How Many Amps Does a 0.5 HP three-phase Motor Draw at 208V?

Q: What is 0.5 HP in watts?

0.5 HP equals 373 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 438.82 W.

A 0.5 HP three-phase motor at 208V draws approximately 1.43 amps per line during normal operation (85% efficiency, PF 0.85). Startup current can be 5-7 times the NEC Table 430.250 FLC (12-16.8 A) for a few seconds, which affects breaker and wire sizing.

Common applications for 0.5 HP motors: commercial HVAC fans, rooftop-unit blowers, pump motors, three-phase industrial equipment.

0.5 HP three-phase motor at 208V

1.43 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)2.4 A

Conductor min ampacity (NEC 430.22, 125% of FLC)3 A

Electrical input (HP × 746 ÷ efficiency)438.82 W

Try: 1/2HP at 208V (3Φ) 1/3HP at 208V (3Φ) 1/4HP at 208V (3Φ) 3/4HP at 208V (3Φ)

Volts

Motor Type

Running Amps (per line)

1.43

NEC Table 430.250 FLC (code sizing)

2.4

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)

(0.5 × 746) ÷ (√3 × 208 × 0.85 × 0.85) = 373 ÷ 260.29 = 1.43 A per line

Convert HP to watts: 0.5 × 746 = 373W
Denominator: √3 × 208 × 0.85 × 0.85 = 1.73 × 208 × 0.85 × 0.85 = 260.29
Result: 373 ÷ 260.29 = 1.43 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 0.5 HP three-phase motor at 208V, the table value is 2.4 A (direct column match under 430.6(A)(1)).

The 1.43 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 2.4 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: 2.4 × 1.25 = 3 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 2.4 A (roughly 12 to 16.8 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)	1.43 A per line	Continuous at full load
NEC Table 430.250 FLC (Code reference)	2.4 A	Sizing base, not metered
Locked rotor (typical, 5-7×)	12-16.8 A	2-5 seconds

Operating Cost

Motor mechanical output is 373 W (0.5 HP × 746). Electrical input at the terminals is higher because no motor is 100% efficient: 373 ÷ 0.85 = 438.82 W. At $0.17/kWh, running cost is $0.07/hour or $17.90/month at 8 hours/day. Full breakdown at 438.82 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 208V per line and PF 0.85:

Efficiency	Amps at 208V (per line)	Watts Consumed	Waste Heat
75%	1.62 A	497.33 W	124.33 W
80%	1.52 A	466.25 W	93.25 W
85%	1.43 A	438.82 W	65.82 W
90%	1.35 A	414.44 W	41.44 W
95%	1.28 A	392.63 W	19.63 W

Other HP Values at 208V (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	0.3583 A	off-table	n/a
1/6 HP	0.4778 A	off-table	n/a
1/4 HP	0.7165 A	off-table	n/a
1/3 HP	0.9552 A	off-table	n/a
1/2 HP	1.43 A	2.4 A	12-16.8 A
3/4 HP	2.15 A	3.5 A	17.5-24.5 A
1 HP	2.87 A	4.6 A	23-32.2 A
1.5 HP	4.3 A	6.6 A	33-46.2 A
2 HP	5.73 A	7.5 A	37.5-52.5 A
3 HP	8.6 A	10.6 A	53-74.2 A
5 HP	14.33 A	16.7 A	83.5-116.9 A
7.5 HP	21.5 A	24.2 A	121-169.4 A
10 HP	28.66 A	30.8 A	154-215.6 A
15 HP	42.99 A	46.2 A	231-323.4 A
20 HP	57.32 A	59.4 A	297-415.8 A
25 HP	71.65 A	74.8 A	374-523.6 A
30 HP	85.98 A	88 A	440-616 A
40 HP	114.64 A	114 A	570-798 A
50 HP	143.3 A	143 A	715-1,001 A
75 HP	214.95 A	211 A	1,055-1,477 A

Same HP, Other Voltages (three-phase)

speed 0.5 HP at 120V = 2.48A per line speed 0.5 HP at 220V = 1.35A per line speed 0.5 HP at 230V = 1.3A per line speed 0.5 HP at 240V = 1.24A per line speed 0.5 HP at 400V = 0.7452A per line speed 0.5 HP at 480V = 0.621A per line

Related Calculations

bolt 439W electrical input to amps at 208V payments 439W running cost cable Wire size for 3A at 208V (3Φ) (NEC 430.22 minimum: 125% of 2.4A NEC Table 430.250 FLC)

Frequently Asked Questions

How many amps does a 0.5 HP three-phase motor draw at 208V?expand_more

At the terminals, a 0.5 HP three-phase motor at 208V draws about 1.43 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: 2.4 A.

Does motor efficiency affect amp draw?expand_more

Yes. A 0.5 HP three-phase motor at 90% efficiency draws 1.35 A at the terminals versus 1.62 A at 75% efficiency. Higher efficiency means lower running amps and lower electrical input wattage for the same mechanical output.

What is locked rotor amps for a 0.5 HP three-phase motor?expand_more

Locked-rotor (startup) current typically runs 5-7 times the NEC Table 430.250 FLC for a squirrel-cage induction motor: 12-16.8 A for 2-5 seconds. The exact value depends on the NEMA code letter stamped on the motor nameplate.

What branch-circuit context does a 0.5 HP three-phase motor at 208V 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 0.5 HP three-phase motor at 208V 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 0.5 HP in watts?expand_more

0.5 HP equals 373 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 438.82 W.

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.