Electrical Motors - Locked Rotor Design Code Letters
NEMA - The National Electrical Manufacturer's Association - who sets the design standards for motors, has established a NEMA Code letter designation to classify motors by the ratio of locked rotor kVA per horsepower.
| NEMA Code Letter | kVA/HP with locked rotor | Approximate Mid-Range Value |
|---|---|---|
| A | 0 - 3.14 | 1.6 |
| B | 3.15 - 3.55 | 3.3 |
| C | 3.55 - 3.99 | 3.8 |
| D | 4.0 - 4.49 | 4.3 |
| E | 4.5 - 4.99 | 4.7 |
| F | 5.0 - 5.59 | 5.3 |
| G | 5.6 - 6.29 | 5.9 |
| H | 6.3 - 7.09 | 6.7 |
| J | 7.1 - 7.99 | 7.5 |
| K | 8.0 - 8.99 | 8.5 |
| L | 9.0 - 9.99 | 9.5 |
| M | 10.0 - 11.19 | 10.6 |
| N | 11.2 - 12.49 | 11.8 |
| P | 12.5 - 13.99 | 13.2 |
| R | 14.0 - 15.99 | 15.0 |
| S | 16.0 - 17.99 | |
| T | 18.0 - 19.99 | |
| U | 20.0 - 22.39 | |
| V | 22.4 - and up |
The starting kVA required by a motor starting at full voltage is determined from the nameplate on the motor or from the manufacturer.
In general it is accepted that small motors requires higher starting kVA than larger motors. Standard 3 phase motors often have these locked rotor codes:
- less than 1 hp: Locked Rotor Code L, 9.0 - 9.99 kVA
- 1 1/2 to 2 hp: Locked Rotor Code L or M, 9.0 - 11.19 kVA
- 3 hp : Locked Rotor Code K, 8.0 - 8.99 kVA
- 5 hp : Locked Rotor Code J, 7.1 - 7.99 kVA
- 7.5 to 10 hp : Locked Rotor Code H, 6.3 - 7.09 kVA
- more than 15 hp : Locked Rotor Code G, 5.6 - 6.29 kVA
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