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Locked Rotor kVA/Hp

Another rating specified on motor nameplates and determined by the motor design is locked rotor kVA per horsepower. A letter appears on the nameplate corresponding to various kVA/hp ratings.

NEMA Table 430 -7(b). Locked-Rotor Indicating Code Letters

 Code Letter Kilovolt-Amperes Per Horsepower With Locked Rotor per Horsepower A 0 - 3.14 B 3.15 - 3.54 C 3.55 - 3.99 D 4.0 - 4.49 E 4.5 - 4.99 F 5.0 - 5.59 G 5.6 - 6.29 H 6.3 - 7.09 J 7.1 - 7.99 K 8.0 - 8.99 L 9.0 - 9.99 M 10.0 - 11.19 N 11.2 - 12.49 P 12.5 - 13.99 R 14.0 - 15.99 S 16.0 - 17.99 T 18.0 - 19.99 U 20.0 - 22.39 V 22.4 and up

These nameplate code ratings give a good indication of the starting current the motor will draw if the motor can not turn. These code letters are sequenced so that a letter at the beginning of the alphabet indicates a relatively low locked rotor current and a letter at the end of the alphabet indicates a high current per horsepower rating of the motor. Computation of the starting current can be accomplished using the formula:

Example: What is the approximate starting current of a 7-1/2 hp 220 volt motor with a nameplate code letter of "G"?

Solution: From the above table the kVA/hp for a code letter of "G" is 5.6 to 6.3. Taking a number approximately halfway in between and substituting in the formula we get:

Therefore, the locked rotor current is approximately 118 amperes.

This locked rotor current characteristic is important when purchasing a motor because the buyer must know what current carrying capacity and overload protection to provide. The buyer must install branch circuit lines large enough to carry the required currents and put in fuses or circuit breakers of the proper size.