Full Load Amps (FLA) is the maximum continuous current a motor draws when delivering its rated horsepower at its rated voltage and frequency. When you look at a motor nameplate, this single number dictates almost every downstream electrical decision you will make, from the gauge of copper you pull through the conduit to the trip setting on your thermal overload relay. Misinterpreting this value or confusing it with starting current metrics is one of the most common reasons for nuisance breaker trips and burned-out windings in both DIY and industrial installations.
What FLA Amperage Actually Means (and What It Isn't)
Electric motors convert electrical energy into mechanical work. As the mechanical load on the motor shaft increases, the motor must draw more current to maintain its speed and torque. FLA is measured at 100% rated mechanical load. If a 5 HP motor is driving a pump that requires exactly 5 HP to move the fluid, the motor will draw its FLA. If the pump valve is throttled down and only requires 3 HP, the motor draws less current than the FLA.
Think of FLA like a car cruising at 65 MPH on the highway, while starting current is the massive surge of fuel needed to get that same car moving from a dead stop.
- LRA (Locked Rotor Amps): The massive current spike (typically 5 to 8 times the FLA) that occurs the exact millisecond power is applied before the rotor begins to turn. You use LRA to calculate voltage drop during startup, not for wire sizing.
- RLA (Rated Load Amps): Found almost exclusively on HVAC compressors. This is a mathematically derived value used for overload protection, not a direct measurement of maximum continuous current like FLA.
- MCA (Minimum Circuit Ampacity): A pre-calculated wire sizing requirement found on HVAC equipment. It already includes the 125% safety multiplier, whereas raw FLA requires you to do the math.
How FLA Changes Your Circuit Design
In the electrical trade, FLA is the foundational variable for NEC Article 430 calculations. It changes your installation in three critical ways:
- Conductor Sizing: Wires must be sized to handle 125% of the motor's full-load current to prevent insulation degradation from continuous heat buildup.
- Overload Protection: Thermal overload relays or heaters are calibrated to a tight percentage (usually 115% to 125%) of the FLA. Their sole job is to protect the motor from burning itself out if it is mechanically overloaded.
- Short-Circuit and Ground-Fault Protection: The branch circuit breaker or fuse is sized much larger than the FLA—often up to 250%—to allow the motor to survive the massive LRA inrush current during startup without tripping.
A critical code nuance that catches many off guard: according to NEC 430.6(A), you must use the NEC tables (Table 430.250) to find the standard full-load current for sizing wires and breakers, but you must use the actual nameplate FLA for sizing the overload relays. This accounts for minor manufacturing variations in motor efficiency.
Worked Example: Sizing Wire and Breakers Using FLA
Let's walk through a real-world calculation for a 5 HP, 230V, 3-phase AC motor. The manufacturer's nameplate states an FLA of 15.0A, a Service Factor (SF) of 1.15, and an LRA of 95A.
Step 1: Sizing the Conductors (NEC 430.22)
First, we look up the standard full-load current for a 5 HP, 230V, 3-phase motor in NEC Table 430.250, which gives us 15.2A. We multiply this by 125% for continuous duty: 15.2A × 1.25 = 19.0A. Checking the 75°C column of NEC Table 310.16, 14 AWG copper is only rated for 20A, but standard practice and terminal temperature limits usually push us to 12 AWG THHN (rated 25A at 75°C), giving us a safe, code-compliant margin.
Step 2: Sizing the Branch Circuit Breaker (NEC 430.52)
To prevent the breaker from tripping during the 95A LRA startup surge, we use the maximum allowance for an inverse-time breaker, which is 250% of the Table 430.250 value. 15.2A × 2.50 = 38.0A. Per NEC 240.6, we round up to the next standard breaker size, which is a 40A breaker.
Step 3: Setting the Overload Relay (NEC 430.32)
Here, we switch to the nameplate FLA of 15.0A. Because the motor has a 1.15 Service Factor, the code allows us to set the overload at 125% of the nameplate FLA. 15.0A × 1.15 = 17.25A. If using an adjustable electronic overload relay, you dial it precisely to 17.2A.
| Component | Calculation Basis | Math | Final Selection |
|---|---|---|---|
| Conductors | 125% of Table 430.250 (15.2A) | 15.2 × 1.25 = 19.0A | 12 AWG THHN Copper |
| Branch Breaker | 250% of Table 430.250 (15.2A) | 15.2 × 2.50 = 38.0A | 40A Inverse-Time Breaker |
| Overload Relay | 125% of Nameplate FLA (15.0A) | 15.0 × 1.15 = 17.25A | Relay dialed to 17.2A |
Where You Meet FLA in Practice
You will encounter FLA amperage most frequently in three specific scenarios:
- Industrial Control Panels (UL 508A): When building a motor control center, the sum of all motor FLAs dictates the main busbar sizing and the main feeder breaker. Panel builders use software that aggregates these values to ensure the enclosure's thermal limits aren't exceeded.
- Variable Frequency Drive (VFD) Sizing: A common and costly mistake is sizing a VFD strictly by the motor's horsepower. VFDs must be sized by their continuous current output rating matching or exceeding the motor's FLA. A 5 HP VFD might only output 14A, which will fault continuously if your specific 5 HP motor has an FLA of 15.0A. Always match the ampacity, not just the HP label.
- Generator and UPS Sizing: When sizing a backup generator for a facility, the running load is calculated using the aggregate FLA of all motors expected to run simultaneously, while the starting kVA is calculated using the LRA.
For deeper technical specifications on motor ratings and nameplate requirements, the NEMA MG 1 standard remains the definitive manufacturing reference in North America.
FLA Amperage FAQ
Is FLA the same as the breaker size?
No. The breaker size is almost always significantly larger than the FLA. While the FLA represents the continuous running current, the breaker must be large enough to tolerate the massive inrush current (LRA) that occurs for a few seconds every time the motor starts. If you sized a breaker exactly to the FLA, it would trip instantly upon startup. The overload relay, not the breaker, is the device sized closely to the FLA to protect the motor from thermal damage.
What happens if a motor runs above its FLA?
If a motor continuously draws current above its FLA, it is mechanically overloaded. The excess current generates proportional heat (I²R losses) inside the stator windings. Over time, this heat degrades the enamel insulation on the copper wire, eventually leading to a short circuit between windings and catastrophic motor failure. This is exactly what the thermal overload relay is designed to prevent by cutting power when current exceeds roughly 115% to 125% of the FLA.
How do I find the FLA if the nameplate is missing or unreadable?
If the nameplate is gone, you should use the NEC Table 430.250 (Full-Load Currents in Amperes, Alternating-Current Motors) to find the standard FLA based on the motor's horsepower, voltage, and phase configuration. Alternatively, if you know the exact efficiency and power factor, you can calculate it using the formula: FLA = (HP × 746) / (1.732 × Voltage × Efficiency × Power Factor) for a 3-phase motor. However, the NEC table is the legally recognized method for electrical installations.
Does FLA change if the supply voltage drops?
Yes. A motor is essentially a constant-power device. If the supply voltage drops below the rated nameplate voltage (for example, a 230V motor receiving only 208V due to voltage drop on a long wire run), the motor must draw more current to produce the same mechanical horsepower. Running a motor at 10% below rated voltage will typically cause it to draw roughly 10% more current than its rated FLA, which can lead to overheating and premature tripping of the overload relay.
