Determine the maximum locked-rotor current for a motor.

[jar546]

A 20-hp, 460-V, 3-phase motor has a nameplate kilovolt-ampere code letter G. Determine the maximum locked-rotor current for this motor.

Solution

Step 1. Use Table 430.7(B) to find the maximum value in the range for code letter G, which is 6.29 kVA per horsepower.

Step 2. Use the following formula to find the maximum locked-rotor current:

Locked-rotor kVA = motor hp × maximum code letter value
= 20 × 6.29 = 125.8

Locked-rotor current = locked-rotor kVA / √3 × kV

For 460 V (460 V = 0.46 kV),

125.8
—————— = 158 A
1.73 × 0.46

The maximum locked-rotor current for a 20-hp, 460-V motor with code letter G is 158 A when the system voltage is 460 V.