Full Load Amps (FLA) is the maximum continuous current a motor is designed to draw when delivering its rated mechanical horsepower at its rated voltage and frequency. In a real installation, the FLA value dictates the exact trip setting for your thermal overload relays and establishes the baseline for calculating your branch circuit conductor ampacity. If you misread this number or confuse it with starting current, you risk either nuisance-tripping your breakers on startup or silently cooking your motor windings under heavy load.
What is FLA and Why It Dictates Your Motor Protection
When a manufacturer tests an electric motor, they measure exactly how much electrical current it takes to produce the mechanical work stamped on the plate (e.g., 5 HP). That measured current at maximum rated torque is the FLA. According to Fluke's motor nameplate guidelines, this value assumes the motor is operating at its nameplate voltage (e.g., 230V) and frequency (60Hz). If your actual line voltage sags to 208V, the motor will draw more current than the FLA to produce the same horsepower, which is why voltage drop calculations on long feeder runs are critical.
Do not confuse nameplate FLA (Full Load Amps) with NEC FLC (Full Load Current). FLA is the specific measured value printed on your motor's physical sticker. FLC is the standardized value found in NEC Table 430.250. Per the National Electrical Code, you use the nameplate FLA to size your thermal overload relays, but you must use the NEC table FLC to size your branch circuit wires and short-circuit breakers.
The Big Three: FLA vs. LRA vs. RLA
The most common mistake DIYers and junior technicians make is looking at a nameplate and sizing a standard thermal-magnetic breaker to the FLA. This ignores the physics of motor starting. Here is how the three main current ratings differ:
| Metric | Stands For | What It Means | Used For Sizing... |
|---|---|---|---|
| FLA | Full Load Amps | Current drawn at 100% rated mechanical load. | Thermal overload relays, VFD programming. |
| LRA | Locked Rotor Amps | Current drawn when the rotor is stalled (startup surge). | Short-circuit breaker magnetic trip thresholds, contactor ratings. |
| RLA | Rated Load Amps | Current drawn under typical HVAC compressor operating conditions. | HVAC compressor contactors and specific refrigeration circuit sizing. |
While FLA tells you what the motor draws while doing its job, LRA tells you what it draws for the first 2 to 5 seconds while accelerating the load from zero to operating RPM. As noted by the Electrical Engineering Portal, LRA is typically 5 to 8 times higher than FLA. If your breaker is sized strictly to FLA, the LRA inrush will instantly trip the breaker's magnetic mechanism before the motor ever reaches full speed.
Worked Numeric Example: Sizing Wire and Overloads for a 5HP Motor
Let's walk through a real bench calculation for a 5 HP, 230V, 3-phase, 60Hz induction motor driving a workshop dust collector.
NEC Table 430.250 FLC: 15.2A
- Size the Overload Relay (NEC 430.32): Overloads protect the motor from burning up. They must be sized at 125% of the nameplate FLA.
Calculation: 14.0A × 1.25 = 17.5A. You will dial your adjustable bimetallic overload relay exactly to 17.5A. - Size the Branch Conductors (NEC 430.22): Wires must handle 125% of the NEC table FLC.
Calculation: 15.2A × 1.25 = 19.0A. Looking at the 75°C column of NEC Table 310.16, 14 AWG THHN is rated for 20A, but standard practice and voltage drop considerations usually push us to 12 AWG THHN (rated 25A at 75°C) for a robust, low-resistance run. - Size the Short-Circuit Breaker (NEC 430.52): The breaker protects the wire from dead shorts, not the motor from overloads. For an inverse-time breaker, the code allows up to 250% of the NEC table FLC to accommodate the LRA startup surge.
Calculation: 15.2A × 2.5 = 38.0A. The next standard breaker size up per NEC 240.6 is 40A.
If you had mistakenly used a 20A breaker based on the 14.0A FLA, the 85A LRA startup surge would have tripped the 20A breaker's instantaneous magnetic latch immediately.
Where You Meet FLA in Practice
You will encounter and rely on FLA in several specific scenarios on the jobsite or at the workbench:
- VFD Parameter Setup: When programming a Variable Frequency Drive (like a Yaskawa J1000 or Allen-Bradley PowerFlex), Parameter E2-01 (Motor Rated Current) must be set exactly to the nameplate FLA so the drive's internal electronic thermal protection functions correctly.
- Soft Starter Sizing: Solid-state soft starters are rated by their continuous current capacity. You must select a soft starter whose amp rating meets or exceeds the motor's FLA, not its horsepower rating, due to varying efficiency across motor brands.
- Current Transformer (CT) Selection: When installing a motor monitoring relay or an IoT energy sensor (like a Shelly EM or Emporia Vue), you select CT clamps based on the FLA. A 100A CT on a motor with a 5A FLA will yield poor resolution; a 20A CT provides much tighter data granularity.
- Generator Sizing: When calculating the load for a standby generator, the running wattage is derived directly from the FLA (Watts = √3 × Volts × FLA × Power Factor), while the starting kVA requirement is derived from the LRA.
Real-World Scenario: The Compressor Breaker Nuisance Trip
The Setup: A hobbyist is wiring a newly acquired 3HP, 240V single-phase air compressor in their garage. They look at the motor nameplate, see FLA: 17A, and decide to wire it using 12 AWG NM-B cable and a standard 20A thermal-magnetic breaker from the hardware store, reasoning that 20A is safely above the 17A running current.
The Numbers: Nameplate FLA = 17A. Nameplate LRA = 102A. NEC Table 430.248 FLC = 20A.
The Outcome: The hobbyist flips the 20A breaker on. The compressor motor emits a loud hum, the lights in the garage dim significantly, and less than half a second later, the breaker trips with a sharp snap. The motor never reaches operating speed. The hobbyist resets it, tries again, and gets the exact same result.
What Went Wrong: The DIYer sized the breaker for the running current (FLA) instead of the starting surge (LRA). A standard 20A thermal-magnetic breaker has an instantaneous magnetic trip threshold typically set between 5x and 10x its rating (100A to 200A). The compressor's 102A LRA pushed right into that magnetic trip zone. Furthermore, single-phase motors draw heavily on the grid during startup. The correct fix, per NFPA 70 (NEC) Article 430, is to recognize that the breaker's job here is strictly short-circuit protection. The correct breaker size is 250% of the NEC FLC (20A × 2.5 = 50A). The hobbyist needed to install a 50A HACR-rated breaker (which has a delayed magnetic trip curve specifically designed for motor inrush) while retaining the 12 AWG wire. The motor itself is protected from continuous overloads by the internal thermal switch or an external overload relay, rendering the 50A breaker safe for the 12 AWG wire in this specific motor circuit topology.
FAQ: Common FLA Questions from the Workbench
Can a motor draw more than its FLA without failing?
Yes, briefly. Motors have a Service Factor (SF) printed on the nameplate, commonly 1.15. If a 10A FLA motor has a 1.15 SF, it can safely draw up to 11.5A continuously under high ambient temperatures or slight voltage sags without degrading the insulation class. However, your overload relay should still be set based on the base FLA unless the manufacturer specifies otherwise.
Why is the FLA different for 230V vs 460V on the same motor?
What happens if my measured running current is 10% higher than the nameplate FLA?
If your clamp meter reads 10% over FLA while the motor is under its normal mechanical load, you likely have a voltage drop issue, a misaligned mechanical coupling causing excess friction, or deteriorating bearings. If the current remains elevated, the motor will eventually exceed its thermal limits, bake the winding varnish, and fail prematurely.
