Motor Current Calculator - Single & Three-Phase Motor Amps / FLA
Motor current, or Full Load Amps (FLA), is computed from motor power, voltage, efficiency, and power factor. For single-phase: I = P / (V × η × PF). For three-phase: I = P / (sqrt(3) × V × η × PF). The calculator below handles both, with HP and kW input, and common voltage presets.
Use nameplate FLA for final overload settings. This calculator is for estimating motor amperage, branch sizing, and sanity checks. Code work still belongs with the adopted code book and the authority having jurisdiction.
Quick Reference - Common Motor FLA Values
These are NEC-style table currents from Article 430 tables, not manufacturer-specific nameplate currents. This is the table electricians actually look up when they need a code design current instead of a lab-perfect formula.
| Motor size | Expected FLA | Field note |
|---|---|---|
| 1/4 HP | 5.8 A at 115 V, 2.9 A at 230 V | Use NEC 430.248 for single-phase |
| 1/2 HP | 9.8 A at 115 V, 4.9 A at 230 V | Small shop motors and fractional HP pumps |
| 1 HP | 16 A at 115 V, 8 A at 230 V; 4.2 A at 230 V 3-phase, 2.1 A at 460 V | Common small machines |
| 2 HP | 24 A at 115 V, 12 A at 230 V; 6.8 A at 230 V 3-phase, 3.4 A at 460 V | Use nameplate for capacitor-start motors |
| 3 HP | 34 A at 115 V, 17 A at 230 V; 9.6 A at 230 V 3-phase, 4.8 A at 460 V | Often the practical edge of small single-phase |
| 5 HP | 56 A at 115 V, 28 A at 230 V; 15.2 A at 230 V 3-phase, 7.6 A at 460 V | Single-phase 5 HP needs real wiring |
| 10 HP | 100 A at 115 V, 50 A at 230 V; 28 A at 230 V 3-phase, 14 A at 460 V | Usually three-phase in commercial work |
| 25 HP | 68 A at 230 V 3-phase, 34 A at 460 V, 27 A at 575 V | Starter and overload settings matter |
| 50 HP | 130 A at 230 V 3-phase, 65 A at 460 V, 52 A at 575 V | Check feeder and voltage drop |
| 100 HP | 248 A at 230 V 3-phase, 124 A at 460 V, 99 A at 575 V | NEC 430.250 table value |
| 200 HP | 480 A at 230 V 3-phase, 240 A at 460 V, 192 A at 575 V | Engineering review, not a quick web-only call |
HP to kW Conversion
One mechanical horsepower is 745.7 W, or 0.7457 kW. One kW equals 1.341 HP. Common conversions: 1 HP = 0.746 kW, 2 HP = 1.49 kW, 5 HP = 3.73 kW, 10 HP = 7.46 kW, 20 HP = 14.9 kW, and 50 HP = 37.3 kW.
There is one catch that trips people up. The HP rating is shaft output power. The motor pulls more electrical input power than that because the motor has losses. That is why efficiency sits in the denominator of the current formula. A 10 HP motor does not consume only 7.46 kW at the terminals unless it is 100% efficient, and no real motor is.
Efficiency and Power Factor - What To Use
Efficiency varies with motor size. Small motors under 1 HP often sit around 70% to 82%. Motors from 1 to 10 HP usually run around 82% to 90%. A 10 to 100 HP industrial induction motor is commonly 90% to 94%. Large motors over 100 HP may be 94% to 96%. Premium efficiency motors, IE3 and IE4 under IEC language or NEMA Premium in the US, can push a few percentage points higher across the board.
Power factor is a different animal. Standard induction motors often run 0.75 to 0.88 at rated load. Loaded motors have better PF than lightly loaded motors. A motor spinning with very little mechanical load can have ugly power factor even if the current does not look frightening. Synchronous motors and permanent magnet synchronous motors can sit around 0.95 to 1.0.
If you have the motor nameplate, use the nameplate values. If you do not, 0.85 efficiency and 0.85 power factor are sane defaults for a quick motor amp calculator. They will usually land within 10% of reality for a general industrial motor. For final breaker, cable, starter, or generator sizing, stop guessing and get the actual data sheet.
Running Current vs Starting Current vs FLA
FLA, full load amps, is the maximum current the motor draws at rated load, rated voltage, and rated frequency. Running current is the actual current at the load you are running. It may be less than FLA if the motor is lightly loaded. It may climb above FLA if the driven load is binding, overloaded, or the voltage is poor.
Starting current is another matter. Locked rotor amps, LRA, can be 5 to 7 times FLA for a direct-on-line start, usually for 1 to 10 seconds. Inrush current is the magnetic hit at energization and can be 10 to 15 times FLA for milliseconds. The calculator gives FLA. For circuit conductor sizing, use FLA times 1.25 as a common branch-circuit starting point. For starting current, look at starters: DOL, star-delta, soft starter, or VFD.
Wire Sizing and Breaker Selection
NEC motor work is not the same as sizing a receptacle circuit. NEC 430.6 points you to motor full-load current tables for many sizing calculations, and NEC 430.22 commonly sizes branch-circuit conductors at 125% of FLA for continuous-duty motors. So a 5 HP motor with an FLA around 28 A needs wire ampacity of at least 35 A. In real installations that often means 8 AWG copper THHN once terminal ratings, temperature, grouping, and local practice are considered.
Common examples: a 1 HP motor around 8 A can land on 14 AWG and a 15 A breaker in a simple case. A 5 HP motor around 28 A often pushes to 8 AWG and a 40 A breaker. A 10 HP motor around 50 A may use 6 AWG and a 70 A breaker. A 25 HP motor around 120 A can push into 2/0 AWG territory with a large breaker, depending on the exact code table and installation.
Overload protection in the motor starter is not the same as the upstream breaker. Overloads protect the motor from sustained overcurrent and are often set around 115% to 125% of FLA under NEC 430.32. The breaker protects against short circuits and ground faults. Mixing those two up is how motors nuisance trip or burn up.
Single-Phase vs Three-Phase Motors
Single-phase motors show up in residential and light commercial work, usually up to about 10 HP. They need a start capacitor, split-phase mechanism, or another starting method because single-phase power does not create a naturally rotating field. They are less efficient and can have higher starting current relative to running current.
Three-phase motors are the industrial default. They are self-starting, smoother, more efficient, and available from fractional HP to thousands of HP. Most factories worldwide run three-phase at 380, 400, 415, 440, or 480 V. For the same HP and voltage, three-phase draws about 58%, or 1/sqrt(3), of the current that single-phase would draw before motor differences are considered. That is why serious motor loads almost always end up on three-phase power.
Indian Motor Standards
Indian industrial motors typically run at 415 V three-phase on LT supply. You will also see 440 V on motor nameplates and older plant equipment. Very large motors may move to HT supplies such as 11 kV. Residential and small commercial single-phase motors commonly run at 230 V.
Indian motor nameplates usually quote kW, voltage, frequency, full-load current, power factor, and efficiency. BIS IS 12615 covers line-operated three-phase AC motors with IE efficiency classes, broadly matching the IE1, IE2, IE3, and IE4 language engineers see in IEC work. Motors above 1.1 kW also sit inside India's efficiency and labelling framework, so nameplate efficiency is not decorative. Standard induction motors often show PF around 0.8 to 0.85. If the nameplate gives FLA, use it. If it does not, use the calculator as an estimate and leave yourself room for starting current.
Common Mistakes
- Using HP nameplate as electrical input power. HP is shaft output; efficiency losses make input power higher.
- Forgetting power factor, especially on single-phase motor ampere calculation.
- Using sqrt(3) in a single-phase formula.
- Sizing breakers at exactly FLA instead of applying the required sizing factor.
- Ignoring starting current when selecting starters, contactors, generators, and transformers.
Motor Amperage Calculator FAQ
How do I calculate motor current?
Convert the motor output power to watts first. For horsepower, multiply HP by 745.7. For a single-phase motor, current equals watts divided by voltage, efficiency, and power factor. For a three-phase motor, divide by sqrt(3) times line voltage, efficiency, and power factor. Use the motor nameplate when you have it. A calculator gets you close, but the nameplate FLA is the number inspectors and motor starters usually care about.
What is the difference between FLA and running current?
FLA means full load amps. It is the current the motor is expected to draw at rated load, rated voltage, and rated frequency. Running current is the current the motor is actually drawing under the load you put on it today. A pump throttled back may run below FLA. A conveyor jam can run above it. FLA is the design number. Running current is what the clamp meter tells you on site.
How many amps does a 5 HP motor draw?
A calculated 5 HP single-phase motor at 230 V, 85% efficiency, and 0.85 power factor draws about 22.4 A. NEC table values are often higher because they are conservative design currents. NEC Table 430.248 lists a 5 HP single-phase motor at 230 V around 28 A. If you are sizing conductors or protection under NEC rules, use the code table or nameplate rule that applies to the job.
What is the formula for three-phase motor current?
Three-phase motor current is I = P / (sqrt(3) x V x efficiency x power factor). P is input-equivalent shaft power in watts after converting from HP or kW. V is line-to-line voltage. The sqrt(3) term belongs in three-phase because the phases are separated by 120 electrical degrees. Do not use sqrt(3) for single-phase. That mistake can undersize a single-phase circuit badly.
What efficiency should I use if I do not know the motor exact value?
Use 0.85 for a general-purpose motor when you do not have a nameplate. Small motors under 1 HP may be closer to 0.70 to 0.82. Motors from 1 to 10 HP often land around 0.82 to 0.90. Larger industrial motors are commonly 0.90 and up. Premium efficiency motors can run higher. If you are buying gear, get the manufacturer data sheet. If you are troubleshooting, read the nameplate.
How do I size a breaker for a 10 HP motor?
Start with FLA, then apply the rule your jurisdiction uses. This calculator shows a simple 125% branch-circuit sizing number: breaker target = FLA x 1.25, rounded up to a standard breaker size. NEC motor short-circuit and ground-fault protection can allow different percentages depending on breaker type, motor type, and starting method. Overload protection in the starter is a separate setting, usually 115% to 125% of motor FLA.
What is locked rotor amps and why does it matter?
Locked rotor amps, or LRA, is the current drawn when the rotor is not turning and full voltage is applied. Direct-on-line induction motor starts can pull 5 to 7 times FLA for a few seconds. Magnetic inrush can be even higher for milliseconds. Breakers, contactors, starters, generators, and transformers need to tolerate that starting event. A circuit that looks fine at running current can still trip hard on start.
How do I convert HP to kW for motor calculations?
Mechanical horsepower converts at 1 HP = 745.7 W, or 0.7457 kW. So 5 HP is 3.73 kW and 10 HP is 7.46 kW. One kW equals 1.341 HP. Remember the HP rating is shaft output power. Electrical input power is higher because the motor has efficiency losses. That is why efficiency belongs in the denominator of the current formula.
What wire size do I need for a 25 HP motor?
It depends on voltage, phase, conductor material, insulation temperature, installation method, ambient correction, and local code. A common NEC-style example is a 25 HP motor with about 120 A FLA. Branch conductors at 125% need around 150 A ampacity, often pushing you into large copper conductors such as 2/0 AWG depending on the exact table and terminals. Do not size large motors from a web page alone.
Why does three-phase draw less current than single-phase for the same HP?
At the same voltage, three-phase power delivers more real power for the same line current because the three phases share the load. The formula includes sqrt(3), so the current is roughly 1/sqrt(3), or 58%, of the comparable single-phase current before efficiency and power factor differences. Three-phase motors also tend to run smoother, start better, and have better power factor. That is why factories use three-phase whenever they can.
Related Electrical Calculators
For nearby work, use the kW to Amps Calculator, Amps to Watts Calculator, Power Factor Calculator, Voltage Drop Calculator, Wire Gauge Calculator, Transformer Calculator, and Star-Delta Starter Calculator. More tools are in the Electrical Power Calculator hub.
Sources
- NFPA 70, National Electrical Code - Article 430 covers motors, motor circuits, and controllers.
- IEC 60034-1 rotating electrical machines - rating and performance reference for rotating machines.
- BIS IS 12615 - Indian efficiency classes and performance specification for line-operated three-phase AC motors.
- NEMA MG 1 Motors and Generators - selection, performance, safety, testing, and construction reference.
- IEEE 100 Authoritative Dictionary of IEEE Standards Terms - electrical terminology reference.