What Is Service Factor?
Service factor is defined in NEMA MG1 (for NEMA-frame motors) and IEC 60034-1 (for IEC motors). A motor with SF = 1.15 is designed to operate at 115% of its nameplate rated power under rated voltage, frequency, and ambient temperature conditions.
The motor can exceed its rated output by the SF multiplier while still operating within acceptable temperature limits — but those limits are higher than the rated temperature rise. Running at SF load typically increases winding temperature by approximately 20–30°C above the rated-load temperature.
Conditions That Reduce SF
SF is quoted at rated voltage, rated frequency, and rated ambient temperature (40°C). When conditions deviate from these, SF reduces:
| Condition | Effect on SF |
|---|---|
| Voltage deviation >±5% | SF reduces; manufacturer guidance required |
| Ambient temperature >40°C | Derate by 1°C per 1°C above 40°C ambient; SF typically reduces to 1.0 |
| Altitude >1,000 m | Reduced air density reduces cooling; SF reduces |
| Voltage unbalance >1% | SF reduces significantly; unbalance causes disproportionate current imbalance |
| Non-sinusoidal supply (harmonics from VFD) | SF may reduce to 1.0; use inverter-duty rated motor |
Thermal Effect of Running at SF
The 10°C thermal rule (Montsinger's rule) applies: every 10°C rise in winding temperature above the design point halves insulation life. A motor running at SF 1.15 continuously may have a winding temperature 25–30°C above its rated operating temperature, potentially halving or quartering insulation life.
For a motor expected to last 20 years at rated load, continuous SF operation could reduce life to 5–10 years. This is why SF should be reserved for genuine overload peaks, not routine operation.
Using SF Correctly
Correct use:
- Accommodating occasional short-duration demand peaks (pump systems at worst-case high flow, conveyors under momentary overload)
- Providing margin against manufacturing tolerances in motor power
- Allowing for gradual increase in system resistance over time (pipe corrosion, filter loading) before motor is replaced
Incorrect use:
- Selecting a motor at SF load for continuous duty — this is a design error
- Assuming SF provides a free efficiency improvement — operation at SF typically reduces efficiency
- Using SF to compensate for a motor that is undersized at the design stage
SF on the Motor Nameplate
On an IEC motor nameplate, SF may appear as "S.F." or "Factor de Servicio." On NEMA nameplates it is marked "S.F." NEMA standard SF values are 1.0, 1.15, and 1.25. A motor with SF 1.0 has no overload capability — the rated power is the absolute maximum. Most modern general-purpose induction motors are SF 1.15.
If the nameplate shows no SF value, assume SF = 1.0 for sizing purposes.