SettingsSettings for this calculationUS
The rated output on the nameplate — the shaft power, in the unit chosen below.
Motors are rated by what they deliver at the shaft, not what they draw. North American motors are rated in horsepower, in steps such as 1/3, 1/2, 3/4, 1, 1 1/2 and 2; IEC motors in kilowatts — 0.37, 0.55, 0.75, 1.1, 1.5, 2.2. Enter fractions of a horsepower as decimals: 3/4 hp is 0.75.
The unit the nameplate states the rating in.
Horsepower here is mechanical horsepower, 745.7 W. A continental European plate stating PS or CV means metric horsepower, about 1.4% smaller; convert it to kW first rather than entering it as hp.
The rated voltage the motor runs at, as connected.
North American single-phase motors are rated 115 V and 230 V for 120 V and 240 V circuits, and many are dual-voltage: the same motor draws twice the current connected for 115 V as for 230 V. UK, European and Australian single-phase motors are rated 230 V. Choose another voltage to type one in.
From the nameplate or the maker's data sheet; it opens on a real motor's figure — 68.5% in North America, 70% elsewhere.
Single-phase motors are markedly less efficient than three-phase ones of the same output: WEG's US general-purpose single-phase range runs from 50% at 1/4 hp to 80% at 3 hp. The page opens on a real motor so that the opening answer is one a data sheet prints: a North American reader on WEG's US 3/4 hp, 1,745 rpm motor at 68.5%, listed at 5.3 A on 230 V, and every other reader on WEG's 0.75 kW, 1,435 rpm, 50 Hz W22 with start and run capacitors at 70.0%, listed at 5.06 A on 230 V. Replace it with your own motor's figure.
From the nameplate or the maker's data sheet; it opens on the same motor's figure — 0.67 in North America, 0.92 elsewhere.
The motor's design moves it more than its size does. In WEG's US table the four-pole motors (about 1,750 rpm) from 1/4 to 1 hp sit between 0.59 and 0.68, while its two-pole motors (about 3,500 rpm) of 3/4 hp and up sit between 0.91 and 0.97. WEG's European W22 single-phase motors, built with start and run capacitors, list between 0.75 and 0.99 at full load — 0.92 for the 0.75 kW four-pole motor. A low power factor is why a small motor of the first kind draws far more current than its shaft output divided by the voltage suggests.
Full-load running current
5.3 A
Full-load running current = rated output ÷ (volts × efficiency × power factor). Worked from WEG's own efficiency and power factor, it comes within 2% of the current WEG lists for 13 of its 15 general-purpose US motors and within 5% for the other two, the smallest. It is only as good as the efficiency and power factor entered, and it is not the figure a US circuit is sized from: the National Electrical Code's own single-phase motor table governs that for most motors, and the nameplate for some — a sealed (hermetic) refrigeration or air-conditioning compressor among them — and the table is not reproduced here.
- Rated output at the shaft
- 0.56 kW
- Rated output in watts
- 559.27 W
- Electrical input, output ÷ efficiency
- 0.82 kW
- Apparent power drawn, volts × amps
- 1.22 kVA
- Voltage used
- 230 V
They open the calculator with your figures already in it
Single-Phase Motor Full-Load Current Calculator (Amps from hp or kW): 5.3 A — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Single-phase motor input current from rated output: I = P(out) ÷ (V × efficiency × power factor), the electrical input being the shaft output divided by the efficiency, and single-phase current that input divided by volts times power factor
- WEG Rolled Steel General Purpose 1-phase Motors, performance data (AutomationDirect catalogue page tMTR-54): full-load amps, efficiency and power factor per motor — 3/4 hp 1,745 rpm .7518ES1B56C-S at 10.6/5.86-5.3 A (115/208-230 V), 68.5% and 0.67, the figures this page opens on; 1 hp 1,745 rpm 00118ES1B56C-S at 13.6/7.52-6.8 A, 71.0% and 0.68; 1 hp 3,500 rpm 00136ES1B56C-S at 10.1/5.64-5.06 A, 70.0% and 0.92
- Mechanical horsepower is 550 ft·lbf/s = 745.699872 W exactly
- NFPA 70-1968, the National Electrical Code, Section 430-6(a) (reprinted with permission by the US Mine Safety and Health Administration): where a motor's current is used to size conductors, switches and branch-circuit protection, the code's full-load current tables are used instead of the nameplate current, and running overload protection is based on the nameplate. Its exceptions in the same section: 430-6(b), a sealed (hermetic-type) motor-compressor takes the full-load current marked on the nameplate of the equipment it is in, for the branch-circuit conductors, the controller and the protection; 430-6(c), a torque motor its nameplate locked-rotor current; and multispeed motors their own rules. Current editions number it 430.6(A)(1) and the single-phase table 430.248, which this page does not reproduce
- WEG W22 Single-Phase Electric Motor, Commercial Catalogue, European Market (Cod. 50069268, Rev. 00, 01/2017), page 5, Electrical Data, W22 Single-Phase CSR (starting and run capacitor), IV pole, 50 Hz: 0.75 kW (1 HP), frame 80, 1,435 rpm, efficiency 70.0% and power factor 0.92 at full load, 5.06 A at 230 V — the motor a UK, Australian or international reader opens on; II pole 0.75 kW, 2,915 rpm, 75.0% and 0.91, 4.78 A
Inputs used
- Motor rating
- 0.75
- Rated in
- Horsepower (hp)
- Voltage the motor is connected for
- 230 V (UK, European and Australian supply; North American motor rating on 240 V)
- Voltage you are working at (V)
- 220
- Efficiency at full load (%)
- 68.5
- Power factor at full load
- 0.67
Intermediate steps
- Rated output at the shaft
- 0.56 kW
- Rated output in watts
- 559.27 W
- Electrical input, output ÷ efficiency
- 0.82 kW
- Apparent power drawn, volts × amps
- 1.22 kVA
- Voltage used
- 230 V
Confidence note: Full-load running current = rated output ÷ (volts × efficiency × power factor). Worked from WEG's own efficiency and power factor, it comes within 2% of the current WEG lists for 13 of its 15 general-purpose US motors and within 5% for the other two, the smallest. It is only as good as the efficiency and power factor entered, and it is not the figure a US circuit is sized from: the National Electrical Code's own single-phase motor table governs that for most motors, and the nameplate for some — a sealed (hermetic) refrigeration or air-conditioning compressor among them — and the table is not reproduced here.
What this calculation does not cover
- This is the RUNNING current at rated load, worked from the efficiency and power factor you enter. A single-phase motor starting across the line draws several times this for the first moments — the nameplate's locked-rotor figure or code letter says how much — and that inrush is what decides how a starter, a breaker's trip curve or a generator copes with it.
- Efficiency and power factor are full-load figures, and a single-phase motor's change sharply with load. Lightly loaded, its power factor falls and the current stays well above the share of rated output it is delivering, so a clamp reading on an oversized motor will not scale with the load.
- In the United States, motor branch-circuit conductors and short-circuit protection are sized from the National Electrical Code's own full-load current table for single-phase motors, Table 430.248, not from this calculation or the nameplate — except where the code sends a motor to its nameplate instead. The 1968 text this page cites already does so for a sealed (hermetic-type) motor-compressor, such as the compressor in refrigeration or air-conditioning equipment, using the current marked on that equipment's nameplate (430-6(b)), and gives torque and multispeed motors rules of their own; the edition in force sets the list, and the electrician applies it. The table is not reproduced here, and its figures are not the ones this page prints. Overload protection, by contrast, follows the nameplate current.
- Nothing here sizes a cable, a breaker, a fuse, an overload or a starter. In the UK that is BS 7671 and the motor maker's instructions, in Australia AS/NZS 3000, and in North America NEC Article 430 or Canada's own electrical code — each applied by the electrician who designs and certifies the circuit.
- The voltage is the motor's rated voltage, assumed to arrive at its terminals. On a long run the voltage at the motor is lower, and a motor asked for the same torque at a lower voltage draws more current; check the run's voltage drop separately.
- A dual-voltage motor draws twice the current when connected for the lower voltage: the same 3/4 hp motor that draws 5.3 A on 230 V draws about 10.6 A on 115 V. Enter the voltage it is actually connected for.
Add the equipment this sizes
This result is a specification — 5.3 A — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations5
- Single-phase motor input current from rated output: I = P(out) ÷ (V × efficiency × power factor), the electrical input being the shaft output divided by the efficiency, and single-phase current that input divided by volts times power factor
- WEG Rolled Steel General Purpose 1-phase Motors, performance data (AutomationDirect catalogue page tMTR-54): full-load amps, efficiency and power factor per motor — 3/4 hp 1,745 rpm .7518ES1B56C-S at 10.6/5.86-5.3 A (115/208-230 V), 68.5% and 0.67, the figures this page opens on; 1 hp 1,745 rpm 00118ES1B56C-S at 13.6/7.52-6.8 A, 71.0% and 0.68; 1 hp 3,500 rpm 00136ES1B56C-S at 10.1/5.64-5.06 A, 70.0% and 0.92
- Mechanical horsepower is 550 ft·lbf/s = 745.699872 W exactly
- NFPA 70-1968, the National Electrical Code, Section 430-6(a) (reprinted with permission by the US Mine Safety and Health Administration): where a motor's current is used to size conductors, switches and branch-circuit protection, the code's full-load current tables are used instead of the nameplate current, and running overload protection is based on the nameplate. Its exceptions in the same section: 430-6(b), a sealed (hermetic-type) motor-compressor takes the full-load current marked on the nameplate of the equipment it is in, for the branch-circuit conductors, the controller and the protection; 430-6(c), a torque motor its nameplate locked-rotor current; and multispeed motors their own rules. Current editions number it 430.6(A)(1) and the single-phase table 430.248, which this page does not reproduce
- WEG W22 Single-Phase Electric Motor, Commercial Catalogue, European Market (Cod. 50069268, Rev. 00, 01/2017), page 5, Electrical Data, W22 Single-Phase CSR (starting and run capacitor), IV pole, 50 Hz: 0.75 kW (1 HP), frame 80, 1,435 rpm, efficiency 70.0% and power factor 0.92 at full load, 5.06 A at 230 V — the motor a UK, Australian or international reader opens on; II pole 0.75 kW, 2,915 rpm, 75.0% and 0.91, 4.78 A
Which documents these citations point at
- National Electrical Code (NFPA 70) (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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