Full Load Amps (FLA) is the maximum continuous current a motor or electrical appliance is designed to draw when operating at its rated voltage and delivering its full rated mechanical output. If you are wiring a 5 HP air compressor, setting up a Variable Frequency Drive (VFD), or sizing an overload relay, the FLA is the single most critical baseline number on the equipment nameplate. It tells you exactly what the motor should pull under maximum continuous rated load, which in turn dictates your wire gauge, breaker size, and thermal protection settings.
What FLA Changes in a Real Installation (The Math)
A common mistake among junior technicians is looking at the FLA and immediately installing a breaker of that exact size. Doing so will result in a tripped breaker every time the motor starts. Under NEC Article 430, FLA is not your final breaker size; it is the mathematical seed used to calculate your branch circuit components.
Assume we have a 5 HP, 230V, 3-phase motor with a nameplate FLA of 15.0A and a Service Factor (SF) of 1.15. Here is how FLA dictates the installation:
- Wire Sizing (NEC 430.22): Branch circuit conductors must be sized at 125% of the FLA.
15.0A × 1.25 = 18.75A. Looking at the 75°C column of NEC Table 310.16, 12 AWG THHN copper (rated for 25A) is the minimum legal wire size. - Breaker Sizing (NEC 430.52): To handle the inrush current without nuisance tripping, the maximum inverse-time breaker size is 250% of the FLA.
15.0A × 2.5 = 37.5A. Per NEC 240.6, you round up to the next standard breaker size, yielding a 40A breaker. - Overload Relay Setting (NEC 430.32): The thermal overload relay on the motor starter protects the motor itself. For a 1.15 SF motor, it is typically set at 115% of FLA.
15.0A × 1.15 = 17.25A. You would dial the overload relay precisely to 17.25A.
If you ignored the FLA and simply put this motor on a 15A breaker with 14 AWG wire, the inrush current would trip the breaker instantly, and the continuous run current would overheat the wire, creating a severe fire hazard.
Where You Meet FLA in Practice
You will encounter FLA in three primary scenarios on the bench or in the field:
- VFD Parameter Setup: When commissioning a Variable Frequency Drive (like an Allen-Bradley PowerFlex 525 or Yaskawa A1000), you must manually enter the motor's FLA into the drive's parameters (e.g., Parameter P031 on the Yaskawa). The VFD uses this value to build an internal thermal model of the motor. If you enter a value higher than the actual FLA, the drive will fail to trip during a mechanical jam, potentially burning up the motor windings.
- Motor Control Centers (MCCs) and Contactors: When selecting a NEMA or IEC contactor, you match the FLA to the contactor's continuous current rating. For our 15A FLA motor, a NEMA Size 1 contactor (rated for 27A at 230V) is sufficient. However, if the motor starts and stops 50 times an hour (high duty cycle), you would upsize to a NEMA Size 2 (45A) to prevent the internal contacts from pitting and welding shut prematurely.
- Field Troubleshooting with a Clamp Meter: When a motor is running, you clamp a True-RMS multimeter around one of the phase wires. If the meter reads 18A on a motor with a 15A FLA, the motor is mechanically overloaded, the supply voltage is low, or there is a bearing failure causing excess friction. According to Fluke's motor diagnostic guidelines, comparing real-time amperage to nameplate FLA is the fastest way to diagnose mechanical vs. electrical faults.
The Great Nameplate Confusion: FLA vs. LRA, RLA, and MCA
Motor and compressor nameplates are densely packed with acronyms. Confusing FLA with startup or compressor-specific metrics is a frequent cause of improperly sized circuits. Think of FLA like your car's engine RPM when cruising at 75 MPH on the highway—it's the sustainable, continuous operating state. Other metrics represent extreme or specialized conditions.
| Acronym | Stands For | What It Means | Typical Multiplier vs FLA |
|---|---|---|---|
| FLA | Full Load Amps | Max continuous current at rated load and voltage. | 1.0x (Baseline) |
| LRA | Locked Rotor Amps | Current drawn at the exact moment of startup (0 RPM) before back-EMF builds. | 5x to 8x FLA |
| RLA | Rated Load Amps | Specific to HVAC compressors; derived from FLA but tested under specific refrigerant conditions. | Usually slightly lower than FLA |
| MCA | Minimum Circuit Ampacity | HVAC-specific wire sizing number. Replaces the 125% FLA math for packaged units. | 1.25x of largest motor + sum of others |
Frequently Asked Questions About Full Load Amps
Is FLA the same as the breaker size I should install?
No. FLA is strictly a motor performance metric, not a circuit protection metric. Because electric motors draw massive inrush currents (LRA) for a few seconds during startup, the branch circuit breaker must be sized significantly higher than the FLA to prevent nuisance tripping. As shown in the NEC Article 430 example above, a 15A FLA motor typically requires a 40A breaker. The actual continuous overload protection is handled by a separate thermal overload relay or VFD internal logic, which is dialed in close to the FLA.
Why does my motor draw more amps than the FLA on the nameplate?
If your clamp meter reads higher than the nameplate FLA while the motor is running, the motor is working harder than its rated mechanical design. Common culprits include:
- Voltage Drop: If the supply voltage drops below the nameplate rating (e.g., measuring 208V on a 230V motor), the motor will draw proportionally more current to maintain the same mechanical power output (Watts = Volts × Amps).
- Mechanical Overload: The driven equipment (pump impeller, conveyor belt, fan blade) is binding, jammed, or pushing more mass than designed.
- Single-Phasing: In a 3-phase system, if one phase is lost or severely unbalanced, the remaining two phases will spike well past the FLA to keep the rotor turning, usually leading to rapid thermal failure.
What happens if I run a motor continuously above its FLA?
Running above FLA generates excess heat in the stator windings. Motor insulation is rated by temperature class (e.g., Class F is rated for 155°C). For every 10°C you exceed the insulation's thermal limit, the lifespan of the motor's dielectric insulation is cut in half. Eventually, the insulation melts, the windings short out to the stator core or each other, and the motor fails catastrophically. This is exactly why the thermal overload relay is set to trip just above the FLA—to sacrifice the process and shut down the motor before the insulation is destroyed.
How do I measure FLA accurately in the field?
You cannot measure FLA on an unloaded motor. A motor spinning freely on a bench with no mechanical load attached will draw a fraction of its FLA (often just 20% to 30%). To measure true FLA, the motor must be coupled to its actual driven load, and the system must be operating at its maximum designed capacity (e.g., a pump pushing against maximum head pressure, or a crusher processing maximum material). Use a True-RMS clamp meter, clamp around one single phase conductor at a time, and wait for the motor to reach full operating temperature, as winding resistance changes as the copper heats up.
