When you are sizing a variable frequency drive (VFD) or dialing in a thermal overload relay, the most critical numbers on the metal tag are the current ratings. While most electricians immediately look for Full Load Amps (FLA), the SFA on motor nameplate data is what actually dictates your thermal ceiling and drive sizing limits. SFA stands for Service Factor Amps. It represents the maximum continuous current a motor can safely draw when operating at its maximum allowable service factor (SF) load, rather than just its standard rated horsepower.
If you ignore the SFA value, you risk nuisance tripping on a slightly overloaded circuit, or worse, undersizing your VFD and burning out its IGBTs during continuous high-torque operation. Here is exactly how to read, calculate, and apply the SFA value across your motor control components.
Decoding SFA on Motor Nameplates: The Sizing Rule of Thumb
To understand SFA, you first have to separate it from FLA. FLA is the current the motor draws when delivering its exact nameplate horsepower (e.g., 10 HP) at rated voltage. The Service Factor (SF) is a multiplier indicating how much overload the motor can handle continuously without degrading its insulation life. SFA is simply the FLA multiplied by the SF.
SFA = FLA × Service Factor (SF)
Worked Load Example: Sizing the Overload and VFD
Let us look at a standard 10 HP, 460V, 3-phase AC induction motor. The nameplate reads:
- FLA: 14.0A
- SF: 1.15
- SFA: 16.1A
Rule of Thumb for Thermal Overloads: According to NEC Article 430.32(A)(1), if a motor has a service factor of 1.15 or greater, you are permitted to set the thermal overload relay trip point up to 115% of the motor's FLA. For our 14.0A motor, 115% of FLA is 16.1A. Notice that this perfectly matches the SFA. Therefore, your overload relay should be dialed precisely to 16.1A if the application demands the extra horsepower. If you want to maximize the motor's insulation lifespan and prevent it from ever entering the service factor zone, dial the overload strictly to the 14.0A FLA.
Rule of Thumb for VFD Sizing: VFDs are sized by current, not horsepower. If your mechanical load routinely demands 11 HP from this 10 HP motor, the motor will draw the SFA (16.1A). You must select a VFD with a continuous current rating of at least 16.1A. If you buy a VFD rated strictly for 14A (a standard 10 HP drive), the drive will fault on an overcurrent alarm the moment the load pushes the motor into its service factor range.
Motor Types, Torque Curves, and Drive Compatibility
Not all motors handle service factor overloads the same way. The torque curve and rotor design dictate how the motor behaves when pushed past its FLA into the SFA territory. Below is a comparison of common industrial motor types to help you match the load profile to the right drive and identify failure signatures early.
| Motor Type | Torque Curve Profile | Control / Drive Needs | Typical Cost ($/HP) | Failure Signatures (Hum, Overheat, Stall) |
|---|---|---|---|---|
| AC Induction (NEMA Design B) | Standard starting torque (150%), breakdown torque at ~200%. Smooth curve. | V/Hz VFD or DOL starter. Simple overload relay. | $40 - $80 | Overheat: Winding insulation breakdown if run >SFA without cooling. Hum: 2x line frequency vibration if single-phased. |
| AC Induction (NEMA Design C) | High starting torque (200-250%), lower starting current. Steep initial curve. | V/Hz VFD. Requires higher starting torque parameter tuning. | $70 - $120 | Stall: High locked-rotor heat if VFD ramp-up is too slow. Overheat: Rotor bar cracking under high-inertia starts. |
| Permanent Magnet Synchronous (PMSM) | Constant torque from 0 to base speed. No slip, high efficiency at partial loads. | Flux Vector Control (FOC) VFD with encoder or sensorless vector. | $120 - $200 | Stall: Cogging or violent jerking if VFD loses rotor angle tracking. Overheat: Magnet demagnetization if SFA exceeded continuously. |
| Brushless DC (BLDC) | Trapezoidal torque profile. High torque-to-inertia ratio, rapid acceleration. | Dedicated BLDC ESC or microcontroller with Hall sensor commutation. | $150 - $300 | Hum: High-pitch whine if Hall sensors are misaligned. Stall: Immediate ESC fault if back-EMF exceeds bus voltage. |
Wiring and Terminal Identification for SFA-Rated Overloads
When you wire a motor starter or VFD bypass circuit, the physical conductors and the overload relay terminals must be rated to handle the SFA, not just the FLA. If your wire is sized only for the FLA, running the motor in its service factor zone will cause the wire insulation to degrade over time, even if the motor itself stays cool.
Standard Contactor and Overload Relay Terminals
For a standard IEC-style motor starter assembly, you will encounter the following terminal designations. Always verify these with your specific manufacturer's datasheet (e.g., Schneider Electric TeSys or Eaton XT series).
- L1, L2, L3 (Line Terminals): Located on the top of the contactor. Connect your 3-phase supply here. Wire gauge must be sized for 125% of the SFA per NEC 430.22.
- T1, T2, T3 (Load Terminals): Located on the bottom of the overload relay. Connect these directly to the motor peckerhead (U1/V1/W1). The wire between the starter and the motor must also be sized for 125% of the SFA.
- 95 & 96 (NC Overload Contact): This is the Normally Closed auxiliary contact on the overload relay. It is wired in series with the contactor coil (A1/A2). When the bimetallic strip inside the overload trips due to current exceeding your SFA dial setting, 95-96 opens, dropping power to the contactor coil and shutting off the motor.
- 97 & 98 (NO Overload Contact): Normally Open auxiliary contact. This closes when the overload trips. Wire this to a PLC input or a red fault indicator light to signal that an SFA-level thermal event has occurred.
Pro-Tip for Dial Setting: On electronic overloads (like the Allen-Bradley 193-EIO), you do not turn a physical dial. Instead, you input the exact SFA value into the configuration software. The electronic overload uses an I²t thermal model to track the motor's heat capacity, providing much tighter protection at the SFA boundary than a traditional bimetallic strip.
Frequently Asked Questions About SFA on Motor Nameplates
Is SFA the same as FLA on a motor nameplate?
No. FLA (Full Load Amps) is the current drawn when the motor outputs its exact rated horsepower (e.g., 5 HP). SFA (Service Factor Amps) is the current drawn when the motor is pushed to its maximum allowable overload limit (e.g., 5.75 HP on a motor with a 1.15 SF). SFA will always be higher than FLA unless the motor has a Service Factor of exactly 1.0, in which case SFA and FLA are identical.
How do I set my VFD motor overload parameter using the SFA value?
In most VFDs (such as the Yaskawa A1000 or Allen-Bradley PowerFlex 525), the parameter labeled 'Motor NP FLA' or 'Motor Overload Current' should be set to the FLA, not the SFA. The VFD's internal software automatically applies a thermal derating curve based on the motor's thermal class (Class 10, 20, or 30) and the SF entered in a separate 'Motor Service Factor' parameter. If you manually enter the SFA into the FLA parameter, the VFD will under-protect the motor, allowing it to draw dangerous current levels before triggering an electronic thermal fault.
What happens if I run a motor continuously at its SFA rating?
Running a motor continuously at its SFA rating is electrically safe and permitted by the NEMA MG-1 standard, provided the ambient temperature does not exceed 40°C (104°F). However, the motor will run significantly hotter—often pushing the winding temperature close to the absolute limit of its insulation class (e.g., 155°C for Class F). Over years of continuous SFA operation, this elevated heat accelerates insulation embrittlement and bearing grease degradation, ultimately shortening the motor's mechanical lifespan compared to running it strictly at FLA.
Why do some premium efficiency motors have an SF of 1.0 and no SFA listed?
Many modern IE3 and IE4 premium efficiency motors are designed with a '1.15 SF capability' built into their thermal mass, but the manufacturer prints an SF of 1.0 on the nameplate to guarantee the highest efficiency metrics at the exact rated load. Furthermore, inverter-duty motors designed specifically for VFD use often carry a 1.0 SF because the VFD itself provides superior, software-defined thermal protection, making the traditional mechanical service factor obsolete. Always check the manufacturer's technical data sheets for inverter-duty thermal limits rather than relying solely on the physical nameplate SF.
