How Many Amps Does a 1/3 HP single-phase Motor Draw at 240V?

Q: What is 1/3 HP in watts?

1/3 HP equals 248.64 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 292.52 W.

A 1/3 HP single-phase motor at 240V draws approximately 1.43 amps during normal operation (85% efficiency, PF 0.85). Startup current can be 5-7 times the NEC Table 430.248 FLC (18-25.2 A) for a few seconds, which affects breaker and wire sizing.

Common applications for 1/3 HP motors: small pool pumps, light-commercial fans, booster pumps.

1/3 HP single-phase motor at 240V

1.43 Amps running

Calculated running current at the motor terminals at the assumed 85% efficiency and PF 0.85. 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 430.248 FLC (code sizing base)3.6 A

Conductor min ampacity (NEC 430.22, 125% of FLC)4.5 A

Electrical input (HP × 746 ÷ efficiency)292.52 W

Try: 1/4HP at 240V 1/6HP at 240V 1/2HP at 240V 1/8HP at 240V

Volts

Motor Type

Running Amps (meter)

1.43

NEC Table 430.248 FLC (code sizing)

3.6

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 (single-phase)

I_(A) = (HP × 746) ÷ (V × Eff × PF)

(1/3 × 746) ÷ (240 × 0.85 × 0.85) = 248.64 ÷ 173.4 = 1.43 A

Convert HP to watts: 1/3 × 746 = 248.64W
Denominator: 240 × 0.85 × 0.85 = 173.4
Result: 248.64 ÷ 173.4 = 1.43 amps

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 430.248 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 1/3 HP single-phase motor at 240V, the table value is 3.6 A (the 115V column covers 110-120V systems 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. 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 3.6 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 430.248 value: 3.6 × 1.25 = 4.5 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 430.248 FLC of 3.6 A (roughly 18 to 25.2 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	Continuous at full load
NEC Table 430.248 FLC (Code reference)	3.6 A	Sizing base, not metered
Locked rotor (typical, 5-7×)	18-25.2 A	2-5 seconds

Operating Cost

Motor mechanical output is 248.64 W (1/3 HP × 746). Electrical input at the terminals is higher because no motor is 100% efficient: 248.64 ÷ 0.85 = 292.52 W. At $0.17/kWh, running cost is $0.05/hour or $11.93/month at 8 hours/day. Full breakdown at 292.52 W.

Amps by Motor Efficiency (single-phase)

Motor efficiency directly affects amp draw. A more efficient motor draws less current for the same HP output. Values below are the calculated single-phase running current at 240V and PF 0.85:

Efficiency	Amps at 240V	Watts Consumed	Waste Heat
75%	1.63 A	331.52 W	82.88 W
80%	1.52 A	310.8 W	62.16 W
85%	1.43 A	292.52 W	43.88 W
90%	1.35 A	276.27 W	27.63 W
95%	1.28 A	261.73 W	13.09 W

Other HP Values at 240V (single-phase)

Running current is the calculated single-phase draw 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 430.248 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 single-phase mode.

HP	Running Amps (calculated)	NEC Table 430.248 FLC	LRA Estimate (5-7× FLC)
1/8 HP	0.5378 A	off-table	n/a
1/6 HP	0.7172 A	2.2 A	11-15.4 A
1/4 HP	1.08 A	2.9 A	14.5-20.3 A
1/3 HP	1.43 A	3.6 A	18-25.2 A
1/2 HP	2.15 A	4.9 A	24.5-34.3 A
3/4 HP	3.23 A	6.9 A	34.5-48.3 A
1 HP	4.3 A	8 A	40-56 A
1.5 HP	6.45 A	10 A	50-70 A
2 HP	8.6 A	12 A	60-84 A
3 HP	12.91 A	17 A	85-119 A
5 HP	21.51 A	28 A	140-196 A
7.5 HP	32.27 A	40 A	200-280 A
10 HP	43.02 A	50 A	250-350 A
15 HP	64.53 A	off-table	n/a
20 HP	86.04 A	off-table	n/a
25 HP	107.55 A	off-table	n/a
30 HP	129.07 A	off-table	n/a
40 HP	172.09 A	off-table	n/a
50 HP	215.11 A	off-table	n/a
75 HP	322.66 A	off-table	n/a

Same HP, Other Voltages (single-phase)

speed 1/3 HP at 120V = 2.87A speed 1/3 HP at 208V = 1.65A speed 1/3 HP at 220V = 1.56A speed 1/3 HP at 230V = 1.5A speed 1/3 HP at 400V = 0.8604A speed 1/3 HP at 480V = 0.717A

Related Calculations

bolt 293W electrical input to amps at 240V payments 293W running cost cable Wire size for 4.5A at 240V (NEC 430.22 minimum: 125% of 3.6A NEC Table 430.248 FLC)

Frequently Asked Questions

How many amps does a 1/3 HP single-phase motor draw at 240V?expand_more

At the terminals, a 1/3 HP single-phase motor at 240V draws about 1.43 amps at 85% efficiency and 0.85 power factor. For NEC branch-circuit sizing use the NEC Table 430.248 full-load current instead: 3.6 A.

What branch-circuit context does a 1/3 HP single-phase motor at 240V typically sit in?expand_more

At 240V single-phase, motor branches are dedicated circuits, not general-purpose receptacles. Common NEMA configurations at this voltage are the 6-series (6-15, 6-20, 6-30, 6-50) and the 14-series (14-30, 14-50) when a neutral is needed, sized to the motor branch-circuit OCP the installer picks under NEC 430.52(C)(1). The NEC Table 430.248 FLC of 3.6 A is the reference number an electrician feeds into 430.22 and 430.52, not a receptacle pick on its own. Verify against the nameplate and local code.

Does motor efficiency affect amp draw?expand_more

Yes. A 1/3 HP single-phase motor at 90% efficiency draws 1.35 A at the terminals versus 1.63 A at 75% efficiency. Higher efficiency means lower running amps and lower electrical input wattage for the same mechanical output.

How much does a 1/3 HP motor cost to run?expand_more

Operating cost is based on electrical input, not mechanical HP output. At 85% efficiency, a 1/3 HP motor draws about 292.52 W at the terminals. At $0.17/kWh (US residential average, last reviewed April 2026), that is $0.05/hour or $11.93/month at 8 hours/day.

What is 1/3 HP in watts?expand_more

1/3 HP equals 248.64 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 292.52 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.