How Many Amps Does a 50 HP three-phase Motor Draw at 400V?

Q: What is 50 HP in watts?

50 HP equals 37,300 watts of mechanical output (1 HP = 746 W). The electrical input at the terminals is higher because no motor is 100% efficient: at 85% efficiency the input is about 43,882.35 W.

A 50 HP three-phase motor at 400V draws approximately 74.52 amps per line during normal operation (85% efficiency, PF 0.85). This HP and voltage combination is outside NEC Table 430.250, so there is no code-authoritative LRA multiplier for branch-circuit sizing; refer to the motor nameplate for both running current and startup characteristics.

50 HP three-phase motor at 400V

74.52 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 FLCoff-table (see nameplate)

Electrical input (HP × 746 ÷ efficiency)43,882.35 W

Try: 50HP at 400V (3Φ) 40HP at 400V (3Φ) 30HP at 400V (3Φ) 25HP at 400V (3Φ)

Volts

Motor Type

Running Amps (per line)

74.52

NEC Table 430.250 FLC (code sizing)

off-table

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)

(50 × 746) ÷ (√3 × 400 × 0.85 × 0.85) = 37,300 ÷ 500.56 = 74.52 A per line

Convert HP to watts: 50 × 746 = 37,300W
Denominator: √3 × 400 × 0.85 × 0.85 = 1.73 × 400 × 0.85 × 0.85 = 500.56
Result: 37,300 ÷ 500.56 = 74.52 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.

Off-Table: No Code-Anchored Sizing

This combination is off-table because 400V is not a listed three-phase voltage in NEC Table 430.250. The table lists three-phase motors at 200V, 208V, 230V, 460V, and 575V, three-phase power is not typically distributed at 120V, 240V single-leg, 277V, or 400V in the United States. 50 HP is a listed horsepower, but not at this voltage.

Because there is no table FLC to anchor the NEC 430.22 conductor and 430.52(C)(1) OCP math, this page deliberately does not show branch-circuit sizing values for this variant. Multiplying the 74.52 A calculated running current by 125% or 250% would produce numbers that look authoritative but are not what the code requires.

What to do instead:

Use one of the voltages NEC Table 430.250 actually lists for 50 HP three-phase: 50 HP at 208V 3Φ, 50 HP at 230V 3Φ, 50 HP at 480V 3Φ, 50 HP at 575V 3Φ.
Pull the motor nameplate FLC and have a licensed electrician apply the 430.22 (conductor) and 430.52(C)(1) (OCP) rules against that number. NEC 430.6(A)(1) Exception permits using the next-higher listed HP where the motor rating is between table values; your inspector may also accept nameplate-based sizing for unusual HP ratings.

Operating Cost

Motor mechanical output is 37,300 W (50 HP × 746). Electrical input at the terminals is higher because no motor is 100% efficient: 37,300 ÷ 0.85 = 43,882.35 W. At $0.17/kWh, running cost is $7.46/hour or $1,790.40/month at 8 hours/day. Full breakdown at 43,882.35 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 400V per line and PF 0.85:

Efficiency	Amps at 400V (per line)	Watts Consumed	Waste Heat
75%	84.45 A	49,733.33 W	12,433.33 W
80%	79.17 A	46,625 W	9,325 W
85%	74.52 A	43,882.35 W	6,582.35 W
90%	70.38 A	41,444.44 W	4,144.44 W
95%	66.67 A	39,263.16 W	1,963.16 W

Other HP Values at 400V (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	0.1863 A	off-table	n/a
1/6 HP	0.2484 A	off-table	n/a
1/4 HP	0.3726 A	off-table	n/a
1/3 HP	0.4967 A	off-table	n/a
1/2 HP	0.7452 A	off-table	n/a
3/4 HP	1.12 A	off-table	n/a
1 HP	1.49 A	off-table	n/a
1.5 HP	2.24 A	off-table	n/a
2 HP	2.98 A	off-table	n/a
3 HP	4.47 A	off-table	n/a
5 HP	7.45 A	off-table	n/a
7.5 HP	11.18 A	off-table	n/a
10 HP	14.9 A	off-table	n/a
15 HP	22.35 A	off-table	n/a
20 HP	29.81 A	off-table	n/a
25 HP	37.26 A	off-table	n/a
30 HP	44.71 A	off-table	n/a
40 HP	59.61 A	off-table	n/a
50 HP	74.52 A	off-table	n/a
75 HP	111.77 A	off-table	n/a

Same HP, Other Voltages (three-phase)

speed 50 HP at 120V = 248.39A per line speed 50 HP at 208V = 143.3A per line speed 50 HP at 220V = 135.48A per line speed 50 HP at 230V = 129.59A per line speed 50 HP at 240V = 124.19A per line speed 50 HP at 480V = 62.1A per line

Related Calculations

bolt 43,882W electrical input to amps at 400V payments 43,882W running cost

Frequently Asked Questions

How many amps does a 50 HP three-phase motor draw at 400V?expand_more

At the terminals, a 50 HP three-phase motor at 400V draws about 74.52 amps per line at 85% efficiency and 0.85 power factor. This specific HP and voltage combination is outside NEC Table 430.250, so NEC branch-circuit sizing must come from the motor nameplate and a licensed electrician, not from the calculated value above.

What branch-circuit context does a 50 HP three-phase motor at 400V 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 50 HP three-phase motor at 400V 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 locked rotor amps for a 50 HP three-phase motor?expand_more

Locked-rotor startup current is typically 5-7 times the NEC table full-load current. Because this HP/voltage combination is not listed in NEC Table 430.250, there is no code-authoritative starting number for the multiplier; refer to the motor nameplate NEMA code letter for the actual LRA value.

What is 50 HP in watts?expand_more

50 HP equals 37,300 watts of mechanical output (1 HP = 746 W). The electrical input at the terminals is higher because no motor is 100% efficient: at 85% efficiency the input is about 43,882.35 W.

How does NEC 430.22 size conductors for a 50 HP three-phase motor?expand_more

NEC 430.22 sizes branch-circuit conductors at 125% of the table full-load current, but this HP/voltage combination is not listed in NEC Table 430.250. The correct input for the 430.22 formula here is the motor nameplate FLC, applied by a licensed electrician.

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.