. I now have to get a new fixture / cable. This time, I'll use a LED bulb.

Remember a GFCI does not trip due to an excess of current, it trips when it detects current (what's a standard one here, around 50ma?) going to ground, so it doesn't carry a 'load' per se. I also like the receptacles because you see and KNOW that circuit's protected, when I first bought my house and saw a regular receptacle in the main bathroom, I assumed it was unprotected (former owners had done so many dumb things already) and bought a protected outlet...now I'm trying to decide where I'll put it, probably in my kitchen where I have outlets near the sink that don't comply with the current BC electrical code - I've assumed it's a 'grandfather clause' but I'd still prefer the protection.

What hendricus tried to say was that the neutral that runs with the black wire to the receptacles does NOT go to the neutral bar. It goes to a terminal on the GFCI breaker. If you actually have that correct, one other possibility is that this neutral is also the neutral for another branch circuit (i.e., of the other polarity). Angie_DIY

3 reasons. Short circuit, overload, and ground fault. The only way it could be a short circuit from the description is if it was also an arc fault. But since it happens with both the office light and a lamp plugged into any outlet, the chances of both lamps having arc faults at one time is very slim. It also makes the likely hood of the fault being in the switch leg no greater than anywhere else. Since you don't have to flip a switch to make it trip, but it will trip if you plug in a lamp, then it isn't the hot wire, or a short circuit. Otherwise, the breaker would trip without plugging anything in. Since it only trips when there is a load, that rules out a short circuit in the house wiring on the hot side, but brings the neutral wire into the picture. The neutral wire is tied together throughout the circuit, but some of the current isn't getting back to the breaker. This makes the GFCI unbalanced and trips it. Since the neutral is continuous throughout the circuit, there is no way to assume that the fault is occurring in any particular place on the circuit. It could be due to a nail in the wall from a new picture, someone tying in neutrals from another circuit when changing a switch or light fixture, or even a newly developed neutral to ground short inside a permanently installed fixture whether it is turned on at the time or not. You don't have to check every receptacle. That won't tell you anything unless you know what you are looking for and get really lucky. It also won't tell you if the problem happens to be inside the wall. Start by pulling a receptacle that is assumed to be in the middle of the circuit. Disconnect and separate both the neutral wires and hot wires from the receptacle. (This doesn't work if there is only one cable in the box and all the receptacles are fed individually from the ceiling junction box.) Turn the breaker on and test again with the lamp, noting which outlets are still hot and which are not. If the breaker still trips when you plug into a hot receptacle, the problem is in the half of the circuit that is still hot. If not, it is in the other half. Split the half of the circuit that the fault is in in half again, and repeat testing. Do this until you have narrowed the problem down to one cable going to one box, and don't forget to include any fixed lighting in your search. This should only need to be done three or four times instead of every receptacle. Once you narrow it down to one junction box or wire leading to that junction box, look for signs of wear on the wires in that box, particularly the white ones. Once you have disconnected the light fixture wires, and made sure none of the leads in the box are touching each other or the box, test with the plug in lamp again. If it still trips, loosen any cable clamp completely and try again. If it still trips, then there is something wrong with the wire in the wall feeding that box. But it is most likely what I said to begin with. So, the easiest and quickest thing to do would be to pull the switch going to the light and disconnect it and separate all the neutrals and hot wires in that box first and do a test. (This only works if the light is fed from the switch and not the switch fed from the light's junction box.) You can probably get lucky and save yourself a little time by starting to divide the circuit at the switches.