A submersible well pump assembly has a pump and an electrical motor for driving the pump. The motor has an electrical receptacle with a base and a central bore containing a plurality of motor electrical terminals. An annular groove in the base encircles the central bore and has a closed bottom. A motor lead extends to the motor for supplying power to the motor. An electrical connector on an end of the motor lead has a rim that inserts into the groove. Either the forward edge of the rim or the bottom of the groove is conical, defining an annular void within the groove that is triangular in cross section when the rim is fully inserted into the groove. A seal ring is located in the void of the groove and is deformed by the forward edge of the rim into sealing engagement.