3-Phase Power & Motor Sizing Calculator

Calculate required amperage, wire size (AWG), and breaker size for 3-phase industrial motors. Includes voltage drop check for long cable runs. Based on NEC Article 430.

Free Tool · NEC Art. 430 · Wire & Breaker Sizing · Voltage Drop

Motor Parameters

Motor Power Rating

⚡

Supply Voltage (Line-to-Line)

460 V is the most common US industrial voltage. 480 V is the nominal utility supply; motors are rated 460 V to allow for line losses.

Power Factor — 85%

60%75%85%95%+

Typical 3-phase induction motors: 80–90% at full load.

Motor Efficiency — 91%

70%80%91%98%

NEMA Premium motors: 91–96%. Standard efficiency: 85–91%.

Circuit Protection & Voltage Drop

Branch Circuit Breaker Type

🔌

NEC 430.52: Inverse-time breakers up to 250% FLC to handle starting inrush.

Motor Service Factor (SF)

One-Way Cable Length (for Voltage Drop)

📏 ft

Distance from panel to motor. NEC recommends ≤3% voltage drop on branch circuits.

Conductor Material

≈

Results

NEC FLC (Table 430.250)

—

use for conductor sizing

Calculated FLA

—

formula-based (reference only)

Min. Wire Size (AWG)

—

NEC 430.22 · 125% FLC

Max. Breaker Size

—

NEC 430.52 max

Overload Relay Setting

—

NEC 430.32

Starting Inrush (~6× FLA)

—

DOL starting estimate

Voltage Drop

Detailed Summary

Amperage
NEC FLC (Table 430.250)	—
Calculated FLA	—
Conductor sizing (125% FLC)	—
Starting inrush (~6×)	—
Circuit Protection (NEC Art. 430)
Max. breaker (430.52)	—
Overload relay max (430.32)	—
Min. disconnect (430.102)	—
Conductors
Min. wire size	—
Wire ampacity	—
Voltage Drop
Calculated voltage drop	—
Voltage drop %	—
NEC recommendation	—
Inputs Used
Motor power	—
Voltage	—
Power factor / efficiency	—
Cable / conductor	—

How 3-Phase Motor Sizing Works

Sizing a 3-phase motor circuit requires four steps per NEC Article 430: calculate full-load current (FLC), size conductors at 125% FLC, size the breaker, and set the overload relay. Use NEC Table 430.250 FLC for conductors and breakers, and nameplate FLA for the overload relay only.

1 Full-Load Current

The formula calculates FLA from kW, voltage, efficiency, and power factor. For conductor and breaker sizing, always use NEC Table 430.250 — values are intentionally conservative.

FLA (3φ) = (kW × 1000) ÷ (√3 × V × PF × η) √3 = 1.732

2 Wire Sizing (430.22)

Branch circuit conductors must be sized at minimum 125% of NEC FLC. For 10 HP / 460V: 14 A × 1.25 = 17.5 A → #12 AWG copper (20 A rated).

Min. Ampacity = NEC FLC × 1.25 Select AWG from NEC Table 310.16 (75°C Cu column)

3 Breaker Sizing (430.52)

Breakers are oversized to handle starting inrush (5–8× FLC). NEC 430.52 allows inverse-time breakers up to 250% of NEC FLC. Next standard size up is permitted.

Max Breaker (ITCB) = NEC FLC × 250% Standard sizes (240.6): 15,20,25,30,35,40, 45,50,60,70,80,90, 100,125,150,175,200A

4 Overload & VDrop

Overload (NEC 430.32) uses nameplate FLA — 125% for SF≥1.15, 115% for SF=1.0. Voltage drop check: ≤3% on branch circuit recommended.

OL Relay (SF≥1.15): Nameplate FLA × 125% VD (3φ) = √3 × I × R × L/1000

NEC Table vs. Nameplate

Use NEC Table 430.250 FLC for conductors (430.22) and breakers (430.52). Use motor nameplate FLA for overload relay (430.32). Never cross-use these values. Always have a licensed electrician verify all motor circuit designs before installation.

Example Calculations — 3 Real-World Scenarios

These worked examples show the calculator in action across three common U.S. industrial situations. All use 460V, 3-phase, copper conductors, inverse-time breaker (250% FLC), and NEMA SF 1.15.

🔵 CNC Milling Machine

Scenario — 10 HP VMC, 100 ft run

Motor: 10 HP / 460V 3-phase PF / Eff: 85% / 91% Cable: 100 ft copper Breaker: ITCB (250%)

NEC FLC: 14 A · Wire: #12 AWG
Breaker: 35 A · VDrop: ~0.8%

100 ft is well within the ≤3% limit. #12 AWG copper at 20 A ampacity meets the 17.5 A minimum (14 A × 1.25). Overload relay set at ≤17.5 A (nameplate FLA × 125%).

🟠 Batch Plant Mixer Motor

Scenario — 50 HP, 350 ft long run

Motor: 50 HP / 460V 3-phase PF / Eff: 85% / 93% Cable: 350 ft copper Breaker: ITCB (250%)

NEC FLC: 65 A · Wire: #4 AWG
Breaker: 175 A · VDrop: ~2.9%

At 350 ft the voltage drop is at the NEC 3% limit — marginal. For critical production motors, upsize to #3 AWG to bring drop below 2.5%. Starting inrush ~390 A — confirm bus capacity.

🔴 Press Brake Hydraulic Pump

Scenario — 25 HP, 250 ft run, Al wire

Motor: 25 HP / 460V 3-phase PF / Eff: 80% / 90% Cable: 250 ft aluminum Breaker: ITCB (250%)

NEC FLC: 34 A · Wire: #2 AWG Al
Breaker: 90 A · VDrop: ~2.4%

Aluminum wire requires 2 AWG sizes larger than copper for equivalent capacity. At 250 ft VDrop stays below 3%. Use AL/CU rated terminations only — aluminum wiring at copper-only terminals is a fire hazard.

NEC Table 430.250 — 3-Phase AC Motor Full-Load Currents

Full-load currents from NEC Table 430.250 for induction-type squirrel cage motors. Required for conductor and overcurrent device sizing per NEC 430.6(A)(1).

HP	208 V	230 V	460 V	575 V	Min. Wire (Cu)	Max. Breaker
1/2	2.4	2.2	1.1	0.9	#14 AWG	15 A
1	4.6	4.2	2.1	1.7	#14 AWG	15 A
2	7.5	6.8	3.4	2.7	#14 AWG	15 A
5	16.7	15.2	7.6	6.1	#14 AWG	20 A
10Common	30.8	28.0	14.0	11.0	#12 AWG	35 A
25	74.8	68.0	34.0	27.0	#8 AWG	90 A
50Heavy	143	130	65.0	52.0	#4 AWG	175 A
100Large	273	248	124	99.0	#1 AWG	300 A
200	528	480	240	192	4/0 AWG	600 A

10 HP

460V FLC14 A

Min. Wire#12 AWG

Max. Breaker35 A

25 HP

460V FLC34 A

Min. Wire#8 AWG

Max. Breaker90 A

50 HP

460V FLC65 A

Min. Wire#4 AWG

Max. Breaker175 A

AWG Copper Wire Ampacity — 75°C (NEC Table 310.16)

#14

15 A

≤ 2 HP / 460V

#12

20 A

≤ 10 HP / 460V

#10

30 A

≤ 20 HP / 460V

50 A

≤ 30 HP / 460V

65 A

≤ 40 HP / 460V

85 A

≤ 50 HP / 460V

100 A

≤ 75 HP / 460V

115 A

Medium load

130 A

≤ 100 HP / 460V

2/0

175 A

Medium-large

3/0

200 A

≤ 150 HP / 460V

4/0

230 A

≤ 200 HP / 460V

Frequently Asked Questions

NEC 430.52 permits inverse-time breakers up to 250% of NEC FLC to handle motor startup inrush (5–8× FLC). The wire is protected by the overload relay (continuous overcurrent) and the breaker handles true short circuits. Wire ampacity must still be ≥125% FLC per NEC 430.22.

FLC = NEC Table 430.250 values — use for conductor sizing (430.22) and breaker sizing (430.52). FLA = motor nameplate — use for overload relay (430.32) only. Never cross-use these values.

480V is the nominal utility supply. Motors are rated 460V to account for voltage drop in distribution. Always use the 460V column of NEC Table 430.250 for 480V systems.