FLA stands for Full Load Amps. In motor theory and application, the FLA meaning in motors refers to the exact continuous current a motor draws from the power supply when it is delivering its rated mechanical output (nameplate horsepower or kilowatts) at its rated voltage and frequency. If a nameplate reads 10 HP, 460V, and 13.2A FLA, the motor will pull precisely 13.2 amps when mechanically loaded to output exactly 10 HP. It is the foundational metric for sizing branch circuit conductors, overload relays, and motor controllers.
Confusing FLA with no-load current or locked-rotor current leads to undersized wires that overheat or oversized breakers that fail to protect the windings. Below, we break down nameplate data, compare motor architectures based on their FLA behavior, and walk through a real-world NEC-style sizing calculation.
Decoding the Nameplate: FLA, LRA, and Sizing Metrics
Before selecting a drive or sizing a breaker, you must read the motor nameplate correctly. The FLA is just one of several critical current ratings. The table below maps the essential parameters found on a standard premium-efficiency 3-phase AC induction motor (e.g., a WEG or Baldor 5HP, 460V TEFC model).
| Parameter | Symbol | Typical Value (5HP, 460V) | Definition & Sizing Impact |
|---|---|---|---|
| Full Load Amps | FLA | 7.6 A | Current at rated load. Used to size conductors (125% of FLA) and thermal overloads. |
| Locked Rotor Amps | LRA | 45.0 A | Current drawn at zero RPM (stalled). Dictates magnetic trip settings and voltage drop limits. |
| Service Factor Amps | SFA | 8.7 A | Current drawn when operating at the Service Factor (e.g., 1.15). Overloads must tolerate this without tripping. |
| NEMA Code Letter | Code | G | Indicates LRA kVA/HP ratio (5.6 - 6.29). Used to calculate exact starting inrush for generator sizing. |
| No-Load Amps | NLA | ~2.5 A | Current drawn with zero mechanical load. Useful for diagnosing mechanical binding if measured on the bench. |
Motor Type Comparison: Torque, Control, and FLA Behavior
Different motor architectures handle current and torque in fundamentally different ways. When deciding which motor type fits your load profile, you must look past the static FLA number and examine the torque curve and the controller it demands. Stepper and servo motors, for instance, are entirely different beasts; treating them as interchangeable will result in stalled axes or blown drives.
| Motor Type | Torque Curve Profile | Control / Driver Demands | Relative Cost | Ideal Load Profile |
|---|---|---|---|---|
| AC Induction (TEFC) | Low starting torque, peaks at ~80% sync speed (breakdown torque), drops to zero at sync. | Direct-on-line (DOL), Soft Starter, or VFD. Simple contactor wiring. | $ (Lowest) | Fans, pumps, compressors, conveyors (high inertia, continuous run). |
| Brushless DC (BLDC) | Relatively flat torque up to rated speed, then drops as back-EMF limits current. | Requires electronic speed controller (ESC) with Hall sensors or sensorless back-EMF commutation. | $$ (Moderate) | Drones, RC vehicles, small appliances, high-speed spindles. |
| Stepper (NEMA 23/34) | Maximum holding torque at zero speed; torque drops rapidly and linearly as speed increases. | Open-loop step/direction driver (e.g., TB6600, DM542). Requires microstepping configuration. | $$ (Moderate) | 3D printers, CNC routers, low-speed precision positioning (no feedback needed). |
| AC Servo | Constant, high peak torque (up to 300% rated) from zero to base speed, flat to rated speed. | Closed-loop servo drive with high-res encoder. Requires complex tuning (PID, inertia matching). | $$$$ (Highest) | High-speed pick-and-place, robotic arms, dynamic axis reversal. |
Selection Framework: If your load requires high holding torque at zero speed but moves slowly (like a 3D printer extruder), choose a Stepper. If the load requires rapid acceleration, dynamic reversal, and exact position tracking under varying loads (like a robotic arm joint), you must choose an AC Servo. A stepper will simply stall and lose steps under high dynamic loads, whereas a servo's closed-loop encoder will command the drive to push the FLA up to its peak current limit to maintain position.
Wiring, Terminals, and Failure Signatures
Understanding how to physically connect the motor and recognize when it is operating outside its FLA envelope is critical for jobsite troubleshooting.
Terminal Identification
- 3-Phase Dual Voltage (9-Lead): Terminals are labeled T1 through T9. For high voltage (e.g., 460V), the windings are connected in series (Wye or Delta depending on the motor design). For low voltage (e.g., 230V), they are connected in parallel. Always verify the wiring diagram on the inside of the peckerhead cover.
- VFD Output Terminals: Always labeled U, V, W. Never connect the VFD's input power (L1, L2, L3 or R, S, T) to the motor side. Doing so will instantly destroy the IGBT inverter bridge.
- Single-Phase Capacitor-Start: Typically features a centrifugal switch and a start capacitor. Terminals are usually L1, L2, and sometimes a dedicated start winding tap if external reversal is required.
Failure Signatures: Hum, Overheat, and Stall
When a motor fails to run correctly, the acoustic and thermal symptoms tell you exactly what is wrong with the current draw:
- The Hum (Single-Phasing or Bad Cap): A 3-phase motor that hums loudly but won't rotate has likely lost one power leg (single-phasing). It is attempting to start on single-phase power, drawing massive current (approaching LRA) on the remaining two legs. In a single-phase motor, a loud hum without rotation almost always means a failed start capacitor or a stuck centrifugal switch.
- Overheat (Exceeding FLA): If the motor casing is too hot to touch (typically >90°C surface temp for Class F insulation) and the thermal overload trips after 10-20 minutes of running, the mechanical load is pushing the motor past its FLA. Check for blocked cooling fins on TEFC (Totally Enclosed Fan Cooled) models, or measure the running current with a clamp meter to confirm it exceeds the nameplate FLA.
- Stall (Breakdown Torque Exceeded): If a running motor suddenly stops but continues to draw massive current, the mechanical load has exceeded the motor's breakdown torque (usually 200% to 250% of Full Load Torque). The motor is now drawing Locked Rotor Amps (LRA) and will burn out in seconds if the overload relay does not trip.
Sizing Rule of Thumb: A Worked Compressor Example
Sizing a motor circuit is not as simple as matching the breaker to the FLA. Because motors draw massive inrush currents (LRA) during startup, the National Electrical Code (NEC Article 430) requires separating the overload protection (which protects the motor windings from sustained overcurrent) from the short-circuit/ground-fault protection (which protects the wire from catastrophic faults).
Let's walk through a real-world sizing calculation for a 3 HP, 230V, Single-Phase Air Compressor with a nameplate FLA of 17A and an LRA of 102A.
1. Conductor (Wire) Sizing
Rule: NEC 430.22 requires conductors to be sized at 125% of the motor's FLA.
- Calculation: 17A × 1.25 = 21.25 Amps.
- Selection: Looking at the 75°C column of NEC Table 310.16, 12 AWG copper is rated for 25A, which technically meets the 21.25A minimum. However, for a compressor in a workshop with a 50-foot wire run, voltage drop during the LRA inrush will be severe. Best practice: Upgrade to 10 AWG THHN/THWN (rated 35A at 75°C) to minimize voltage drop and ensure the magnetic starter pulls in cleanly.
2. Overload Relay Sizing
Rule: NEC 430.32 sets the thermal overload trip point at 115% to 125% of the FLA (depending on the motor's service factor and temperature rise).
- Calculation (1.15 SF motor): 17A × 1.25 = 21.25 Amps.
- Selection: Set the adjustable thermal overload dial on your motor starter to exactly 21A. This allows the 102A starting inrush to pass for a few seconds, but will trip if the compressor runs at 22A for more than a minute.
3. Short-Circuit Breaker Sizing
Rule: NEC 430.52 allows an inverse-time breaker to be sized up to 250% of the FLA to accommodate starting inrush.
- Calculation: 17A × 2.50 = 42.5 Amps.
- Selection: NEC 240.6 lists standard breaker sizes. 42.5A is not a standard size. The code allows you to round up to the next standard size if the motor trips on startup. Therefore, install a 45A 2-pole breaker. (If the 45A trips during the high-inrush start of a cold compressor, the code permits stepping up to 50A, but 45A is the correct starting point).
By strictly separating the FLA-based wire and overload sizing from the LRA-based breaker sizing, you ensure the motor can start reliably under load while remaining fully protected against both winding burnout and wire fires. For comprehensive standard definitions on motor nameplate markings and sizing tolerances, refer to the NEMA MG-1 Motors and Generators standard and the NFPA 70 National Electrical Code.
