Full Load Amps (FLA) represent the exact current a motor draws when delivering its rated mechanical horsepower at rated voltage and frequency. If you are wiring an air compressor, a lathe, or an HVAC blower, the FLA printed on the metal nameplate is the single most important number for sizing your branch circuit. It dictates the continuous thermal limits of your wiring and the calibration of your overload heaters, ensuring the motor doesn't melt its windings during a hard day's work.
Think of FLA like the redline RPM on a car's tachometer when climbing a steep hill in top gear—it is the maximum continuous safe operating limit under full mechanical strain, not the momentary peak surge you see when you first hit the gas.
What FLA Amps Actually Change in Your Circuit
In standard branch circuits (like a wall outlet), you size the wire to match the breaker. Motor circuits completely invert this logic. Because motors draw a massive, momentary surge of current to start (Locked Rotor Amps, or LRA), a standard breaker would trip instantly if sized to the running current. Therefore, the National Electrical Code (NEC) Article 430 separates overload protection (which protects the motor from running too hot over time) from short-circuit protection (which protects the wire from instant faults).
Here is exactly what the FLA changes in your installation:
- Wire Sizing: The branch circuit conductors must be sized to carry 125% of the FLA continuously.
- Overload Sizing: The thermal overload relay inside your motor starter is dialed to trip at 115% to 125% of the FLA.
- Breaker Sizing: The short-circuit breaker is sized much larger (up to 250% of FLA) to allow the startup surge, meaning the wire is not protected by the breaker against minor overloads—the overload relay handles that.
People frequently confuse FLA with LRA (Locked Rotor Amps). LRA is the stalled-startup current, often 5 to 7 times higher than FLA. You use LRA to calculate voltage drop and verify your breaker won't nuisance-trip on startup, but you never use LRA to size your continuous wire. In HVAC, you will also see RLA (Rated Load Amps) on hermetic compressors; RLA is essentially the compressor manufacturer's tested equivalent of FLA.
The Worked Example: Sizing a 5 HP, 230V 3-Phase Motor
Let's walk through a real-world jobsite scenario. You are wiring a 5 HP, 230V, 3-phase table saw motor. You read the motor nameplate and record the following data:
Step 1: Sizing the Branch Circuit Wire
Per NEC 430.22, conductors supplying a single motor must have an ampacity of at least 125% of the motor's FLA.
- Calculation: 15.2A × 1.25 = 19.0 Amps.
- Selection: Looking at NEC Table 310.16 (75°C column), 14 AWG copper is rated for 20A. However, 14 AWG is mechanically fragile and rarely used in industrial conduit. The standard, robust pick is 12 AWG THHN copper (rated 25A at 75°C), which easily clears the 19A minimum and provides better voltage drop performance during the 95A LRA startup surge.
Step 2: Sizing the Thermal Overload Relay
The overload protects the motor windings. Per NEC 430.32, for a motor with a 1.15 Service Factor, the overload can be set at up to 130% of FLA. If it were a standard 1.0 SF motor, the limit is 115%.
- Calculation (1.15 SF): 15.2A × 1.30 = 19.76 Amps.
- Selection: You need a thermal overload relay with an adjustable trip range that encompasses 19.76A. A Schneider Electric TeSys LRD21 (adjustable range 12A to 18A) is too small. You step up to the TeSys LRD22 (adjustable range 16A to 24A) and dial it precisely to 19.8A.
Step 3: Sizing the Short-Circuit Breaker
Per NEC 430.52, the maximum rating for an inverse-time circuit breaker is 250% of the FLA.
- Calculation: 15.2A × 2.50 = 38.0 Amps.
- Selection: The NEC allows you to round up to the next standard breaker size if the exact calculation doesn't match a standard rating. The next standard size is 40A. Therefore, you install a 40A 3-pole molded case circuit breaker (e.g., Square D FA34040). Notice that your 12 AWG wire (25A ampacity) is on a 40A breaker—this is perfectly legal and required in motor circuits because the overload relay provides the thermal protection.
Where You Meet FLA in Practice (And Where People Get It Wrong)
Understanding motor current ratings is critical across several specific applications. Here is where FLA dictates your hardware choices in the real world:
1. HVAC and Mini-Splits (FLA vs. MCA)
If you are wiring a ductless mini-split or central AC condenser, you will rarely see "FLA" on the outdoor unit's data plate. Instead, you will see MCA (Minimum Circuit Ampacity). MCA is a pre-calculated value that already includes the 125% multiplier for the largest compressor motor plus the sum of the fan motor FLAs. Mistake to avoid: Do not multiply the MCA by 1.25 again. If the MCA says 22A, you simply size your wire for 22A (10 AWG) and use the MOCP (Maximum Overcurrent Protective Device) rating printed right below it for your breaker.
2. Variable Frequency Drives (VFDs)
When installing a VFD (like a Yaskawa J1000 or Allen-Bradley PowerFlex), you must pay attention to two different FLA values: Input FLA and Output FLA. The Output FLA must match or exceed your motor's nameplate FLA. However, the Input FLA is often lower than the output FLA due to power factor and efficiency dynamics. Mistake to avoid: Sizing the supply wire feeding the VFD based on the motor's output FLA. Always size the VFD supply wire based on the VFD's rated Input current, applying the 125% NEC rule to that specific number.
3. Missing Nameplates and NEC Table 430.250
If you are retrofitting an old machine and the motor nameplate is painted over or missing, you cannot guess the FLA. NEC Table 430.250 provides standardized FLA values based on horsepower and voltage. For a 5 HP motor at 230V 3-phase, the table dictates 15.2A. You use this table value for wire and breaker sizing, but you must use a clamp meter to measure actual running current before finalizing the overload relay dial.
Decision Tree: Sizing Components Based on Nameplate FLA
Use this decision matrix to terminate your component selection process. Find your component, apply the NEC multiplier to your nameplate FLA, and pick the nearest standard hardware.
| Component to Size | NEC Rule & Multiplier | Calculation (Using 15.2A FLA) | Concrete Hardware Pick |
|---|---|---|---|
| Branch Circuit Wire | 125% of FLA (NEC 430.22) | 15.2 × 1.25 = 19.0A | 12 AWG THHN Copper (25A @ 75°C) |
| Thermal Overload Relay | 115% - 130% of FLA (NEC 430.32) | 15.2 × 1.30 = 19.76A | TeSys LRD22 (Dialed to 19.8A) |
| Inverse-Time Breaker | Max 250% of FLA (NEC 430.52) | 15.2 × 2.50 = 38.0A | 40A 3-Pole Breaker (Next standard size up) |
| Dual-Element Fuse | Max 175% of FLA (NEC 430.52) | 15.2 × 1.75 = 26.6A | 30A Class RK5 Fuse (Next standard size up) |
| Motor Disconnect Switch | Rated for HP, not just Amps | Must handle 5 HP @ 230V | 30A / 10 HP Disconnect (Fused or Unfused) |
Frequently Asked Questions About Motor Current Ratings
Can I use a standard 20A breaker for a 15.2A FLA motor?
No. While 15.2A is technically under 20A, a standard 20A inverse-time breaker will likely nuisance-trip when the motor experiences the 95A LRA startup surge. Motor circuits require the breaker to be sized up to 250% of the FLA specifically to tolerate the startup inrush current without tripping, while the overload relay protects against sustained running overloads.
What happens if the supply voltage drops below the nameplate rating?
If your 230V motor is actually receiving 208V from the panel, it will draw more current to produce the same mechanical horsepower. The actual running amps will exceed the nameplate FLA. If this happens, your thermal overload relay will eventually trip. To fix this, you must either adjust the transformer taps to raise the voltage, or install a motor rated for 208V operation.
Does the Service Factor (SF) change my wire size?
No. The Service Factor (e.g., 1.15) indicates the motor can safely handle 15% more mechanical load than its rated HP for short periods. While a 1.15 SF allows you to increase the overload relay trip setting up to 130% of FLA (per NEC 430.32), the branch circuit wire must still be sized strictly at 125% of the nameplate FLA. Do not multiply the FLA by the SF when sizing wire.
Final Recommendation for Motor Circuit Sizing
When wiring any motor circuit, never default to standard branch circuit rules. Always start with the metal nameplate. Read the exact FLA, multiply it by 1.25 to select your 75°C-rated THHN wire from NEC Table 310.16, and purchase an adjustable thermal overload relay (like the Schneider TeSys LRD series) that allows you to dial in the exact 125% trip point. Size your short-circuit breaker up to 250% of the FLA to clear the startup surge, and verify your final installation with a clamp meter measuring actual running current under full mechanical load before locking out the panel.
