Code Q&A

Jan 26, 2007 8:20 AM, By Mike Holt

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.