How to Convert 240 Single Phase to 480 3 Phase

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If all you have is single-phase 240-volt current and you need 480-volt three-phase current, you can step the voltage up to 480 volts using a transformer. Once at 480 volts, the single-phase current must be converted to three-phase using a phase converter. Rotary phase converters use an electric motor with capacitors to generate two additional phases of current, while static converters use electronics to do the same thing. One possible application is running three-phase motors on shop equipment at 480 volts.

    Turn off power to the disconnect switch that will supply the step-up transformer and move the switch to the off position. Remove the cover on the disconnect switch to expose the load terminals. Cut a piece of cable to run from the switch to the transformer and strip 6 inches of sheathing from each end with the cable knife. Insert the cable into the switch through the cable clamp so that 1/4 in of sheathing on the cable is inside the box. Tighten the cable clamp.

    Strip 1 inch of insulation from each of the two colored wires. Insert the end of the bare ground wire into the green grounding terminal and tighten the terminal with a screwdriver. Repeat this with the neutral wire and the neutral terminal if required. Insert the end of a colored wire into a load terminal and tighten the terminal. Repeat for the other colored wire and the other load terminal. Replace the disconnect switch box cover.

    Remove the cover from the transformer wiring panel. Insert the cable into the wiring panel and strip 1-inch of insulation from the wires. Insert the ground wire into the green ground terminal and tighten the terminal.

    Consult the transformer primary wiring schematic and select the primary terminals required for the voltage level supplied by the utility. You may have to use different terminal combinations based on the voltage level at your location. Transformers with two primary windings may require connecting the windings in series for 240 volt operation. Make the connections described by the transformer primary wiring schematic.

    Cut a new piece of cable to run between the transformer and the phase converter. Strip sheathing from both ends of the cable and remove 1 inch of insulation from the wire ends. Connect the ground wire to the transformer ground terminal as described previously. Consult the transformer wiring schematic and make the necessary connections to the secondary terminals for 480 volt output. Replace the transformer wiring panel cover.

    Remove the cover from the phase converter wiring box and insert the cable into the box through the cable clamp. Insert the ground wire into the ground terminal and tighten the screw. Connect the two colored wires to the two load terminals and tighten the screws.

    Turn on the power and the transformer disconnect switch. Start the phase converter and let it come up to speed. Set the multimeter to a voltage range higher than 480 volts. Touch the multimeter leads to two of the three output terminals and read the voltage. Acceptable readings range from 456 to 504 volts between any two output terminals on the phase converter. Replace the wiring box cover.


    • If your application uses motors over 30 horsepower, you should consult your electric utility to ensure the utility owned transformer supplying your power is adequate for the task.

      Depending on the location of the transformer and the phase converter, a separate disconnect switch may be required at the location of the phase converter.

      Disconnect switches must be within sight of the equipment they supply according to the National Electric Code.


    • Read the transformer wiring schematic and follow it. Different connections are required for different voltages and transformers are designed to handle differing input and output voltages. Make sure your transformer connections are correct.


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.

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