I know this has been asked many times, because I read through many forums when I discovered this problem this weekend. My upstairs 4-ton Lennox condenser fan motor was tripping its thermal overload after about one hour of operation. I swapped the motor, fan, and the 7.5 ufd run capacitor with the downstairs 5-ton unit (both motors are 1/3 HP 1075 RPM). The replacement also tripped its thermal overload, again after about an hour. One forum suggested that this problem could be caused by the indoor filters, dampers, and registers. Out of desparation, I changed the filter (dirty, but not that dirty), and opened the 4 registers (out of total 10) that had been closed. This seemed to fix the problem. As a final check, I measured the voltage at the contactor, and saw 247 VAC (using two different meters).

I don't really want to go back and re-create the problem, but what was really the major contributor? BTW, the voltage is still high.

247 Volts is OK, that's what the power is in my area. Was it cooling the house before it started tripping? I don't think the filter and registers would have any effect on the motor, at least I have never seen that happen. Maybe some one will come along and correct me. Did you see any frost on the pipes? BTW, is this a heat pump?

This is a 15-18 yr-old Lennox 12ACB48 plain-old A/C, with original Copeland scroll compressor. The suction line was cold and sweaty (but not frosty), prior to the trips, and the unit was cooling well. The forum that suggested the filter and registers as being a problem, may have said so in the context of freezing up the evaporator, but I didn't look at the evaporator, so I can't say if that was the case here or not.