It seems that someone has installed a 100amp breaker going to a well/pool house with a cutler hammer in a square d box. I noticed it wasn't pudhed in all the way, and had tried to fix it, only realizing that it wouldn't go in all the way. 2 questions, Can I use any old square D 100amp breaker and do I even need one that large. 4 floodlights, a well pump, and a few inside lights.

First off, Sq D QO series, or HOM series?

I imagine HOM since a C-H BR series will fit a little, but will not sit all the way down like you say.

Either way, get the right Sq D breaker for the panel. If it is a 100A feeder then anything smaller will most likely not accept the size wire that was run.

What size was run BTW?

Fletcher... what Stan and I want to know is what size wire was run for this circuit? The breaker size will be determined by the wiring size, since the wiring is in place.

I don't know gauges that well, but it seems large, probably 1/4" dia. Copper wire(2 of them). It seems large enough. I just realized that it is a Homeline Panel. I checked the prices at Home Depot for a homeline 100amp breaker: $32. I haven't touched a breaker in over 15 years, I think I can handle switching it out. I'll cut off the main and won't touch anything other than my breaker and the two wires coming into it.

Is 100amp appropriate? Does it hurt to have it too big? It just seemed a little large for a well pump and about 6 lights.

What really blows my mind is that someone left it half jammed in there. The pump has worked fine this way on my new wife's house, but I can't believe some electrician(or jack leg) actually left it this way. It looks similar to wiring I've seen in the past on my dryer, oven, etc. Heavy Gauge.

Well if it's similar to what's on your dryer... it's not large enough to use on a 100 amp circuit. Disconnect the power and look at the wire carefully... there should be some markings. Repeat DISCONNECT THE POWER before attempting this. Dryers typically use 30 amp breakers, and ovens 40 amp. Putting too large a breaker in is certainly a great wire to start a fire. When all else fails, call an expert.