How do I calculate the amps for a 3-phase motor?

FLA = HP × 746 / (√3 × Voltage × Motor Efficiency × Power Factor). For a 5 HP motor at 480V, efficiency 0.90, PF 0.86: FLA = 5 × 746 / (1.732 × 480 × 0.90 × 0.86) = 3,730 / 645.2 = 5.78 amps. NEC Table 430.250 gives a conservative 7.6A for the same rating — use NEC values for conductor and overcurrent device sizing.

What size wire do I need for a 5 HP three-phase motor at 480V?

NEC FLA for 5 HP at 460V = 7.6A. Wire must be rated for FLA × 1.25 = 9.5A minimum. 14 AWG (rated 15A at 60°C) is sufficient. However, for motors, always check voltage drop for longer runs — use 12 AWG for runs over 50 feet. The circuit breaker can be up to 250% of FLA: 7.6 × 2.5 = 19A → use a 20A breaker.

Why does NEC motor current differ from my calculated value?

NEC Table 430.250 FLA values are intentionally conservative — they're based on older, less efficient motor designs and include safety margins for starting conditions, temperature variation, and motor aging. Actual nameplate current on modern high-efficiency motors is often lower than the NEC table. For conductor sizing, always use the larger of NEC table or nameplate FLA.

How much starting current does a three-phase motor draw?

Three-phase induction motors typically draw 6–8× their full-load amperage during the first few cycles of startup (inrush current). A 10 HP motor with 14A FLA may draw 84–112A for 0.1–3 seconds at startup. This is why motor circuits use time-delay fuses (up to 175% of FLA for dual-element) or inverse-time breakers (up to 250% of FLA) that can withstand this brief inrush without tripping.

How Three-Phase Motor Amperage Is Calculated: HP, Voltage, Efficiency, and Power Factor

Learn the three-phase motor current formula — how to calculate full-load amps from horsepower, voltage, motor efficiency, and power factor, with NEC table values for quick reference.

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Three-Phase Motor Current Formula

Full-Load Amperage (FLA) = HP × 746 / (√3 × V × η × PF)

Where: - HP = motor horsepower rating - 746 = watts per horsepower - √3 ≈ 1.732 - V = line-to-line voltage (typically 208, 230, 460, or 480V) - η = motor efficiency (decimal, typical 0.88–0.95) - PF = power factor (decimal, typical 0.85–0.92 for most motors)

Simplified (using NEC-standard assumptions): FLA ≈ HP × 746 / (√3 × V × 0.90 × 0.88)

Worked Example: 10 HP Motor at 480V

Standard assumptions: η = 0.91, PF = 0.86

FLA = 10 × 746 / (1.732 × 480 × 0.91 × 0.86) = 7,460 / (1.732 × 480 × 0.7826) = 7,460 / 650.8 = 11.5 amps

NEC Table 430.250 lists 10 HP at 460V = 14A (slightly conservative — NEC values include safety margin)

NEC Table 430.250: Three-Phase Induction Motor FLA

*(Representative values — always check current NEC edition)*

HP	208V	230V	460V	575V
1	5.7	5.2	2.6	2.1
1.5	8.0	7.4	3.7	3.0
2	10.4	9.4	4.7	3.7
3	14.8	13.6	6.8	5.5
5	22.6	20.8	10.4	8.3
7.5	32.0	29.2	14.6	11.7
10	41.0	37.6	18.8	15.0
15	56.0	51.6	25.8	20.6
20	73.0	67.2	33.6	26.9
25	88.0	81.2	40.6	32.5
30	106.0	97.2	48.6	38.9
40	140.0	129.0	64.5	51.6
50	169.0	155.0	77.5	62.0
75	246.0	226.0	113.0	90.5
100	322.0	296.0	148.0	118.0

Wire and Overcurrent Sizing (NEC Article 430)

Wire size for motors: NEC requires conductors rated at 125% of motor FLA Wire ampacity ≥ FLA × 1.25

Example: 10 HP, 460V (FLA = 14A) Required wire ampacity: 14 × 1.25 = 17.5A → use 12 AWG (rated 20A)

Overcurrent protection (fuse/breaker): Up to 250% of FLA (inverse time breaker) Maximum breaker: 14 × 2.50 = 35A → use 35A breaker

Starting Current (Inrush)

Motors draw 6–8× their full-load amperage during startup: - 10 HP motor (14A FLA): Startup inrush = 84–112A - This is why motor circuits use time-delay fuses or inverse-time breakers

Soft starters and VFDs reduce startup inrush significantly: - Soft starter: Limits inrush to 2–4× FLA - VFD: Limits inrush to 1–1.5× FLA (smoothest start)

Related Guides

Breaker Size Guide: How to Size Circuit Breakers for Any Load
Learn how to size circuit breakers correctly using NEC rules. Covers the 80% continuous load rule, motor circuit sizing at 250%, AFCI/GFCI requirements, and common household circuits.

Frequently Asked Questions

How do I calculate the amps for a 3-phase motor?: FLA = HP × 746 / (√3 × Voltage × Motor Efficiency × Power Factor). For a 5 HP motor at 480V, efficiency 0.90, PF 0.86: FLA = 5 × 746 / (1.732 × 480 × 0.90 × 0.86) = 3,730 / 645.2 = 5.78 amps. NEC Table 430.250 gives a conservative 7.6A for the same rating — use NEC values for conductor and overcurrent device sizing.
What size wire do I need for a 5 HP three-phase motor at 480V?: NEC FLA for 5 HP at 460V = 7.6A. Wire must be rated for FLA × 1.25 = 9.5A minimum. 14 AWG (rated 15A at 60°C) is sufficient. However, for motors, always check voltage drop for longer runs — use 12 AWG for runs over 50 feet. The circuit breaker can be up to 250% of FLA: 7.6 × 2.5 = 19A → use a 20A breaker.
Why does NEC motor current differ from my calculated value?: NEC Table 430.250 FLA values are intentionally conservative — they're based on older, less efficient motor designs and include safety margins for starting conditions, temperature variation, and motor aging. Actual nameplate current on modern high-efficiency motors is often lower than the NEC table. For conductor sizing, always use the larger of NEC table or nameplate FLA.
How much starting current does a three-phase motor draw?: Three-phase induction motors typically draw 6–8× their full-load amperage during the first few cycles of startup (inrush current). A 10 HP motor with 14A FLA may draw 84–112A for 0.1–3 seconds at startup. This is why motor circuits use time-delay fuses (up to 175% of FLA for dual-element) or inverse-time breakers (up to 250% of FLA) that can withstand this brief inrush without tripping.

Last updated 7/28/2026