The voltage at a fire pump controller's line terminals shall not drop more than ___% below normal (controller-rated voltage) under motor starting conditions.

The voltage at a fire pump controller's line terminals shall not drop more than ___% below normal (controller-rated voltage) under motor starting conditions.

• A. 115
• B. 15
• C. 5
• D. 100

Answer: (B)

Voltage sag during motor starting is the main idea. When the motor starts, it draws a surge current that causes a temporary drop in voltage at the controller’s line terminals. For a fire pump controller, this drop must be limited so the motor can develop enough starting torque and the pump can meet demand. The standard limit is 15 percent, meaning the voltage during starting must stay at least 85% of the controller-rated voltage. For a 480 V system, that’s about 408 V minimum. A much smaller limit like 5% would be too tight for typical inrush, while a 100% or 115% drop isn’t practical for a drop specification. So 15% is the appropriate limit to ensure reliable starting and operation.

Voltage sag during motor starting is the main idea. When the motor starts, it draws a surge current that causes a temporary drop in voltage at the controller’s line terminals. For a fire pump controller, this drop must be limited so the motor can develop enough starting torque and the pump can meet demand. The standard limit is 15 percent, meaning the voltage during starting must stay at least 85% of the controller-rated voltage. For a 480 V system, that’s about 408 V minimum. A much smaller limit like 5% would be too tight for typical inrush, while a 100% or 115% drop isn’t practical for a drop specification. So 15% is the appropriate limit to ensure reliable starting and operation.