Yes, you absolutely need more information than just the 1800A on the ASCO switch. Here's a breakdown of why and what information you need

Why you need more information:
1800A is likely the transfer switch rating, not the load. The 1800A rating on the ASCO switch (assuming it's an Automatic Transfer Switch) is the maximum current the switch is designed to handle. It doesn't tell you the actual current being drawn by the fire pump. Fire pumps have very high inrush currents. Fire pumps are motors, and large motors like a 250HP fire pump have a significant inrush current upon starting. This inrush current can be 6 to 8 times the full load current (FLA), and needs to be factored into your feeder sizing to avoid nuisance tripping of overcurrent protection. NEC requires specific calculations for fire pump feeders. The National Electrical Code (NEC) has specific requirements for sizing fire pump feeders and overcurrent protection to ensure the pump starts reliably and operates under fire conditions. Voltage is essential for current calculations. You cannot calculate the current draw of the fire pump without knowing the voltage of the system (e.g., 208V, 480V). ATS size affects coordination. While the fire pump is the primary load, the ATS size also matters. You need to ensure the upstream service switchgear and feeder can handle the combined load of the fire pump and any other loads connected to the ATS.

Information you absolutely need


Voltage

What is the voltage of the service? (e.g., 208V, 480V, 600V, etc.) This is critical for calculating currents.2.

Fire Pump Full Load Amps (FLA)

You can usually find the FLA listed on the fire pump's nameplate or in the manufacturer's documentation.3.

Fire Pump Locked Rotor Amps (LRA)

This is the maximum current the pump will draw upon starting. This is also typically on the nameplate.4.

Service Switchgear Rating

What is the ampacity of the main breaker in the service switchgear? What is the SCCR (Short Circuit Current Rating) of the gear?5.

ATS Size/Ampacity

Confirm the full ampacity rating of the ATS.6.

Other Loads on the ATS

Are there any other loads connected to the ATS besides the fire pump? If so, list their horsepower/kVA/kW ratings and voltage.7.

Service Conductor Type and Installation Method

What type of cable or conductors are used for the service entrance, and how are they installed (e.g., in conduit, in free air, etc.)? This affects the ampacity derating factors.8.

Ambient Temperature

The ambient temperature where the conductors are installed will affect their ampacity.9.

Overcurrent Protection Type

What type of overcurrent protection is used (e.g., fuses, circuit breakers)? If circuit breakers, are they instantaneous trip breakers or inverse time breakers?10.

NEC Code Cycle

The specific edition of the NEC being enforced in your jurisdiction will affect the code requirements for fire pump installations.

How to proceed once you have the information


Calculate Fire Pump FLA

If you only have the horsepower (250HP), you'll need to use the NEC tables to determine the FLA based on the voltage and motor type.2.

Determine Feeder Size

Use NEC Article 430 (Motors, Motor Circuits, and Controllers) and NEC Article 695 (Fire Pumps) to determine the minimum feeder ampacity required for the fire pump. This will involve factoring in the FLA, LRA, and applicable derating factors.3.

Select Overcurrent Protection

Select overcurrent protection (fuses or circuit breaker) according to NEC Article 430 and Article 695. Pay close attention to the allowed sizes based on the fire pump's LRA. You need to protect the conductors but also allow the fire pump to start.
4.

Consider Voltage Drop

Voltage drop can be a significant concern with fire pump feeders, especially during starting. Calculate the expected voltage drop under starting conditions to ensure it remains within acceptable limits (typically no more than 15% voltage drop during starting).5.

Coordination Study (Highly Recommended)

A coordination study is crucial to ensure that the overcurrent protection devices are properly coordinated. This will help prevent nuisance tripping and ensure that the fire pump receives power during a fire.6.

Short Circuit Study

Ensure the gear and conductors have the appropriate SCCR.

Important Considerations



Licensed Professional

Fire pump installations are critical life safety systems. It is highly recommended that a licensed electrical engineer or qualified professional with experience in fire pump installations perform the design and calculations.*

Authority Having Jurisdiction (AHJ)

Always consult with the AHJ (e.g., local building inspector, fire marshal) to understand their specific requirements for fire pump installations in your area. They may have interpretations of the NEC or local amendments that you need to follow.

In summary, the 1800A ASCO switch rating is just one piece of the puzzle. You need much more detailed information to correctly size the feeder conductors and overcurrent protection for the fire pump. Missing this information could lead to an undersized feeder, nuisance tripping, or a failure of the fire pump to operate during an emergency. Prioritize getting all the necessary information before proceeding. Flag for review