About

Incorrectly sizing motor circuit protection is a fire risk. This tool calculates the Full Load Amperage (FLA) of a 3-phase AC motor, a critical step for selecting wire gauges, conduit sizes, and overcurrent protection devices. Unlike basic estimators, this calculator accounts for PF (Power Factor) and η (Efficiency) to provide precise electrical load data.

Engineers and electricians use this logic to comply with standards like the NEC (National Electrical Code). While nameplate data should always be the primary reference, this tool provides accurate estimates for planning, load studies, and retrofitting when the physical nameplate is inaccessible or illegible.

Formulas

The core calculation for 3-phase current utilizes the relationship between electrical power, voltage, and the motor's specific characteristics.

I = Pwatts√3 × V × PF × η

Where:

I = Current in Amperes (A)
P = Power in Watts (1 HP ≈ 746 W)
V = Line-to-Line Voltage
PF = Power Factor (0 to 1)
η = Efficiency (0 to 1)
√3 ≈ 1.732 (Phase constant)

Reference Data

Power (HP)	208V Amps	230V Amps	460V Amps	575V Amps
0.5	2.4	2.2	1.1	0.9
1	4.6	4.2	2.1	1.7
5	16.7	15.2	7.6	6.1
10	30.8	28	14	11
20	59.4	54	27	22
50	143	130	65	52
100	273	248	124	99
200	528	480	240	192

Frequently Asked Questions

In 3-phase power systems, the square root of 3 (approx 1.732) relates the line voltage to the phase voltage. It accounts for the 120-degree phase shift between the three conductors, ensuring the total power is calculated correctly across all phases.

No. This tool calculates Full Load Amps (FLA), which is the current drawn while running at rated power. Starting current (Locked Rotor Amps) is typically 6 to 8 times higher than the FLA depending on the NEMA design code of the motor.

If the efficiency is unknown, a safe standard estimate for modern motors is 85% to 92% (enter 0.85 - 0.92). Older motors may be closer to 80%. Using a lower efficiency value results in a higher (safer) estimated amperage.

The wire size provided is an estimation based on standard NEC Table 310.16 ampacity for 75°C copper conductors. Real-world installation requires adjustments for voltage drop, conduit fill, ambient temperature, and terminal ratings. Always consult a licensed electrician.