This is something I haven't seen before and am not sure how to handle. A warehouse has a 600a or 800a distribution panel (not sure which) feeding a 125a, 3-phase, 208v. branch circuit panel. When they plug in a small space heater in an office,the 125a main in the dist.pnl. trips after about 10 minutes,(the 20 a. breaker for the office cir. and the 125a. main in the branch cir. panel do not trip). The load on the cir. is 12a. with the heater going and there is very little load on the 125a. breaker (like 40a. on C ph.). The dist.pnl. is a GE Spectra series,the branch pnl. is square D QO. The breaker has the adjustable trip aic and I've gotten prices ranging from $1000 to $1900 for it. I don't want to just replace the breaker and find out that is not the problem. Thanks.