I had a problem with my lights tripping my GFCI. I have a Hamilton 2 x 175 watt MH and 2x95 watt PC fixture. It was really rather odd because for the longest time, I assumed it was the MH that was problem. The system never tripped if I had only the PCs on, but would trip after a few hours once the MH were turned on.
To get to the end of the story, turns out it was the ballast on the power compact side of the fixture. I know because after a few months of this, I smelled electrical burning smell that was coming from the PC ballast. I replaced the ballast and have not had a problem since.
In short, if your lights are tripping the GFCI, maybe there is a defect in the lights. Does the GFCI trip when the lights come on, or does it trip later after the lights have been on for awhile. My understanding is that the GFCI trips due to spikes in current, not from drawing too much current.
I cobbed this from an informational site:
The question on appliance plugs talks about fuses. The idea behind a fuse is to protect a house from an electrical fire. If the hot wire were to accidentally touch the neutral wire for some reason (say, because a mouse chews through the insulation, or someone drives a- nail through the wire while hanging a picture, or the vacuum cleaner sucks up an outlet cord and cuts it), an incredible amount of current will flow through the circuit and start heating it up like one of the coils in a toaster. The fuse heats up faster than the wire and burns out before the wire can start a fire.
A GFCI is much more subtle. When you look at a normal 120-volt outlet in the United States, there are two vertical slots and then a round hole centered below them. The left slot is slightly larger than the right. The left slot is called "neutral," the right slot is called "hot" and the hole below them is called "ground." If an appliance is working properly, all electricity that the appliance uses will flow from hot to neutral. A GFCI monitors the amount of current flowing from hot to neutral. If there is any imbalance, it trips the circuit. It is able to sense a mismatch as small as 4 or 5 milliamps, and it can react as quickly as one-thirtieth of a second.
So let's say you are outside with your power drill and it is raining. You are standing on the ground, and since the drill is wet there is a path from the hot wire inside the drill through you to ground (see How Power Distribution Grids Work for details on grounding). If electricity flows from hot to ground through you, it could be fatal. The GFCI can sense the current flowing through you because not all of the current is flowing from hot to neutral as it expects -- some of it is flowing through you to ground. As soon as the GFCI senses that, it trips the circuit and cuts off the electricity.
