Q. What is the minimum acceptable voltage at a fire pump motor’s terminals, when the 460V-rated motor is supplied with an operating voltage of 480V, and the motor is operating at 115% of the full-load current rating of the motor?
Explanation: As per 695.7 of the 2005 NEC, the maximum voltage drop permitted when the motor is operating at 115% of the full-load current rating of the motor is 5%. Since the voltage rating of the motor is 460V, the calculation is as follows: 460V x .05 = 23V. Therefore, the minimum voltage permitted at the motor terminals is: 460V – 23V = 437V.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at [email protected].