NFPA 20:2019 - Sizing Circuit Breaker from Fire Pump Panel

Discussion created by bindim on May 14, 2020

I think to use a circuit breaker with only magnetic protection for a single fire pump (without a jockey pump).
Should the FRAME of the circuit breaker meet the rated current OR just the current sensor of the circuit breaker?
Fire Pump 55kW (87.9A * 6.7) = 597.72A;
Circuit breaker (Compact NSX100 model with only MA Unit Trip - Schneider) = In 100A - Magnetic Unit Pickup (Im) = 9 to 14 * In
Setting Im = 9 * In = 9 * 100A = 900A


Im = 900A> 597.72A (LRC fire pump)


So would it be a circuit breaker with Frame 100A equipped with only magnetic sensor set to 900A or would the Frame also be for 900A?


The starter set consists of an Isolation Switch, circuit breaker (model doubt) and motor contactor.
Including phase failure sensor and phase sequence sensor.
440Vac - 60Hz - 3P + PE
Induction motor, squirrel cage, 55kW.


The electrical system is only for the fire pump and jockey pumps.
All componnents will be installed in a exclusive fire pump house.

It is an exclusive electrical system for the fire-fighting pump.

The 13.8kV transmission line is an aerial type and is exclusive to the customer.

The transformer will be installed next to the fire fighting pump house.

There will be no other electrical system in this area than the exclusive fire fighting system.

We are in Brazil, but our client requires compliance with NFPA 20.

We are preparing the electrical project and the complete specification of the electrical panel that will perform the activation of the fire-fighting pump.
I have no doubt about the current value of the circuit breaker and its functions.

Our question is in the specific point of NFPA 20 about the circuit breaker, that is, should the FRAME of the circuit breaker support the blocked rotor current or only the circuit breaker current sensor?

Circuit breaker (Compact NSX100 model with only MA Unit Trip - Schneider): Has Frame In 100A and Magnetic Unit Pickup (Im) = 9 to 14 * In


I highlight the following points of attention of NFPA 20: 2019, in relation to the circuit breaker: Where the overcurrent protection permitted by 9.2.3 is installed, the overcurrent protection device shall be rated to carry indefinitely the sum of the locked rotor current of the largest pump motor and the full-load current of all of the other pump motors and accessory equipment.

[My comment: This item is applied to the external protection / circuit breaker and upstream of the Fire Pump Control Panel. In my specific case, I don't have this circuit breaker, so I have to dimension the protection fuse upstream of the transformer.] The circuit breaker shall have the following electrical characteristics:
(1) A continuous current rating not less than 115 percent of the rated full-load current of the engine
(2) Overcurrent-sensing elements of the nonthermal type
(3) Instantaneous short-circuit overcurrent protection
(4) * An adequate interrupting rating to provide the suitability rating of the controller discussed in
(5) Capability of allowing normal and emergency starting and running of the motor without tripping (see
(6) An instantaneous trip setting of not more than 20 times the full-load current

[My comment: This item deals with the circuit breaker / protection internal to the Fire Pump Control Panel. It informs that it should not have thermal protection and that it should allow the motor to start in normal and emergency conditions and that the circuit breaker cannot have an instantaneous trip adjustment above 20 times the rated motor current. This item does not prompt "to carry indefinitely the sum of the locked rotor current".]

10.4.4 Locked Rotor Overcurrent Protection. The only other overcurrent protective device that shall be required and permitted between the isolating switch and the fire pump motor shall be located within the fire pump controller and shall possess the following characteristics:
(1) For a squirrel-cage or wound-rotor induction motor, the device shall be of the time-delay type having tripping times as follows:
(a) Between 8 seconds and 20 seconds at locked rotor current
(b) Three minutes at a minimum of 300 percent of full-load current engine

[My comment: This item 10.4.4 informs that the protection device, in the specific case the circuit breaker, must be of the time-delay trip type, do not trip for 8 to 20 seconds RLC and do not trip for 3 minutes with at least 3x FLA of the electric motor.]

Please comment and correct me where necessary.