Single-Phase vs Three-Phase FLA Formulas
The core relationship is Power = Voltage x Current x Efficiency x Power Factor. Rearranging for current gives you FLA.
Single-phase: FLA = (HP x 746) / (V x Efficiency x Power Factor)
Three-phase: FLA = (HP x 746) / (V x 1.732 x Efficiency x Power Factor)
The 746 converts horsepower to watts. The 1.732 (square root of 3) accounts for the three-phase power advantage — three-phase motors draw less current per line than single-phase motors of the same power because the current is shared across three conductors.
A 5HP single-phase motor at 230V with 85% efficiency and 0.85 power factor draws about 22.5A. The same motor in three-phase draws about 13A per line — roughly 42% less current per wire, which means smaller wire gauges and cheaper installation.
Power factor matters more than most people expect. A motor with 0.75 power factor draws 13% more current than one at 0.85, even at identical horsepower and efficiency. That extra current generates heat in wiring without doing useful work. If you are sizing cables for a motor with a poor power factor, you need to account for that higher current or risk overheating.
Calculator Results vs NEC Table Values
This calculator gives you the theoretical FLA based on the motor's electrical characteristics. For wire sizing, breaker selection, and code compliance in the US, always cross-reference NEC Table 430.248 (single-phase) or 430.250 (three-phase). Those tables list standardised FLA values that may differ from your motor's actual nameplate amps.
NEC tables assume typical efficiency and power factor for each motor size and voltage. Your specific motor might draw more or less depending on its design, age, and loading. A 10HP three-phase motor at 230V has a NEC table FLA of 28A, but an older motor with poor efficiency could draw 32A or more.
Use the calculator result for quick estimation and load planning. Use NEC tables for final design decisions on a permitted project. The NEC tables exist specifically because nameplate data is not always available or accurate — especially on older equipment where labels have faded or been painted over.
Wire sizing from FLA follows NEC Article 430: conductors must handle at least 125% of the motor FLA. For a motor with a calculator result of 22.5A, you need conductors rated for at least 28.1A. That puts you in 10 AWG copper territory for most insulation types. If you are pulling a long run to a shop motor, voltage drop becomes the limiting factor — use our wire distance calculator to check the drop before selecting your final gauge.
For critical or permitted installations, verify your numbers against NEC tables and consult a licensed electrician. This calculator is an estimation tool, not a substitute for professional design.
Worked Examples
Sizing Wire for a Workshop Table Saw
Context
You are installing a 3HP single-phase table saw in a detached workshop. The motor runs on 230V, has a nameplate efficiency of 86%, and a power factor of 0.84. The panel is 80 feet from the saw.
Calculation
FLA = (3 HP x 746) / (230 V x 0.86 x 0.84)
= 2,238 / 166.15
= 13.5 A
NEC requires conductors rated for 125% of FLA: 13.5 x 1.25 = 16.9 A minimum.
Interpretation
At 13.5A, a 20A breaker and 12 AWG wire handle the load with headroom. But the 80-foot run means voltage drop could be significant on a 230V circuit.
Takeaway
Before pulling wire, check the voltage drop over that 80-foot distance with our wire distance calculator. You may need to bump up to 10 AWG to keep the drop under 3%.
Comparing Three-Phase Motor Options for a Pump Station
Context
A facility needs a 15HP pump motor. You are comparing 208V and 480V three-phase options to decide which requires less expensive wiring. Both have 91% efficiency and 0.87 power factor.
Calculation
At 208V: FLA = (15 x 746) / (208 x 1.732 x 0.91 x 0.87) = 11,190 / 285.2 = 39.2 A
At 480V: FLA = (15 x 746) / (480 x 1.732 x 0.91 x 0.87) = 11,190 / 658.3 = 17.0 A
Interpretation
The 480V motor draws less than half the current of the 208V option. That means smaller wire gauge, smaller conduit, and a smaller breaker — all of which reduce material and labor costs.
Takeaway
Higher voltage pays for itself in wiring savings on any motor above 5HP. To estimate the energy cost difference, run both through our kWh calculator — the efficiency is the same, so the cost per hour is identical, but the installation is far cheaper at 480V.
Frequently Asked Questions
Glossary
Full Load Amps
The current a motor draws when operating at its rated horsepower and voltage under full mechanical load. FLA determines wire size, breaker rating, and overload protection settings for the motor circuit.
Power Factor
The ratio of real power (watts) to apparent power (volt-amps) in an AC circuit. Motors with low power factor draw more current than necessary to do the same work, increasing wire heating and energy waste.
Nameplate Data
The specifications stamped on a motor's identification plate by the manufacturer. Includes rated HP, voltage, FLA, efficiency, power factor, RPM, and frame size. Always the primary reference for circuit design.
Locked Rotor Current
The surge of current a motor draws at the instant of startup, before the rotor begins spinning. Typically 5-8 times the FLA. Determines breaker and starter sizing to prevent nuisance tripping.
Running a motor from a battery-powered inverter? Use our battery size for inverter calculator to find out how much battery capacity the motor needs. Try it now →
Motor FLA is a starting point for proper electrical design, not the finish line. Real-world factors — starting method (DOL vs soft start vs VFD), ambient temperature, altitude, and duty cycle — all affect the final wire and protection sizing. For critical or permitted installations, verify your numbers against NEC tables and consult a licensed electrician. These results are theoretical estimates based on the values you enter, not guarantees of actual motor performance.
Last updated:
Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..
Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.