What Is Motor FLA and How Electricians Use It

Full Load Amps (FLA) is the current a motor draws when running at its rated horsepower and voltage under normal conditions. It is the single number electricians use to size wire, select breakers, and choose overload protection. Get it wrong and you either trip breakers constantly or, worse, undersize wiring and create a fire hazard.

This guide explains what FLA means in practice, where to find it, and how it connects to the electrical decisions that keep motors running safely.

FLA Defined: What the Number Actually Tells You

A motor draws current proportional to its mechanical load. An unloaded motor spins freely and draws only a fraction of its FLA rating. As you add mechanical load (a pump pushing water, a compressor compressing refrigerant, a saw cutting wood), the motor draws more current to produce the required torque. At full rated load, the motor draws its FLA.

FLA is a steady-state number. It does not account for the brief surge of current when a motor first starts: that is LRA (Locked Rotor Amps), which is several times FLA. Mersen's motor fuse guide prints locked rotor currents of 300% to 600% of nameplate current as common with earlier motor efficiencies, 600% to 700% for NEMA Design B, C and D motors under the newer efficiency standards, and as high as 1000% for Design E. How long the surge lasts depends on the motor and the load. Circuit breakers and fuses are designed to tolerate this startup surge without tripping, but they rely on FLA for continuous protection sizing.

Where you have the motor's own nameplate efficiency and power factor, its current can also be computed from them:

FLA = (HP × 746) / (Voltage × Efficiency × Power Factor)

For three-phase motors the formula divides by (V × Efficiency × Power Factor × √3). Those two figures come from your motor's nameplate, not from a fixed factor per horsepower. For sizing, read the printed NEC table instead: a 5 HP single-phase motor at 230V reads 28A, and the same horsepower three-phase at 230V reads 15.2A. Look up your motor with the motor FLA calculator.

NEC Full Load Current Reference Values

The National Electrical Code (NEC) Tables 430.247-430.250 publish standard FLA values by motor type, horsepower, and voltage. These tables, not the motor nameplate, are what the NEC requires electricians to use for sizing branch circuit conductors and overcurrent protection.

Notice the pattern: doubling voltage roughly halves the current for the same horsepower. This is why commercial and industrial facilities use 480V three-phase power: a 10 HP motor at 460V draws only 14A vs 100A at 115V. Lower current means thinner wire, smaller breakers, and less voltage drop over long cable runs. Need the exact watt equivalent? Our HP to watts calculator converts between mechanical, electrical, and metric horsepower.

The NEC table values can differ from the nameplate FLA of your specific motor. A high-efficiency motor might draw less than the table value, while an older or smaller-frame motor might draw more. The NEC requires using the table values for conductor sizing and the nameplate FLA for overload protection sizing.

How to Read a Motor Nameplate

Every motor nameplate contains the information you need, though the abbreviations can be cryptic if you have not seen them before.

1. Find the FLA or "AMPS" rating. Look for "FLA," "Full Load Amps," "Rated Amps," or simply "AMPS." On some motors, you will see two current ratings for different voltages: "115V / 12.0A" and "230V / 6.0A." Use the one matching your supply voltage.
2. Note the voltage. The FLA rating is only valid at the listed voltage. Running a 230V motor on 208V (common in commercial buildings) increases current draw. Siemens' starter catalog prints 208 volt full-load amps as the 230 volt current times 110%. This matters for protection sizing.
3. Check the service factor. A service factor of 1.15 means the motor can continuously deliver 115% of rated HP. At service factor load, current draw exceeds FLA by roughly the same percentage. Some overload relays use this value to set their trip point above FLA.
4. Look for the NEMA design letter. Design B (the most common) has moderate starting current. Design C and D have higher starting current. This affects breaker selection but not continuous FLA.

If the nameplate is faded, damaged, or missing, measure the motor's actual current draw with a clamp meter while it runs under full load. Compare this to the NEC table for the motor's horsepower and voltage. If measured current exceeds the NEC table value by more than 10%, the motor may be overloaded, running on low voltage, or experiencing bearing failure.

FLA in Practice: Wire Sizing, Breakers, and Overload Protection

Wire sizing

Branch circuit conductors must carry at least 125% of the motor's FLA per NEC 430.22. A motor with 28A FLA requires conductors rated for at least 35A. This 125% margin accounts for the motor occasionally running at or near service factor load without overheating the wire.

Use NEC Table 310.16 to select the wire gauge. For 35A on copper THHN in conduit, our wire distance calculator returns 8 AWG on either terminal setting where ampacity sets the gauge. It holds 10 AWG to its 30A small-conductor limit even with 75°C-rated terminations, and with 60°C terminations (NEC 110.14(C)) 8 AWG is the minimum outright. The calculator takes the current, the one-way distance, the voltage and your drop target and returns the recommended gauge, so on a long run where voltage drop sets the gauge it returns a thicker wire.

Branch circuit breaker

The branch circuit breaker must be large enough to handle motor starting current without nuisance tripping, but small enough to protect the circuit from short circuits. NEC 430.52 allows breakers up to 250% of FLA for inverse-time breakers with Design B motors. For 28A FLA: 28 × 2.5 = 70A maximum breaker size, conveniently a standard rating itself. When the percentage lands between standard sizes, NEC 430.52(C)(1) Exception No. 1 permits rounding up to the next standard rating instead.

This is larger than the wire's 35A rating, and that is by design. The breaker protects against short circuits, while overload relays (discussed below) protect against sustained overcurrent. The wire is sized for continuous load (FLA × 1.25), and the brief startup surge does not heat the wire enough to cause damage.

Overload protection

Overload relays (often called heaters) protect the motor from sustained overcurrent due to mechanical overload, bearing failure, or low voltage. They are sized based on the motor nameplate FLA, not the NEC table value. The setting follows the motor's service factor: where fuses are the sole overload protection, Mersen's motor fuse guide prints a maximum of 125% of motor full-load current for a service factor of 1.15 or greater or a marked temperature rise not exceeding 40°C, and 115% for all others.

If a motor repeatedly trips its overload relay, the problem is not the relay: the motor is drawing more current than it should. Common causes: mechanical binding, worn bearings, low supply voltage, or simply trying to drive a load beyond the motor's rating. When the motor's power factor is poor, the apparent power (kVA) exceeds the real power (kW), which also affects how generators and transformers handle the load.

When FLA Matters Most

FLA drives decisions beyond basic wire and breaker sizing. Generator sizing, transfer switch rating, and electrical load calculations all depend on accurate motor current data. If you are planning an off-grid system, every motor in your load audit needs its FLA accounted for when sizing the inverter and battery bank. Underestimating motor loads on a generator causes voltage sag that damages both the generator and the motors connected to it. A well pump, HVAC compressor, and shop equipment all have FLA ratings that must be included in any panel load calculation.

For systems that combine motor loads with battery or solar power, FLA helps you size the inverter. The inverter must handle both the continuous FLA and the LRA startup surge. Use the locked rotor amps calculator to find the surge rating you need, then size your inverter accordingly. Most inverter manufacturers specify a surge rating (lasting 5-10 seconds) that must exceed the motor's LRA.