Motor Full Load Amps (FLA) Calculator
Calculate motor Full Load Amps (FLA) using the formula method or NEC Table 430.250 lookup. Includes circuit breaker sizing and wire gauge recommendations per NEC.
FLA = (10 HP × 0.746 × 1000) / (460V × 1.732 × 0.85 × 0.90)
What is Full Load Amps (FLA)?
Full Load Amps (FLA) is the current a motor draws when operating at its rated horsepower and voltage under full mechanical load. It is the most important value for sizing motor circuit components including conductors, overload relays, fuses, and circuit breakers.
FLA is often used interchangeably with FLC (Full Load Current), though technically FLC is the term used in IEC standards and on motor nameplates, while FLA is the term used in the NEC. MCA (Minimum Circuit Ampacity) is a different value used primarily for HVAC equipment and already includes a 125% multiplier.
The key distinction: FLA is used to select protection devices; it is not the same as the actual running current, which varies with mechanical load.
FLA Calculation Formulas
The full load current can be calculated from motor power using the following formulas:
Three Phase:FLA = (kW × 1000) / (V × 1.732 × PF × Eff)
Single Phase:FLA = (kW × 1000) / (V × PF × Eff)
HP to kW Conversion:kW = HP × 0.746
Practical Example
A 10 HP, 460V, three-phase motor with 90% efficiency and 0.85 power factor:
- kW = 10 × 0.746 = 7.46 kW
- FLA = (7.46 × 1000) / (460 × 1.732 × 0.85 × 0.90)
- FLA = 7460 / 610.5 = 12.2 A
NEC Article 430 — Motor Circuit Protection
NEC Article 430 covers motors, motor circuits, and controllers. Key requirements include:
- NEC 430.6(A)(1): Use NEC Tables 430.247-430.250 for determining conductor ampacity and branch-circuit protection, not the motor nameplate current.
- NEC 430.22: Branch-circuit conductors shall have an ampacity not less than 125% of the motor's full-load current.
- NEC 430.32: Overload protection for motors rated more than 1 HP at 115% (service factor 1.15) or 125% (all others) of nameplate current.
- NEC 430.52: Branch-circuit short-circuit and ground-fault protection — inverse time circuit breakers shall not exceed 250% of FLC (with exceptions for starting).
This calculator uses NEC Table 430.250 (three-phase) and NEC Table 430.248 (single-phase) for direct FLA lookups in NEC Table Mode.
Typical Motor Efficiencies
| Motor Size (HP) | Standard Efficiency | Premium (IE3/NEMA Premium) |
|---|---|---|
| 1 – 5 HP | 82% – 87% | 86% – 90% |
| 5 – 20 HP | 87% – 91% | 90% – 93% |
| 20 – 50 HP | 91% – 93% | 93% – 95% |
| 50 – 100 HP | 93% – 94% | 94% – 96% |
| 100 – 200 HP | 94% – 95% | 95% – 96% |
| 200 – 500 HP | 95% – 96% | 96% – 97% |
Values are approximate for standard TEFC induction motors. Always refer to the manufacturer's nameplate for exact efficiency values.
Frequently Asked Questions
What's the difference between FLA and FLC?
FLA (Full Load Amps) is the term used in the NEC and North American standards. FLC (Full Load Current) is the IEC-equivalent term found on motor nameplates. They represent the same concept — the current a motor draws at rated load — but FLA values from NEC tables may be slightly higher than nameplate FLC to provide a conservative basis for sizing protection devices.
How do I size a circuit breaker for a motor?
Per NEC 430.22, first determine the motor FLA (from NEC tables or nameplate). Multiply by 1.25 (125%) to get the minimum conductor ampacity. The branch-circuit breaker is then sized per NEC 430.52: for an inverse-time circuit breaker, up to 250% of FLC is permitted. The calculator above recommends the minimum standard breaker size that meets the 125% requirement — consult a licensed electrician for final sizing including starting current considerations.
Why does motor current change with load?
An induction motor draws only as much current as needed to overcome the mechanical load on the shaft. At no load, current is low (primarily magnetizing current, typically 25-35% of FLA). As mechanical load increases, the motor slip increases, rotor current rises, and stator current increases proportionally. FLA occurs at the motor's rated mechanical output (nameplate HP). Exceeding FLA for extended periods causes overheating and insulation damage.
Why is the NEC table value different from my calculation?
NEC Table 430.250 uses conservative generic values that account for worst-case motor characteristics. If your calculated FLA differs from the NEC table lookup, the NEC value takes precedence for code compliance purposes per NEC 430.6(A)(1). Always use the NEC table values for breaker and conductor sizing unless you have actual nameplate data.