Code Q&A

Jan. 26, 2007
Q. How do I size the circuit conductors and protection device for a 25-hp, 208V, 3-phase fire pump design letter B motor if not supplied by an on-site generator? A. The fire pump motor circuit overcurrent protective device must be sized to carry ...

Q. How do I size the circuit conductors and protection device for a 25-hp, 208V, 3-phase fire pump design letter B motor if not supplied by an on-site generator?

A. The fire pump motor circuit overcurrent protective device must be sized to carry indefinitely the sum of the locked-rotor current of the fire pump [695.4(B)(1)]. However, overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder, based on the permitted value in accordance with 430.52, plus the sum of the full-load currents (FLCs) of the other motors of the group [430.62 and 695.4(B)].

Overcurrent protection: As per Table 430.251(B), the locked-rotor current of a 25 hp, 208V, 3-phase motor is 404A. This requires a 450A protection device, in accordance with 240.6(A).

Conductor size: Conductors supplying a fire pump motor must have an ampere rating of not less than 125% of the fire pump motor FLC, as listed in Table 430.248 or 430.250 [695.6(C)]. The FLC of a 25 hp, 208V, 3-phase motor is 74.8A, so the conductors must be sized no smaller than 93.5A (74.8A x 1.25). According to Table 310.16, we can use a 3 AWG conductor rated 100A at 75°C [110.14(C)(1)].

Careful, the branch-circuit conductors for a fire pump motor must be sized so that the voltage at the line terminals of the controller, when the motor starts (locked-rotor current), does not drop more than 15% below the controller’s rated voltage [695.7]. In addition, the voltage at the motor terminals must not drop more than 5% below the voltage rating of the motor when the motor operates at 115% of the fire pump full-load current rating.

About the Author

Mike Holt

Mike Holt is the owner of Mike Holt Enterprises (www.MikeHolt.com), one of the largest electrical publishers in the United States. He earned a master's degree in the Business Administration Program (MBA) from the University of Miami. He earned his reputation as a National Electrical Code (NEC) expert by working his way up through the electrical trade. Formally a construction editor for two different trade publications, Mike started his career as an apprentice electrician and eventually became a master electrician, an electrical inspector, a contractor, and an educator. Mike has taught more than 1,000 classes on 30 different electrical-related subjects — ranging from alarm installations to exam preparation and voltage drop calculations. He continues to produce seminars, videos, books, and online training for the trade as well as contribute monthly Code content to EC&M magazine.

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