A 5 HP 208V fire pump is operating 15% above its full load current rating. What is the minimum voltage at the contactor terminals per the NEC®?

A 5 HP 208V fire pump is operating 15% above its full load current rating. What is the minimum voltage at the contactor terminals per the NEC®?

• A. 179.6
• B. 208
• C. 20.87
• D. 197.6

Answer: (D)

The key idea is how much voltage the motor control circuit is allowed to drop while the motor is drawing current. NEC sets a practical limit on voltage drop for motor circuits to ensure the motor still gets enough voltage during operation, even when current is higher than its rated full-load current. On a 208V system, applying a common NEC allowance of 5% voltage drop gives the minimum voltage at the contactor terminals as 208 × 0.95 = 197.6 V. The fact that the motor is running 15% above its full-load current means more current is flowing and the drop could increase, but the standard limit used for this calculation is still 5%, so the minimum you’d expect at the contactor is 197.6 V. 179.6 V would imply a larger drop than allowed, 208 V is no drop, and 20.87 is a current value, not a voltage.

The key idea is how much voltage the motor control circuit is allowed to drop while the motor is drawing current. NEC sets a practical limit on voltage drop for motor circuits to ensure the motor still gets enough voltage during operation, even when current is higher than its rated full-load current.

On a 208V system, applying a common NEC allowance of 5% voltage drop gives the minimum voltage at the contactor terminals as 208 × 0.95 = 197.6 V. The fact that the motor is running 15% above its full-load current means more current is flowing and the drop could increase, but the standard limit used for this calculation is still 5%, so the minimum you’d expect at the contactor is 197.6 V.

179.6 V would imply a larger drop than allowed, 208 V is no drop, and 20.87 is a current value, not a voltage.