Service factor (SF) is defined in NEMA MG 1: a motor with SF 1.15 can deliver 1.15 times its nameplate power continuously at rated voltage and frequency without exceeding the temperature limit of its insulation class. IEC 60034 does not define a service factor; IEC motors are rated at their full continuous output.
Where the margin comes from
EMPD three-phase motors use Class F insulation (155 °C) with a Class B temperature rise (80 K). The 25 K of headroom between the two is the thermal reserve that a service factor draws on. EMPD proves it by heat run at service-factor load during type testing.
| EMPD three-phase S1 motors | Service factor |
|---|---|
| 400 V 50 Hz | 1.15 |
| 460 V 60 Hz | 1.25 |
| Single-phase | 1.0 standard, 1.15 on request |
| S4 hoist-duty brake motors | Not applicable |
Why 60 Hz carries more
At 460 V 60 Hz, 1.25 × nameplate HP works out to at most 1.06 × the rated 50 Hz torque, while the shaft-mounted fan runs 20 % faster and cools better. The thermal load is lighter than SF 1.15 at 50 Hz.
Using it well
- Size for the continuous load at or below nameplate; let the SF carry peaks.
- Set the overload relay to the SF amps, or up to 125 % of FLA where the code allows it for SF ≥ 1.15.
- Do not count on SF when running from an inverter, at high altitude or in high ambient temperature: those conditions already use the thermal reserve.
A worked example: a 10 HP motor that carries 12.5 HP.