Is 240V Single Phase or 3 Phase? What "2 Phase Power" Means
In almost every US home, 240V is single phase. That includes a garage or home shop wired from the house. The outlet has two hot wires, but together they deliver single phase power. A 3 phase motor won't start on single phase power, and it won't run properly on it. To run a 3 phase machine on household 240V, you need a phase converter or a VFD (variable frequency drive) built for single phase input.
240V 3 phase does exist, mostly in industrial buildings. That's why the number 240 alone can't tell you which kind of power you have.
People sometimes call household 240V "2 phase" because of its two hot wires. It's still single phase. If you searched for a "2 phase to 3 phase converter," the product you want goes by another name. At Built for Production, an online retailer of phase converters and VFDs, that's the name we use: a single phase to 3 phase converter.
Why are two hot wires on 240V still single phase?
Household 240V is single phase because both hot wires come from one transformer coil that delivers a single phase voltage. Continental Control Systems, a maker of power meters, says on its two-phase service page that both voltages are "derived from a single phase of a distribution transformer."
Inside the transformer, the neutral wire connects to a tap at the middle of the coil. Your two hot wires, also called hot legs, connect to the two ends. Its electrical service types page lists the readings: 120V from either hot leg to neutral, and 240V between the two hot legs.
AC voltage rises and falls in a wave. The two hot legs sit at opposite ends of the same coil, so the voltage on one rises while the voltage on the other falls. Each leg is 120V from the neutral, in opposite directions, so the two legs are 240V apart. The coil is split at its center tap, which is where the name split phase comes from.
A 3 phase motor needs three voltage waves that each peak at a different time, one third of a wave apart. Two legs from one coil can't make those three staggered waves.
Can a 3 phase machine run on a 240V outlet?
A 3 phase machine won't start on household 240V, and it won't run properly on it. A motor nameplate marked 3 PH needs three hot legs, and a 240V outlet in a house has two. The voltage on the plate, whether it says 220V, 230V or 208-230V, can match your outlet while the phase still doesn't. To run the machine, you need a single phase to 3 phase converter or a VFD with single phase input between the outlet and the machine.
When is 240V 3 phase?
240V is 3 phase when the building has a 240V delta service: three hot legs, each reading 240V to the other two. Continental Control Systems describes 240V delta as mostly an industrial service.
240V delta comes as 3-wire or 4-wire. The 3-wire version has only the three hot legs, with no neutral. The 4-wire version adds a neutral. Two of its legs read 120V to neutral, and the third reads about 208V. The meter maker says this 4-wire setup is found in older manufacturing buildings that run mostly 3 phase motors.
That third leg is called the high leg or wild leg. A 120V device connected to it would get about 208V. The National Electrical Code requires the high leg to be marked orange or identified some other clear way, according to Electrical Contractor magazine. That lets anyone working in the panel pick it out. A licensed electrician decides what connects to which leg.
Is 208V 3 phase?
208V comes from a 3 phase service called 208Y/120V. IAEI Magazine gives the readings: 120V from any hot leg to neutral, and 208V between any two hot legs. A regular 208V outlet connects to only two of those hot legs. Two legs give one voltage between them, so that outlet delivers single phase power.
Continental Control Systems calls 208Y/120V the most common service in North American commercial buildings. Larger facilities use 480V 3 phase, with 277V from each hot leg to neutral. If your shop is a unit in a commercial building, the building may have 3 phase at 208V or 480V. Your unit may still be wired for single phase only, so ask the landlord or the utility which service your unit gets.
A motor rated only for 230V is a poor match for a 208V building. The U.S. Department of Energy's motor tip sheet says motors are designed to run within 10% of their nameplate voltage. Its table pairs a 208V service with about 200V at the motor, because voltage drops along the wires. 200V is 13% below 230V, past that 10% limit. HPAC Engineering reports that NEMA, the motor makers' standards group, cautions against running a 230V-only motor on a 208V system.
A motor rated for 208V shows it on the nameplate in one of three ways:
- 208 in the voltage line, as in 208-230/460, which means the motor is rated for 208V, 230V or 460V
- A full load amps figure listed at 208V. ABB's motor nameplate guide says this means the motor can run at 208V without overheating.
- The words "Usable on 208 Volts," which the DOE sheet says some 230/460V motors carry. NEMA says newer nameplates no longer show this marking.
If the plate shows none of these, ask the machine or motor maker.
What is 2 phase power?
In a US home, "2 phase power" is a nickname for household 240V, which gets the name from its two hot wires. Continental Control Systems says US residential 120/240V is sometimes called "two-phase service," and that the name is wrong. A "2 phase" label on a 240V breaker or heater listing means two poles: two hot connections on single phase.
True two-phase is an older system with two voltages that peak a quarter wave apart. Hammond Power Solutions, a transformer maker, says 3 phase has replaced it on the modern grid. The tech site Hackaday reported in 2018 that parts of Philadelphia and Hartford, Connecticut, still had two-phase power.
2 phase vs 3 phase
| Item | Single phase 240V (split phase) | True two-phase | 3 phase |
|---|---|---|---|
| Hot wires or circuits | 2 hot wires, plus a neutral | 2 circuits | 3 hot wires, plus a neutral on 4-wire service |
| Timing between voltages (one full wave = 360 degrees) | One wave, flipped: one hot leg peaks as the other hits its low point (180 degrees) | 2 voltages, a quarter wave apart (90 degrees) | 3 voltages, a third of a wave apart (120 degrees) |
| Where you find it | Almost every US home, and shops on a home's service | Reported in parts of Philadelphia and Hartford | Examples: commercial buildings (208V), industrial buildings (240V delta), larger buildings (480V) |
| Runs a 3 phase motor | No, it needs a converter or VFD | No | Yes, when the nameplate is rated for that voltage |
Is there a 2 phase to 3 phase converter?
Three kinds of equipment turn household 240V single phase into 3 phase for a machine: a rotary phase converter, a digital phase converter, and a VFD with single phase input. The rotary vs digital vs VFD guide compares all three.
A VFD is set up for one motor, or for several motors that run together at one speed. ABB's Technical Note 204, "Multiple motors controlled by a single drive," sets that limit. If your machine's motors start and stop separately, or it has a magnetic starter between the drive and a motor, look at a rotary or digital converter instead. Schneider Electric's FAQ calls a contactor on a drive's output, used to start and stop the motor while the drive runs, "a misapplication."
Converters built for household service take 208V or 240V single phase. In its manual for older Phase Perfect DPC models, Phase Technologies says input power "should be wired by a qualified electrician into a 208V or 240 V circuit." Phoenix Phase Converters says "On its own, a rotary phase converter produces a three-phase voltage that matches the input voltage." So a rotary converter on 208V puts out about 208V 3 phase. Your machine's motor then has to be rated for 208V, as covered under "Is 208V 3 phase?"
A static converter is a more limited option. It helps the motor start, then disengages, and the motor runs on single phase at reduced power. Phase-A-Matic's PAM Series instruction sheet says that in the hookup where the converter only starts the motor, you get about two thirds of the motor's rated horsepower. It's sized to the machine's largest motor, and the static phase converter guide covers when one fits.
A transformer can't make 3 phase from household single phase. Hammond's FAQ puts it plainly: "Transformers cannot convert a single-phase source to a three-phase source." Our single phase to 3 phase transformer guide explains why.
If your building has true two-phase, Hammond's Scott-T FAQ says that in theory a Scott-T transformer fed with true two-phase power, 90 degrees apart, can produce 3 phase. Whether that works for your building is a question for your utility and a licensed electrician.
How can you tell what power you have?
Check two things:
- The machine. The motor nameplate has a phase line, marked "PH" or "Phase," with a 1 or a 3. Ask a seller for a photo of the plate before you buy a used machine. If the plate shows only 460V or 480V, the machine needs about twice the voltage a 240V service supplies. Take that one to a converter maker's engineers and a licensed electrician before you plan anything.
- The building. Call your utility and ask what service you have, or have a licensed electrician check it at the panel. A reading of 240V between two wires shows up on single phase service and on 240V delta, so one reading doesn't settle it. Any meter testing or panel work is the electrician's job.
For the longer version of both checks, see single phase vs 3 phase power for a home shop.
What to do if you have single phase 240V and a 3 phase machine
- Note the horsepower, volts, full load amps and phase from the nameplate.
- Enter one motor rating (horsepower, full load amps or kW) and the voltage into our phase converter sizing calculator. It shows each maker's own sizing rule and the math.
- Work through our guide to running 3 phase machines on single phase power. It covers whether your building's electrical service can carry a converter and what to expect when one is delivered by truck.
Not sure what power your shop has?
Call us at (855) 923-2610 or email contact@builtforproduction.com with your motor's nameplate: horsepower, volts, full load amps and phase. Questions about wiring or your shop's electrical service go to a licensed electrician and your local inspector. You can also reach us through our contact page.
Documents named in this article
- Continental Control Systems, "Electrical Service Types and Voltages" (updated 2021) and "Two-Phase Electrical Service" (updated 2020)
- Electrical Contractor, Michael Johnston, "The Wild Leg: Special requirements for these systems" (2025)
- IAEI Magazine, David Young, "Basic Electricity, Part 10" (2006)
- U.S. Department of Energy, "Improve Motor Operation at Off-Design Voltages" (2012)
- HPAC Engineering, Larry Clark, "HVAC and Electric Motors" (2015)
- ABB (Baldor), "How to read a NEMA motor nameplate" (2021)
- ABB, Technical Note 204, "Multiple motors controlled by a single drive"
- Schneider Electric, FAQ FA330384
- NEMA, "US DOE Regulation Change Impacting Useable at Nameplate Voltage" (2023)
- Hammond Power Solutions FAQs on two-phase circuits, Scott-T transformers, and single phase to 3 phase conversion
- Hackaday, Al Williams, "A Tale Of Two Phases And Tech Inertia" (2018)
- Phase Technologies, Phase Perfect Phase Converters Operation & Installation Manual (older DPC-A10, DPC-20 and DPC-30 models)
- Phoenix Phase Converters, "Can I Run a 480 Volt Piece of Equipment On a Rotary Phase Converter If My Single-Phase Voltage Is 240?" (2022)
- Phase-A-Matic, PAM Series static phase converter instruction sheet (2021)