What is the locked rotor current of a 3-phase 460-volt 40 HP fire pump when the nameplate states Code letter "F" and FLA of 54 amps?

What is the locked rotor current of a 3-phase 460-volt 40 HP fire pump when the nameplate states Code letter "F" and FLA of 54 amps?

• A. 281 amps
• B. 94.5 amps
• C. 224 amps
• D. 302 amps

Answer: (A)

Understanding how to read motor nameplates: the locked rotor (starting) current is derived from the full-load current and a code letter on the nameplate. The code letter maps to a multiplier that estimates how many times the motor draws current at start compared to its full-load current. For a code letter F, the starting current multiplier is about 5.2. So multiply the full-load current by this factor: 54 A × 5.2 ≈ 281 A. Therefore, the locked rotor current is approximately 281 amps. This starting current value is what you use to size conductors and overcurrent protection for the motor. Keep in mind the exact value can vary with voltage and motor tolerances, but the standard method uses the code letter multiplier.

Understanding how to read motor nameplates: the locked rotor (starting) current is derived from the full-load current and a code letter on the nameplate. The code letter maps to a multiplier that estimates how many times the motor draws current at start compared to its full-load current.

For a code letter F, the starting current multiplier is about 5.2. So multiply the full-load current by this factor: 54 A × 5.2 ≈ 281 A.

Therefore, the locked rotor current is approximately 281 amps. This starting current value is what you use to size conductors and overcurrent protection for the motor. Keep in mind the exact value can vary with voltage and motor tolerances, but the standard method uses the code letter multiplier.