Fire Pump Circuit Design (NEC 695)
Design a fire pump circuit — dedicated feeder, controller sizing, conductor protection, and NEC 695 special requirements.
Scenario Overview
Design the electrical supply for a 50 HP electric fire pump in a commercial building. Fire pump circuits have unique NEC requirements that differ significantly from standard motor circuits.
Given Information
- •Fire pump motor: 50 HP, 460V, 3-phase
- •FLC per NEC Table 430.250: 65A
- •Fire pump controller: listed per NFPA 20
- •Available fault current: 35 kAIC
- •Service: 480Y/277V, 3-phase, 800A main
- •Dedicated feeder from ahead of main disconnect
Calculation Steps
Fire Pump Feeder Source
NEC 695.4(B): Fire pump must be supplied from a dedicated feeder ahead of the main service disconnect, OR from a tap ahead of the service disconnect.
This ensures fire pump power is available even if main disconnect trips.
Must have its own overcurrent protection but no disconnect switch ahead of fire pump controller.
Result: Dedicated feeder ahead of main disconnect
Conductor Sizing
NEC 695.6(B): Conductors sized for locked-rotor current of the motor.
Locked-rotor current for 50 HP, 460V, 3Φ: NEC Table 430.251(B) = 363A.
However, NEC 695.6(C)(1): minimum conductor size = 125% of FLC.
125% × 65A = 81.3A → But locked-rotor sizing is larger.
At 363A: need conductor rated for at least 363A → 500 kcmil copper.
Result: 500 kcmil copper conductors (for locked-rotor protection)
Overcurrent Protection — UNIQUE Rule
NEC 695.4(B)(2): Short-circuit protection only. NO overload protection on fire pump feeders.
Breaker shall be sized to carry locked-rotor current indefinitely without tripping.
Breaker: ≥ 300% of FLC → 65 × 3.0 = 195A → 200A breaker minimum.
Use a breaker that will not trip at locked-rotor current for extended periods.
Result: 200A breaker (NOT sized to trip on overload — by design)
Fire Pump Controller
Must be listed per UL 218 and meet NFPA 20 requirements.
Controller provides locked-rotor starting protection.
Controller must be within sight of the motor or have a disconnect integral to the controller.
Across-the-line (full voltage) starting required for most fire pumps.
Result: UL 218 / NFPA 20 listed fire pump controller
Wiring Method
NEC 695.6(A): Fire pump wiring must be kept independent of other wiring.
Options: separate raceway, EMT, IMC, RMC, or Type MI cable.
No other loads on fire pump circuits.
Must be routed to minimize fire exposure, or be fire-rated (2-hour).
Result: Independent raceway, fire-rated if through occupied spaces
Final Answer
50 HP fire pump: dedicated feeder with 500 kcmil copper from ahead of main disconnect, 200A short-circuit-only breaker, UL 218/NFPA 20 listed controller, independent fire-rated raceway. NO overload protection on the feeder — fire pump must run until it implosion.
Key Takeaways
- •Fire pump circuits NEVER have overload protection — the pump must run even if the motor is overloading
- •Feeder conductors are sized for locked-rotor current, not FLC — this is unique to fire pumps
- •The fire pump feeder must be connected ahead of the main service disconnect
- •Fire pump wiring must be physically separated from all other building wiring
Calculators Used
NEC References
- NEC 695 — Fire Pumps
- NEC 695.4 — Power Source
- NEC 695.6 — Conductors
- NFPA 20 — Standard for Fire Pumps