How to Convert Single Phase to 3 Phase Power

Updated February 21, 2017

Electric utilities generate three-phase power for distribution to the electric grid, but only provide single-phase power to homes, farms and small businesses. Single-phase current will not operate three-phase motors, which are available in larger horsepower ratings than single-phase motors. Farms, small manufacturing companies and even home shop applications sometimes require motors rated higher than 10 horsepower -- the highest standard horsepower single-phase motor available. Phase converters change single-phase current to three-phase current to run three-phase motors. A 240-volt, single-phase supply is required to operate a phase converter through a receptacle or disconnect switch.

Position the rotary phase converter in a location accessible to the 220-to-240 volt supply. The 220-to-240 volt supply must conform to National Electric Code which requires a disconnect switch within sight of the electric motor it supplies -- in this case the rotary phase converter. The disconnect is a switch designed to carry the load required and disconnect it for service or in an emergency.

Turn off the power to the disconnect switch. Remove the screws holding the switch box cover in place and remove the cover. Inside are three terminals, two labelled L1 and L2, and one labelled Neutral or W. A fourth terminal labelled is labelled ground. If it is not labelled, it probably uses a green screw.

Strip 6 inches of insulation from the end of the sheathed cable using the cable stripper. Remove 1 inch of insulation from the ends of the two insulated wires. If one of the wires is white, colour the insulation black with the permanent marker for at least two inches at the end of the wire.

Feed the cable into the switch box through the bushing and clamp and tighten the clamp. Loosen the screws on terminals L1, L2 and Ground. Insert the end of the ground wire into the ground terminal and tighten the screw. Insert the other two wires into L1 and L2 -- the order does not matter -- and tighten the screws. Replace the switch box cover and move the lever to the off position.

Remove the wiring box cover on the rotary phase converter. There are two sets of terminals. One has two terminals for the input current and the other has three terminals for the output current. One or two ground terminals are also provided.

Remove 6 inches of sheathing from the cable ends and 1 inch of insulation from the wires. Insert the cable into the wiring box through the strain relief or clamp and tighten it.

Loosen the screws for the two input terminals and the ground terminal. Insert the ground wire into the ground terminal and the other two wires into the input terminals. Tighten all the terminal screws. Leave the cover off for testing.

Turn on power to the phase converter and move the disconnect switch to the "On" position. Start the phase converter and let it come up to speed.

Set the multimeter to AC volts and select a range higher than 300 volts. Some multimeters are auto-ranging and don't require selecting the voltage range. Place the black test lead in the multimeter common terminal and the red lead in the voltage terminal.

Test the phase converter output between pairs of output terminal by touching one meter lead to one terminal and the other meter lead to another terminal. Make tests between L1 and L2, L1 and L3 and between L2 and L3. Readings will vary between terminals, but should fall into the range specified by the motor manufacturer.


Ideally, the output will not vary more than five per cent between any two terminals, but quality varies. You can reasonably expect output to fall within five to 10 per cent of the input voltage measured between the two input terminals on the phase converter. If you intend to operate your three-phase equipment away from your phase converter, you must incorporate a separate disconnect switch and motor controller for the equipment. Each motor must have a disconnect within sight of the motor.


Always turn the power off to work on a circuit. Turn the phase converter off and the power at the disconnect switch to make connections to the output of the phase converter.

Things You'll Need

  • Rotary phase converter
  • Screwdrivers
  • Cable stripper
  • Sheathed cable
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About the Author

Michael Logan is a writer, editor and web page designer. His professional background includes electrical, computer and test engineering, real estate investment, network engineering and management, programming and remodeling company owner. Logan has been writing professionally since he was first published in "Test & Measurement World" in 1989.