Full Load Amperage (FLA) is the maximum continuous current a motor or electrical device is designed to draw when operating at its rated mechanical load, voltage, and frequency. If you are sizing conductors, picking a branch-circuit breaker, or setting a thermal overload relay, the FLA stamped on the equipment nameplate is your absolute starting point. Ignoring it or confusing it with starting currents is the fastest way to trip a breaker on startup or melt a terminal lug under continuous operation.
What Full Load Amperage Actually Changes in Your Circuit
When you read the FLA off a nameplate, it directly dictates three critical installation parameters under NEC-style guidance (specifically NEC Article 430 for motors):
- Conductor Ampacity: Motor branch circuit wires are not sized to the exact FLA. Because motors are considered continuous or highly inductive loads, the wire must be sized to carry at least 125% of the FLA to prevent insulation degradation from sustained heat.
- Overload Relay Settings: The thermal overload (which protects the motor itself from burning out) is typically dialed to 115% to 125% of the FLA, depending on the motor's service factor and temperature rise.
- Contactor Ratings: The magnetic contactor switching the motor must have an AC-3 (or equivalent) utilization category rating that meets or exceeds the FLA at the operating voltage.
The Big Confusion: FLA vs. LRA vs. MCA vs. RLA
Manufacturers stamp several current values on nameplates, and mixing them up leads to catastrophic sizing errors. Think of FLA as the engine’s cruising RPM on the highway, while LRA (Locked Rotor Amps) is the massive torque spike required to get a stalled car moving from a dead stop.
| Acronym | Stands For | What It Means | How to Use It |
|---|---|---|---|
| FLA | Full Load Amps | Current drawn at max rated mechanical load. | Base number for wire sizing (x1.25) and overload settings. |
| LRA | Locked Rotor Amps | Current drawn if the rotor is physically stalled (or at the exact millisecond of startup). | Used to calculate voltage drop during startup and size soft-starters or VFDs. |
| MCA | Minimum Circuit Ampacity | Manufacturer's pre-calculated wire sizing number (common in HVAC). | Use this directly for wire sizing; do not apply the 125% multiplier again. |
| RLA | Rated Load Amps | Similar to FLA, but specifically tested for HVAC compressor under normal refrigerant loads. | Treat exactly like FLA for contactor and overload sizing in refrigeration. |
For deeper reading on interpreting these specific nameplate data points, Electrical Technology's guide to motor nameplate data provides excellent visual breakdowns of manufacturer labeling standards.
Worked Numeric Example: Sizing a 5 HP Compressor Motor
Let’s move from theory to the workbench. You are wiring a new 5 HP, 230V, 3-phase air compressor motor. You look at the nameplate and find the following data:
- Voltage: 230V AC (3-Phase)
- Full Load Amps (FLA): 15.2A
- Locked Rotor Amps (LRA): 95.0A
- Service Factor (SF): 1.15
Step 1: Sizing the Branch Circuit Wire
Per NEC 430.22, conductors must be sized at 125% of the FLA.
15.2A × 1.25 = 19.0A
We look at the 75°C column of NEC Table 310.16 (since most motor terminals are rated 75°C). 14 AWG is rated 20A, which technically covers 19A, but 12 AWG THHN (rated 25A at 75°C) is the standard minimum physical size used in industrial environments for mechanical durability. Pick: 12 AWG THHN Copper.
Step 2: Setting the Thermal Overload
The overload heater or electronic dial protects the motor windings. For a motor with a 1.15 Service Factor, NEC 430.32 allows sizing up to 125% of FLA.
15.2A × 1.25 = 19.0A
If your overload relay has a dial range of 14A–22A, you set the dial exactly to 19.0A. If the motor runs in a 115°F ambient room, you might dial it down to 115% (17.5A) to prevent winding degradation.
Step 3: Sizing the Short-Circuit Breaker
This is where DIYers fail. The breaker protects the 12 AWG wire from a dead short, not the motor from an overload. Per NEC 430.52, an inverse-time breaker for a standard AC motor can be sized up to 250% of the FLA.
15.2A × 2.50 = 38.0A
The next standard breaker size up per NEC 240.6 is 40A. Pick: 40A 3-Pole Breaker. (Note: A 40A breaker is perfectly safe here because the 19A thermal overload will trip long before the motor burns out, and the 12 AWG wire is protected against catastrophic short circuits by the 40A magnetic trip).
Where You Meet FLA in Practice
You will encounter FLA requirements in several specific jobsite and workbench scenarios:
- HVAC Condenser Units: You will usually see MCA and MOCP (Maximum Overcurrent Protection) on the nameplate instead of raw FLA. If you only see RLA/FLA, you must do the math yourself. Always check the Engineering Toolbox's FLA reference charts if the nameplate is faded or missing.
- VFD (Variable Frequency Drive) Setup: When commissioning a VFD (like an Allen-Bradley PowerFlex or a Yaskawa V1000), parameter
P0305(or equivalent) requires you to manually type in the motor's FLA. The VFD uses this to build its internal thermal protection model. If you type in the LRA by mistake, the drive will never trip on overload and the motor will eventually catch fire. - Generator Sizing: When sizing a backup generator, you must account for the LRA (starting surge) of the largest motor, but the continuous running load is calculated using the sum of all connected motors' FLA.
Decision Tree: Sizing Wire, Breaker, and Overloads from FLA
Use this decision path to terminate your sizing process with a concrete pick. This assumes standard 3-phase AC induction motors and copper THHN conductors in a 75°C environment.
| If Your Goal Is... | Apply This Multiplier to Nameplate FLA | Concrete Pick / Standard (Based on 15.2A FLA Example) |
|---|---|---|
| Sizing Branch Circuit Wire | FLA × 1.25 | 12 AWG Copper THHN (25A ampacity at 75°C) |
| Setting Thermal Overload Relay | FLA × 1.15 (or 1.25 if SF ≥ 1.15) | 19.0A Dial Setting on adjustable bimetallic relay |
| Sizing Inverse-Time Breaker | FLA × 2.50 (Next standard size up) | 40A 3-Pole Molded Case Breaker |
| Sizing Magnetic Motor Starter | FLA × 1.0 (Must exceed FLA) | NEMA Size 2 Contactor (Rated 25A at 230V) |
Frequently Asked Questions
Can I measure FLA with a clamp meter to find out what it is?
Only if the motor is actually driving its maximum rated mechanical load. If you clamp the wires of a 5 HP air compressor that is currently running unloaded (just spinning freely), your meter might read 4A. That is no-load current, not FLA. Always trust the stamped nameplate data for sizing; use the clamp meter only for troubleshooting to see if the motor is currently overloaded compared to the nameplate.
What if the motor nameplate is completely missing or unreadable?
If you cannot read the nameplate, you must use the NEC Table 430.250 (Full-Load Currents for Alternating-Current Motors) as a last resort. For a 5 HP motor at 230V 3-phase, the table lists 15.2A. However, this is a code-minimum baseline; actual modern high-efficiency motors often draw slightly less, while older or specialized motors might draw more. Replace the nameplate or consult the manufacturer datasheet before finalizing a design.
Does FLA change if the voltage drops?
Yes. If your supply voltage sags below the motor's rated voltage (e.g., dropping to 208V on a 230V motor), the motor will draw more current to produce the same mechanical horsepower. This is why NEC guidelines allow for a ±10% voltage tolerance, and why sizing your wire to 125% of the FLA provides a crucial thermal buffer for minor voltage drops on long feeder runs.
