Full Load Amps (FLA) is the steady-state current a motor draws when delivering its rated mechanical horsepower at the rated voltage and frequency. If you are looking at a motor nameplate, the FLA tells you exactly how much continuous electrical current the motor will consume when it is doing the maximum amount of mechanical work it was designed to do, without overheating. In a real circuit, the FLA is the foundational number that dictates your wire gauge, your overload heater settings, and the baseline for sizing your branch circuit breaker.
The Physics and Purpose of Full Load Amps
To understand why FLA matters, you have to look at the thermal limits of the motor windings. When current flows through the copper windings of a stator, it generates heat proportional to the square of the current ($I^2R$ losses). Motor manufacturers design the insulation system (typically Class F or Class H, rated for 155°C or 180°C respectively) to survive the heat generated exactly at the FLA rating, plus a small safety margin defined by the Service Factor.
Think of a motor like a car engine cruising at 65 mph on a flat highway. The engine is under a steady load, burning a predictable amount of fuel, and the cooling system is keeping the temperature perfectly stable. That steady-state fuel burn is your FLA. If you suddenly floor the gas pedal to merge onto a freeway, the engine demands a massive, temporary spike in fuel—that is your inrush or Locked Rotor Amps (LRA). But you cannot sustain that merge RPM indefinitely without overheating the engine. Similarly, a motor cannot sustain currents above its FLA indefinitely without breaking down the varnish insulation on the windings, leading to a short circuit and a dead motor.
FLA vs. LRA vs. RLA: Clearing Up the Nameplate Confusion
The most common mistake DIYers and junior electricians make is confusing FLA with other acronyms stamped on the nameplate. Using the wrong number to size your wire or breaker will result in either nuisance tripping or, worse, a fire hazard. Here is how FLA stacks up against the other common motor current ratings.
| Acronym | Stands For | Definition & Application | Typical Multiplier |
|---|---|---|---|
| FLA | Full Load Amps | Continuous current at rated HP and voltage. Used for overload sizing and baseline thermal calculations. | 1.0x (Baseline) |
| LRA | Locked Rotor Amps | The massive inrush current drawn the instant power is applied before the rotor begins to turn. Used for sizing motor starters and evaluating voltage dip. | 5x to 8x FLA |
| RLA | Rated Load Amps | Specific to HVAC compressors. It is a mathematically derived maximum operating current used strictly for sizing HVAC contactors and wires, not a direct physical measurement like FLA. | Varies by manufacturer |
| MCA | Minimum Circuit Ampacity | Also specific to HVAC. The absolute minimum wire ampacity required to safely feed the unit without overheating the conductors. | 1.25x largest motor + sum of others |
As noted by motor testing experts at Fluke, confusing LRA with FLA when setting a thermal overload relay is a classic error. If you set your overload to trip at the LRA, the motor will literally catch fire before the relay ever opens the circuit. Overloads must always be calibrated to the FLA (or slightly above, per the manufacturer's trip curve).
Where You Meet FLA in Practice: Sizing Wire and Breakers
This is where theory meets the jobsite. When you are wiring a new machine tool, a dust collector, or an air compressor, you must use the FLA to determine your wire size and breaker size. However, there is a massive catch that trips up even experienced hobbyists: Under the National Electrical Code (NEC), you generally do not use the nameplate FLA to size your wire and breakers.
According to EC&M's breakdown of NEC Article 430, you must use the NEC Table FLA (found in Tables 430.247 through 430.250) rather than the nameplate FLA. Why? Because nameplates can be swapped, faded, or inaccurate, but the NEC tables provide a standardized, conservative baseline that ensures safety across all installations. The nameplate FLA is only used for one specific thing: setting the adjustable thermal overload heaters at the motor starter.
Worked Numeric Example: Wiring a 5 HP Air Compressor
Let us say you are installing a 5 HP, 230V, 3-phase air compressor. You look at the motor nameplate and it says FLA: 13.2A.
Step 1: Find the NEC Table FLA
Ignore the 13.2A for wire sizing. Open the NEC to Table 430.250 (Full-Load Current, Three-Phase Alternating-Current Motors). For a 5 HP motor at 230V, the table lists 15.2A. This is your official "FLA" for the rest of the math.
Step 2: Size the Conductors (Wire)
NEC 430.22 requires branch circuit conductors to be sized at 125% of the motor's full-load current.
Calculation: 15.2A × 1.25 = 19.0 Amps.
Looking at the 75°C column of NEC Table 310.16, 14 AWG copper is only good for 20A, but 240.4(D) restricts small conductors. We step up to 12 AWG THHN copper wire (rated 25A at 75°C), which safely handles the 19A requirement and provides mechanical durability.
Step 3: Size the Overload Protection
This is the only place you use the nameplate FLA (13.2A). Per NEC 430.32, the thermal overload relay should be set to no more than 115% of the nameplate FLA (assuming a 1.15 service factor motor).
Calculation: 13.2A × 1.15 = 15.18 Amps. You would select or dial in an overload heater that trips at roughly 15.2A.
Step 4: Size the Branch Circuit Breaker (Short Circuit & Ground Fault)
Motors draw massive inrush current (LRA) when starting. If you used a standard 20A breaker, it would trip instantly every time the compressor kicked on. NEC Table 430.52 allows an inverse-time breaker to be sized up to 250% of the NEC Table FLA.
Calculation: 15.2A × 2.50 = 38.0 Amps.
Per NEC 240.6, you round up to the next standard breaker size. You will install a 40 Amp 3-pole breaker. Notice how the breaker (40A) is much larger than the wire ampacity (25A)? This is legal and required in motor circuits because the overload relay protects the wire from continuous overcurrent, while the breaker only protects against catastrophic short circuits.
Frequently Asked Questions About Motor FLA
What is the FLA on a 1 hp motor?
The FLA of a 1 HP motor depends entirely on the voltage and phase configuration. For a standard 1 HP, 115V single-phase motor (like a table saw or drill press), the NEC Table 430.248 lists the FLA at 16 Amps. If that same 1 HP motor is wired for 230V single-phase, the FLA drops to 8 Amps. For a 1 HP, 230V 3-phase motor, the FLA is much lower, at just 4.2 Amps. Always check the specific voltage column on the NEC tables or the manufacturer's data sheet, as assuming a universal FLA for a given horsepower will result in incorrectly sized components.
How do I calculate FLA from kW?
If you are dealing with a metric motor rated in kilowatts (kW) instead of horsepower, you can calculate the approximate FLA using the three-phase power formula: $I = \frac{P}{\sqrt{3} \times V \times PF \times \eta}$.
Where $P$ is power in watts (kW × 1000), $V$ is line-to-line voltage, $PF$ is power factor (typically 0.85 for induction motors), and $\eta$ is efficiency (typically 0.90). For example, a 3 kW motor at 400V 3-phase: $I = \frac{3000}{1.732 \times 400 \times 0.85 \times 0.90} = \frac{3000}{530} \approx 5.66 Amps$. For exact code compliance, always defer to the manufacturer's published data or Engineering Toolbox motor charts rather than manual calculations.
Is FLA the same as MCA (Minimum Circuit Ampacity)?
No, they are entirely different metrics used in different contexts. FLA is a physical measurement of the current a standard motor draws under full mechanical load. MCA (Minimum Circuit Ampacity) is a calculated, code-mandated minimum wire size rating found almost exclusively on HVAC and refrigeration equipment nameplates. MCA accounts for the largest motor in the HVAC system (multiplied by 125%) plus the sum of all other smaller loads (like condenser fan motors and control transformers). You use FLA to set motor overloads; you use MCA to size the feeder wire to an AC condenser.
Why is my motor drawing more amps than the FLA?
If your clamp meter reads a current higher than the nameplate FLA while the motor is running, the motor is overloaded. Common mechanical causes include a failing bearing creating excess friction, a misaligned drive shaft, a clogged impeller on a pump, or a belt tensioned too tightly. Electrical causes include voltage unbalance (in 3-phase systems, a 2% voltage unbalance can cause a 10% temperature rise) or operating the motor at a lower voltage than its rating, which forces it to draw more current to maintain the same mechanical power output ($P = V \times I$). If the current exceeds the FLA continuously, the thermal overloads should eventually trip to save the windings.
