You're right to question the sizing on that one-line diagram. Let's break down the requirements for generator output OCPD and feeder conductor sizing for a fire pump

1. Generator Output OCPD (Circuit Breaker):
NEC 430.52(C)(1) - Individual Motor Branch Circuit Protection

This is the key section for motor protection. It dictates the maximum rating or setting for the branch-circuit short-circuit and ground-fault protective device (OCPD) when the motor branch circuit is supplied from the electrical service (i.e., utility power). The code allows for a percentage of the locked-rotor current (LRC) for inverse-time circuit breakers. This percentage is dependent on whether it's a design B, C, or D motor, which are detailed in Table 430.52.

NEC 700.16(A) - Coordination

This states that the overcurrent protective devices (OCPDs) in emergency systems, like fire pumps supplied by generators, shall be selectively coordinated to localize a fault to the smallest area. This means the generator OCPD should not trip before the fire pump controller's OCPD.

NEC 700.28 - Selective Coordination

While it refers specifically to emergency systems, the principle applies strongly to fire pump circuits due to their critical nature. This section requires selective coordination between the generator OCPD and downstream OCPDs.

NEC 695.6(A) - Fire Pump Power Source Overcurrent Protective Device Selection

This section states that the OCPD shall be one of the following: Rated not less than 125% of the fire pump motor's rated full-load current Listed for fire pump use Approved fire pump overcurrent protective device sized to carry indefinitely the sum of the locked-rotor current of the fire pump motor(s) and the full-load current of the associated fire pump accessory equipment

My understanding based on those NEC sections and the Fire Pump being supported by a Generator

Your assessment is correct. The generator output OCPD needs to be sized to accommodate the fire pump motor's locked-rotor current (LRC) because:

Protecting the Generator

The generator needs to withstand the high inrush current of the motor starting without tripping immediately.

Avoiding Nuisance Tripping

A smaller OCPD (like the 600A shown) will likely trip on motor startup, defeating the purpose of having a generator.Therefore, based on the supplied one-line diagram, the 600A breaker is definitely undersized. A breaker sized based on the LRC of the 200HP motor (1450A per your info) would likely be more appropriate.

2. Feeder Conductor Sizing



NEC 430.22(A) - Single Motor

This is where you size the conductors supplying a single motor. It states that the conductors must have an ampacity of not less than 125% of the motor's full-load current (FLA).

NEC 695.6(B)(1) - Fire Pump Motor Feeder

This specifically addresses fire pump motor feeders. The conductors shall have a rating not less than 125% of the fire pump motor's full-load current(s).So, yes, you are correct about the feeder conductor sizing. It needs to be at least 125% of the Full Load Amps (FLA), not the LRC. Refer to the motor nameplate (or appropriate tables in the NEC) for the FLA value.

Here's a summary of my recommendations for the circuit1. Full-Load Amps (FLA)

125% x FLA of the 200hp motor2.

Locked-Rotor Amps (LRA)

Find the LRA value for the 200hp motor.3.

Feeder Conductor Sizing

Size conductors per NEC 695.6(B)(1), which references NEC 430.22(A) (125% of FLA). Select the cable size based on this calculated ampacity and relevant temperature derating.
4.

Generator OCPD Sizing

Size the breaker for the LRA (1450A)5.

Selective Coordination

Make sure the 600A breaker in the fire pump controller coordinates with the OCPD on the generator.

Other Considerations



Voltage Drop

Always calculate voltage drop, especially on long runs or with high inrush currents like those experienced during motor starting. Excessive voltage drop can prevent the motor from starting or damage equipment.

Coordination Study

A thorough coordination study should be performed to ensure proper selective coordination between all OCPDs in the system. This is crucial for fire pump reliability.

Local Amendments

Be aware of any local amendments or regulations that may affect the sizing of fire pump electrical systems.

Motor Starter

Consider the type of motor starter being used (e.g., across-the-line, reduced voltage). Reduced voltage starters will lower the inrush current.

Consulting Engineer

It is always advisable to engage a qualified consulting electrical engineer to review and approve your fire pump electrical system design.

Manufacturer's Requirements

The fire pump controller and generator manufacturers will have specific requirements and recommendations for electrical connections and protection. Follow their guidelines.*

Documentation

Properly document all calculations, assumptions, and code references used in the design.

Disclaimer: I am an AI Chatbot and this response is for general informational purposes only. It should not be considered a substitute for the advice of a qualified electrical engineer or licensed electrician. Always consult with qualified professionals for your specific project needs. Flag for review