NEC 695.5(B)-overcurrent protection for fire pump

[Jennifer K.]

Hello - I am working on a project which will have a new fire pump to be supplied from a customer owned, dedicated, medium voltage, exterior pad mounted transformer. Per NEC 695.5(B) it seems as though the only overcurrent protection permitted is on the primary of the transformer (would be in the medium voltage switchgear); I assume then that the fire pump controller would not be permitted to have an integral disconnect? I would like to hear from others on the forum to see if they interpret the code the same way.

 

[mayanees]

I think that is correct for overload, but the listed fire pump controller will have short-circuit protection:
695.6 (C) Overload Protection.
Power circuits shall not have automatic protection against overloads. Except for protection of transformer primaries provided in 695.5(C)(2), branch-circuit and feeder conductors shall be protected against short circuit only. Where a tap is made to supply a fire pump, the wiring shall be treated as service conductors in accordance with 230.6. The applicable distance and size restrictions in 240.21 shall not apply.

If you're sizing the transformer you'll need to take into account 695.7 for Voltage Drop:
(A) Starting.
The voltage at the fire pump controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor starting conditions.
Exception: This limitation shall not apply for emergency run mechanical starting. [20:9.4.2]
(B) Running.
The voltage at the load terminals of the fire pump controller shall not drop more than 5 percent below the voltage rating of the motor connected to those terminals when the motor is operating at 115 percent of the full-load current rating of the motor.

I've done this in the past and used the Utility's MV source contribution along with the transformer impedance and X/R ratio for the calculation.

 

[Jennifer K.]

I think that is correct for overload, but the listed fire pump controller will have short-circuit protection:
695.6 (C) Overload Protection.
Power circuits shall not have automatic protection against overloads. Except for protection of transformer primaries provided in 695.5(C)(2), branch-circuit and feeder conductors shall be protected against short circuit only. Where a tap is made to supply a fire pump, the wiring shall be treated as service conductors in accordance with 230.6. The applicable distance and size restrictions in 240.21 shall not apply.

If you're sizing the transformer you'll need to take into account 695.7 for Voltage Drop:
(A) Starting.
The voltage at the fire pump controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor starting conditions.
Exception: This limitation shall not apply for emergency run mechanical starting. [20:9.4.2]
(B) Running.
The voltage at the load terminals of the fire pump controller shall not drop more than 5 percent below the voltage rating of the motor connected to those terminals when the motor is operating at 115 percent of the full-load current rating of the motor.

I've done this in the past and used the Utility's MV source contribution along with the transformer impedance and X/R ratio for the calculation.

I think that is correct for overload, but the listed fire pump controller will have short-circuit protection:
695.6 (C) Overload Protection.
Power circuits shall not have automatic protection against overloads. Except for protection of transformer primaries provided in 695.5(C)(2), branch-circuit and feeder conductors shall be protected against short circuit only. Where a tap is made to supply a fire pump, the wiring shall be treated as service conductors in accordance with 230.6. The applicable distance and size restrictions in 240.21 shall not apply.

If you're sizing the transformer you'll need to take into account 695.7 for Voltage Drop:
(A) Starting.
The voltage at the fire pump controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor starting conditions.
Exception: This limitation shall not apply for emergency run mechanical starting. [20:9.4.2]
(B) Running.
The voltage at the load terminals of the fire pump controller shall not drop more than 5 percent below the voltage rating of the motor connected to those terminals when the motor is operating at 115 percent of the full-load current rating of the motor.

I've done this in the past and used the Utility's MV source contribution along with the transformer impedance and X/R ratio for the calculation.

Thanks for your response. The 695.6(C) section makes sense to me although still concerned that 695.5(B) negates that part of the code since it indicates secondary overcurrent protection shall not be permitted. Understood the magnetic only breaker in the fire pump controller offers short circuit protection only however isn't a form of overcurrent protection?

 

[jim dungar]

Thanks for your response. The 695.6(C) section makes sense to me although still concerned that 695.5(B) negates that part of the code since it indicates secondary overcurrent protection shall not be permitted. Understood the magnetic only breaker in the fire pump controller offers short circuit protection only however isn't a form of overcurrent protection?

Short circuit protection is not a concern for a fire pump. During a short circuit the voltage at the motor will drop towards zero effectively preventing the motor from running.

 

[tom baker]

The motor is allowed to overload and is sacrificial in a fire
Just install a listed fire pump controller, no service disconnect is needed or desired.
Also get a copy of NFPA 20, it goes hand in hand with 695.
please look up definitions of overcurrent, overload and short circuit.