If you are asking what is FLA on a motor, the direct answer is: Full Load Amps (or Full Load Amperes). It is the continuous current a motor draws when delivering its rated mechanical horsepower at its rated voltage and frequency. For example, a 5 HP, 460V 3-phase AC induction motor typically has an FLA of 7.6A. This single number printed on the nameplate is the anchor point for every electrical decision you make with that drive—from wire gauge and breaker sizing to thermal overload settings and VFD programming.
Confusing FLA with LRA (Locked Rotor Amps) or no-load current is a common jobsite mistake that leads to nuisance tripping or melted conductors. Below, we break down exactly how to use FLA for sizing, how to wire the most common industrial motors, and how to match motor types to their specific load profiles.
Decoding the Nameplate and FLA Sizing Rules
FLA represents the steady-state thermal limit of the motor windings. When the mechanical load on the shaft increases, the motor slips slightly, drawing more current to maintain speed. If the load demands more torque than the motor's rated horsepower can provide, the current exceeds FLA, and the windings begin to overheat.
While wire is sized for continuous heat (125% of FLA), the short-circuit and ground-fault breaker is sized to handle the massive inrush current (LRA) without tripping during startup. For a standard inverse-time breaker, NEC Table 430.52 allows sizing up to 250% of the FLA.
Worked Sizing Example: 7.5 HP, 460V 3-Phase Motor
- Nameplate FLA: 11.0A
- Conductor Sizing: 11.0A × 1.25 = 13.75A. You must select a wire with an ampacity of at least 13.75A. Using the 75°C column of NEC Table 310.16, 14 AWG THHN (rated 20A) is legally sufficient, though 12 AWG is often used on the bench to mitigate voltage drop over long runs.
- Breaker Sizing: 11.0A × 2.50 = 27.5A. The next standard breaker size up is 30A. A 30A breaker will hold during the 60A+ inrush spike but will trip instantly on a dead short.
- Thermal Overload: Set to 115% of FLA (12.65A) to protect the motor windings from slow-cooking under a mild mechanical overload.
| Motor HP | 208V FLA (Amps) | 230V FLA (Amps) | 460V FLA (Amps) | 575V FLA (Amps) |
|---|---|---|---|---|
| 2 | 6.6 | 6.0 | 3.0 | 2.4 |
| 3 | 9.6 | 8.4 | 4.2 | 3.4 |
| 5 | 15.2 | 14.0 | 7.0 | 5.6 |
| 7.5 | 22.0 | 20.0 | 10.0 | 8.0 |
| 10 | 28.0 | 26.0 | 13.0 | 10.4 |
| 15 | 42.0 | 38.0 | 19.0 | 15.2 |
Note: Always use the actual nameplate FLA for thermal overload settings. Use NEC table values only for conductor and breaker sizing when the exact motor is unknown during the design phase. (Source: NFPA 70 / NEC Article 430)
Wiring and Terminal Identification for 3-Phase Induction Motors
The 3-phase AC induction motor is the undisputed workhorse of industrial and heavy commercial applications. Understanding its terminal box (peckerhead) is critical for matching the supply voltage to the motor's winding configuration.
Most fractional and integral horsepower 3-phase motors under 10 HP are dual-voltage (230/460V) and feature 9 leads in the terminal box. These leads are numbered T1 through T9 (or U1-W2 under IEC standards).
9-Lead Dual Voltage Wiring Configurations
- High Voltage (460V) Wye: The internal windings are placed in series. Tie leads 4-7, 5-8, and 6-9 together and insulate them with wire nuts. Connect your 3-phase supply lines (L1, L2, L3) to leads 1, 2, and 3. (Phase sequence dictates rotation; swap any two legs to reverse).
- Low Voltage (230V) Wye: The windings are placed in parallel. Tie leads 1-7 to L1, leads 2-8 to L2, and leads 3-9 to L3. Tie leads 4, 5, and 6 together and insulate them.
- Grounding: Always terminate the equipment grounding conductor (EGC) to the dedicated green grounding screw inside the peckerhead, independent of the power terminals. Bonding the motor frame to the ground grid prevents shock hazards if an internal winding shorts to the casing.
Motor Type Comparison: Torque, Drives, and Load Profiles
Not every application calls for an AC induction motor. When designing a system, you must match the motor's torque curve and control architecture to the mechanical load. Treating a stepper and a servo as interchangeable will result in either a wildly overpriced system or one that stalls under dynamic loads.
| Motor Type | Torque Curve Profile | Required Driver / Controller | Cost per HP | Best Load Profile |
|---|---|---|---|---|
| AC Induction | High starting torque, slight slip under load. Constant torque up to base speed. | Direct-on-line (DOL) contactor or VFD (Volts/Hertz or Vector). | Low ($) | Pumps, fans, conveyors, compressors (continuous duty). |
| BLDC (Brushless DC) | Flat torque curve up to rated speed. High efficiency, low rotor inertia. | Electronic Speed Controller (ESC) with Hall sensors or sensorless back-EMF commutation. | Medium ($$) | Drones, RC vehicles, computer cooling, light traction. |
| Stepper | Maximum torque at zero speed (standstill). Torque drops sharply at high RPM. | Open-loop stepper driver (chopper drive, microstepping). No encoder required. | Medium ($$) | 3D printers, CNC routers, low-speed precision positioning. |
| AC Servo | High peak torque (300% of rated) for acceleration. Constant torque to rated speed. | Closed-loop servo drive with high-resolution absolute encoder feedback. | High ($$$$) | Industrial robotics, high-speed pick-and-place, dynamic CNC axes. |
Which Motor Fits Your Load Profile?
If your load requires continuous, heavy-duty operation with minimal precision (like moving air or water), the AC Induction motor paired with a VFD is the undisputed choice. Modern 2026 SiC (Silicon Carbide) VFDs offer near-unity power factor and regenerative braking capabilities that older IGBT drives lacked.
If your application requires exact positional holding without a feedback loop, choose a Stepper. However, be aware that steppers draw full rated current even when stationary to maintain holding torque, leading to high heat generation. If the load requires rapid acceleration, high-speed traversal, and exact positional verification, you must step up to an AC Servo. Servos only draw the current required to move the load, running cool at standstill, but they demand complex tuning and expensive closed-loop drives. (For deeper standards on motor classifications, refer to the NEMA MG 1 standard).
Diagnosing Failure Signatures: Hum, Overheat, and Stall
When a motor operates outside its designed FLA parameters, it communicates the failure through distinct physical and electrical signatures. Recognizing these early prevents catastrophic winding burnout.
1. The 'Hum' and Single-Phasing
A loud, aggressive 120Hz hum from a 3-phase motor, accompanied by a failure to start or a severe drop in speed, is the classic signature of single-phasing. This occurs when one of the three supply legs loses power (due to a blown fuse, broken wire, or failed contactor pole). The motor attempts to maintain the load using only two legs. The current on the remaining two legs will spike to approximately 1.73 times the normal FLA. The thermal overload may not trip fast enough to save the windings. Fix: Measure phase-to-phase voltage at the motor terminals under load. If one reading is missing or voltage unbalance exceeds 2%, trace the open leg back to the MCC or disconnect.
2. Overheat and Ambient Derating
Motor nameplates are rated for a maximum ambient temperature, typically 40°C (104°F). If you install a motor in a boiler room where the ambient air is 55°C, the motor will overheat and trip its internal thermal protector even if the clamp meter reads exactly the nameplate FLA. Fix: Apply ambient derating curves from the manufacturer, upgrade to Class H insulation (rated for higher internal temps), or force external blower cooling. Furthermore, continuous operation at 105% of FLA will degrade Class F insulation life by half for every 10°C rise in winding temperature.
3. Stall and Locked Rotor Conditions
If the mechanical load jams or the driven equipment seizes, the motor drops to zero RPM. It instantly transitions from drawing FLA to drawing LRA (Locked Rotor Amps), which is typically 600% of FLA. The motor will emit a low-frequency growl and heat up violently. A properly sized thermal overload relay or VFD will detect this massive current spike and trip the circuit within 2 to 10 seconds. If the motor stalls and the breaker does not trip, your short-circuit protection is vastly oversized, or the thermal overload heaters are mismatched to the FLA. Fix: Clear the mechanical bind, verify the overload dial is set exactly to the nameplate FLA, and test the trip mechanism.
