How Many Amps Does a 20 HP three-phase Motor Draw at 575V?

At 575V, 20 horsepower equals roughly 20.73 amps per line of running current on a three-phase circuit. One HP is 746 watts of mechanical output, but motors are not 100% efficient, so the electrical draw is higher. Applying √3 × V_L-L × Eff × PF: 14,920W ÷ (√3 × 575 × 0.85 × 0.85) = 14,920 ÷ 719.56 = 20.73 A.

20 HP three-phase motor at 575V

20.73 Amps per line running

Calculated running current at the motor terminals at the assumed 85% efficiency and PF 0.85, per line on a balanced three-phase circuit. This is a conversion from the nameplate horsepower using those assumptions, not a measured value; a real meter reading depends on the motor's actual nameplate efficiency, loading, temperature, and motor design.

NEC Table 430.250 FLC (code sizing base)22 A

Conductor min ampacity (NEC 430.22, 125% of FLC)27.5 A

Electrical input (HP × 746 ÷ efficiency)17,552.94 W

Try: 20HP at 575V (3Φ) 15HP at 575V (3Φ) 25HP at 575V (3Φ) 10HP at 575V (3Φ)

Volts

Motor Type

Running Amps (per line)

20.73

NEC Table 430.250 FLC (code sizing)

Use the running amps for metering and energy calculations. For branch-circuit sizing, AC motors use the NEC Table 430.248 / 430.250 full-load current under NEC 430.6(A)(1); DC motors use the motor nameplate full-load current under NEC 430.6(A)(3), with Table 430.247 as the reference. Three-phase current is shown per line on a balanced circuit.

Formula (three-phase)

I_(A) = (HP × 746) ÷ (√3 × V_L-L × Eff × PF)

(20 × 746) ÷ (√3 × 575 × 0.85 × 0.85) = 14,920 ÷ 719.56 = 20.73 A per line

Convert HP to watts: 20 × 746 = 14,920W
Denominator: √3 × 575 × 0.85 × 0.85 = 1.73 × 575 × 0.85 × 0.85 = 719.56
Result: 14,920 ÷ 719.56 = 20.73 amps per line

Three-phase current is per line on a balanced circuit. Voltage is line-to-line; the √3 factor comes from the three-phase vector geometry, not a round-trip doubling.

NEC Reference Values

This section lists the Code reference numbers a motor branch circuit is sized from. Final conductor, breaker, disconnect, and overload selection is an install decision a licensed electrician makes against the motor nameplate, the actual install conditions, and the applicable NEC articles, not a decision a conversion page can make for you.

NEC Sizing Base: NEC Table 430.250 FLC

Per NEC 430.6(A)(1), motor branch-circuit conductors, switches, and overcurrent protection are sized from the values in Table 430.248 (single-phase) or Table 430.250 (three-phase), not from the motor nameplate and not from a calculated full-load amps. For a 20 HP three-phase motor at 575V, the table value is 22 A (the 460V column covers 440-480V under 430.6(A)(1)).

The 20.73 A shown in the hero is the calculated running current at 85% efficiency and PF 0.85, per line on a balanced three-phase circuit. This is a conversion from the nameplate horsepower under those assumptions, not a measured value; a real meter reading depends on the motor's actual efficiency, loading, temperature, and design. Use this figure for energy and metering estimates, and use 22 A as the reference FLC when an electrician walks through NEC 430 against the nameplate.

NEC 430.22 Conductor Rule (reference formula)

NEC 430.22 requires motor branch-circuit conductor ampacity of at least 125% of the Code sizing FLC. As a reference calculation against the NEC Table 430.250 value: 22 × 1.25 = 27.5 A. The selected conductor is taken from NEC Table 310.16 at the applicable termination temperature column, with ambient, bundling, and cable-type adjustments applied by the installer. Motor branch-circuit conductors are exempt from the 240.4(D) small-conductor rule via 240.4(G).

NEC 430.52 Overcurrent Protection (code caps)

NEC Table 430.52(C)(1) gives the maximum rating for motor short-circuit and ground-fault protection as a percentage of the Code sizing FLC. The percentage depends on the device type:

Device Type	Maximum % of Table FLC (430.52(C)(1))
Non-time-delay fuse	300%
Dual-element (time-delay) fuse	175%
Inverse-time circuit breaker	250%
Instantaneous-trip circuit breaker	800%

These percentages are maximum caps, not install picks. A real circuit applies the percentage against the Code sizing FLC for the specific device type, rounds up to a standard size per 430.52(C)(1)(a), and is verified against the motor nameplate and the install conditions by the installer. The elevated percentages exist so short-circuit protection does not nuisance-trip on locked-rotor startup inrush.

Locked Rotor (Startup) Current

During the first 2-5 seconds of startup, a squirrel-cage induction motor typically draws 5 to 7 times the NEC Table 430.250 FLC of 22 A (roughly 110 to 154 A). This is why the 430.52(C)(1) percentages above are so much higher than running current: the short-circuit/ground-fault protective device has to ride through locked-rotor inrush without tripping. Actual LRA is set by the motor's NEMA code letter on the nameplate and should be checked there for a real install.

Current	Amps	Duration
Calculated running current (meter)	20.73 A per line	Continuous at full load
NEC Table 430.250 FLC (Code reference)	22 A	Sizing base, not metered
Locked rotor (typical, 5-7×)	110-154 A	2-5 seconds

Operating Cost

Motor mechanical output is 14,920 W (20 HP × 746). Electrical input at the terminals is higher because no motor is 100% efficient: 14,920 ÷ 0.85 = 17,552.94 W. At $0.17/kWh, running cost is $2.98/hour or $716.16/month at 8 hours/day. Full breakdown at 17,552.94 W.

Amps by Motor Efficiency (three-phase)

Motor efficiency directly affects amp draw. A more efficient motor draws less current for the same HP output. Values below are the calculated three-phase running current at 575V per line and PF 0.85:

Efficiency	Amps at 575V (per line)	Watts Consumed	Waste Heat
75%	23.5 A	19,893.33 W	4,973.33 W
80%	22.03 A	18,650 W	3,730 W
85%	20.73 A	17,552.94 W	2,632.94 W
90%	19.58 A	16,577.78 W	1,657.78 W
95%	18.55 A	15,705.26 W	785.26 W

Other HP Values at 575V (three-phase)

Running current is the calculated three-phase draw per line at 85% efficiency and 0.85 PF (a conversion from HP under those assumptions, not a measured value). NEC Table FLC is the value from NEC Table 430.250 used for branch-circuit conductor and OCP sizing under NEC 430.6(A)(1). LRA is estimated at 5-7× the NEC table FLC; rows outside the table show n/a because there is no code-authoritative LRA basis for that HP/voltage/phase combination. Row links open each result page in three-phase mode.

HP	Running Amps (calculated)	NEC Table 430.250 FLC	LRA Estimate (5-7× FLC)
1/8 HP	0.1296 A	off-table	n/a
1/6 HP	0.1728 A	off-table	n/a
1/4 HP	0.2592 A	off-table	n/a
1/3 HP	0.3455 A	off-table	n/a
1/2 HP	0.5184 A	0.9 A	4.5-6.3 A
3/4 HP	0.7776 A	1.3 A	6.5-9.1 A
1 HP	1.04 A	1.7 A	8.5-11.9 A
1.5 HP	1.56 A	2.4 A	12-16.8 A
2 HP	2.07 A	2.7 A	13.5-18.9 A
3 HP	3.11 A	3.9 A	19.5-27.3 A
5 HP	5.18 A	6.1 A	30.5-42.7 A
7.5 HP	7.78 A	9 A	45-63 A
10 HP	10.37 A	11 A	55-77 A
15 HP	15.55 A	17 A	85-119 A
20 HP	20.73 A	22 A	110-154 A
25 HP	25.92 A	27 A	135-189 A
30 HP	31.1 A	32 A	160-224 A
40 HP	41.47 A	41 A	205-287 A
50 HP	51.84 A	52 A	260-364 A
75 HP	77.76 A	77 A	385-539 A

Same HP, Other Voltages (three-phase)

speed 20 HP at 120V = 99.35A per line speed 20 HP at 208V = 57.32A per line speed 20 HP at 220V = 54.19A per line speed 20 HP at 230V = 51.84A per line speed 20 HP at 240V = 49.68A per line speed 20 HP at 400V = 29.81A per line speed 20 HP at 480V = 24.84A per line

Related Calculations

bolt 17,553W electrical input to amps at 575V payments 17,553W running cost cable Wire size for 27.5A at 575V (3Φ) (NEC 430.22 minimum: 125% of 22A NEC Table 430.250 FLC)

Frequently Asked Questions

How many amps does a 20 HP three-phase motor draw at 575V?expand_more

At the terminals, a 20 HP three-phase motor at 575V draws about 20.73 amps per line at 85% efficiency and 0.85 power factor. For NEC branch-circuit sizing use the NEC Table 430.250 full-load current instead: 22 A.

How does NEC 430.22 size conductors for a 20 HP three-phase motor?expand_more

NEC 430.22 requires motor branch-circuit conductors to have an ampacity of at least 125% of the NEC Table 430.250 FLC (not the motor nameplate, and not the calculated running current). As a reference formula against the table value: 22 A × 1.25. The specific conductor is picked from NEC Table 310.16 at the applicable termination temperature column, with ambient and bundling adjustments applied by the installer. Motor branches are exempt from the 240.4(D) small-conductor rule via 240.4(G).

Does motor efficiency affect amp draw?expand_more

Yes. A 20 HP three-phase motor at 90% efficiency draws 19.58 A at the terminals versus 23.5 A at 75% efficiency. Higher efficiency means lower running amps and lower electrical input wattage for the same mechanical output.

What branch-circuit context does a 20 HP three-phase motor at 575V typically sit in?expand_more

Three-phase motor branches are not served from residential receptacles in the US. Three-phase power is distributed to commercial and industrial services, and a 20 HP three-phase motor at 575V needs a dedicated three-phase branch circuit sized by an electrician per NEC 430.22 (conductors) and 430.52(C)(1) (short-circuit / ground-fault protection), against the motor nameplate and install conditions.

What is power factor for motors?expand_more

Motors typically run at PF 0.80-0.85 at full load. At no load, PF drops to 0.30-0.40. Low PF means the wire and breaker carry extra reactive current that does no useful mechanical work, which is why NEC motor sizing uses table FLC (which already accounts for typical PF) rather than a simple watts/volts calculation.

This calculator provides estimates for reference purposes only. Always consult a licensed electrician and verify compliance with the National Electrical Code (NEC) and local electrical codes before performing any electrical work.