Electrical Basics · Motors · Full Load Current · Motor Protection
Motor Full Load Current (FLC): 4 Essential Steps Every Engineer Must Know
Every overload relay, breaker, and cable feeding a motor is sized around one number, its full load current. This guide explains how to calculate motor full load current from HP or kW rating, includes a chart comparing common motor sizes, and gives you a live FLC calculator.
What Full Load Current Actually Means
Full load current, usually shortened to FLC, is the current a motor draws when it is running at its rated horsepower or kilowatt output, at rated voltage. It is not the current at startup, which runs far higher, and it is not the current at partial load, which runs lower. FLC is the reference number used to size overload protection, contactors, and the cable feeding the motor.
Whenever possible, the motor's own nameplate FLC should be used directly, since it reflects the actual design of that specific motor. Calculating FLC from HP or kW becomes necessary when the nameplate is missing, damaged, or when sizing a system before the actual motor has been selected.
How to Calculate Motor Full Load Current: 4 Steps
FLC Formulas by Motor Type
Calculating motor full load current correctly starts with picking the right formula for the motor type you actually have.
Uses line to line voltage and the square root of 3 factor, since three phase power splits current across three conductors.
Most common in: industrial pumps, fans, compressors, and conveyors.
Simpler formula without the √3 factor, since all current flows through a single pair of conductors.
Most common in: small tools, residential pumps, and appliance motors.
The most direct formula, since DC power is simply voltage times current, without any phase or frequency factors involved.
Most common in: battery powered equipment and some legacy industrial drives.
Not a formula at all, but the manufacturer's own tested value, which always takes priority when it is available.
Most common source: the motor's physical nameplate or manufacturer datasheet.
FLC Across Common Motor Sizes
The Full Load Current Formulas
Single Phase: FLC = (P × 746) / (V × PF × Eff)
DC Motor: FLC = (P × 746) / (V × Eff)
Where:
P = rated power in HP (use kW × 1.341 to convert kW to HP first)
V = rated voltage, PF = power factor, Eff = efficiency (as a decimal)
Example: 20 HP three phase motor, 415 V, PF 0.85, efficiency 0.88 FLC = (20 × 746) / (1.732 × 415 × 0.85 × 0.88) = 14,920 / 536.4 ≈ 27.8 A This is a calculated estimate. Real motors vary based on their specific design, so always use the actual nameplate FLC when it is available, and treat a calculated value as a planning estimate rather than a final answer.
Formula Variables by Motor Type
| Motor Type | Uses √3 Factor | Typical Power Factor | Typical Efficiency |
|---|---|---|---|
| Three phase | Yes | 0.80 to 0.90 | 85% to 95% |
| Single phase | No | 0.65 to 0.80 | 60% to 80% |
| DC motor | No, no PF term | Not applicable | 75% to 90% |
Where FLC Calculations Are Used
Motor full load current shows up as a design input across almost every rotating equipment application in a plant.
Overload settings and cable size are both based directly on the pump motor's FLC.
Multiple motor FLC values combine to size the overall feeder for a conveyor line.
High starting current makes accurate FLC critical for correct overload coordination.
FLC feeds directly into breaker and disconnect sizing for rooftop and duct fan units.
High inertia loads need careful FLC based sizing to avoid nuisance overload trips.
Safety critical applications rely on accurate FLC for protection device coordination.
Calculating FLC Correctly
Most mistakes when calculating motor full load current come down to one of these habits.
- Use nameplate FLC whenever it is available: it reflects the real motor, not an estimate.
- Include efficiency and power factor in the formula: skipping them gives an inaccurate, usually too low, result.
- Size overload protection based on FLC: not on horsepower alone.
- Confirm the supply voltage matches the motor's rated voltage: before finalizing any current calculation.
- Don't assume 1 HP always equals exactly 746 watts of real draw: that figure is the mechanical output conversion, not the electrical input.
- Don't use the single phase formula for a three phase motor: the missing √3 factor gives a badly wrong result.
- Don't confuse FLC with starting current: starting current can run five to ten times higher, briefly.
- Don't skip checking standard tables when the nameplate is missing: published reference tables exist for exactly this situation.
Motor FLC Calculator
Enter motor rating, voltage, power factor, and efficiency to estimate full load current.
Quick FAQs: Motor Full Load Current
External References
- Wikipedia: Electric Motor
- NEMA: Motor and Generator Standards (MG 1)
- NFPA 70: National Electrical Code, Motor Circuit Requirements
What we learn today
- Motor full load current is the current drawn at rated output and voltage, used to size overload protection, contactors, and feeder cables.
- Three phase, single phase, and DC motors each use a different formula, differing mainly in whether the √3 and power factor terms appear.
- Efficiency and power factor both belong in the calculation, since rated HP or kW describes mechanical output, not electrical input current.
- Nameplate FLC should always be used over a calculated estimate whenever the actual motor and its nameplate are available.