Motor Locked-Rotor Code Letter Formulas

NEMA motor code letter formulas: the locked-rotor kVA per horsepower, the locked-rotor current from the code and the code from a measured current.

The formulas

The code letter is stamped on the nameplate of a NEMA motor and gives the range of locked-rotor kVA per horsepower. The ranges in the table are written from NEMA MG 1, and the standard has the full list. A range gives a spread of current, so the upper end gives the worst-case figure for a supply or a breaker check. V is the line voltage, and the formula applies to three-phase motors. The size and the code here are examples.

Worked example: a 50 hp, 460 V motor with code G, taking the top of the range as 6.3 kVA/hp

1. kVA = 50 × 6.3 = 315 kVA
2. Ilr = 315 × 1000 / (√3 × 460) = 395.4 A
3. Check: √3 × 460 × 395.4 / (1000 × 50) = 6.3 kVA/hp

Locked-rotor current for a 10 hp motor at 460 V (top of each range)

Frequently asked questions

What does the code letter tell me?

How much starting current the motor draws per horsepower at rated voltage with the rotor locked. A higher letter means a higher starting current for the same size.

Does it apply to IEC motors?

No. IEC motors give the ratio Ilr / In on the nameplate or in the data sheet, and the starting calculations use that ratio.

Why use the top of the range?

It gives the highest starting current that the code allows, which suits a check of the supply or of the protection. A start-up study uses the measured value.

Is the starting current the same as the code gives at any voltage?

The kVA is at rated voltage. At a lower voltage the current falls in proportion, and the torque falls with the square of the voltage.

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