How to wire three phase electric motors

Three-phase electric motors are more efficient than single-phase motors and consume less current for a given horsepower rating. Because a three-phase motor requires less amperage than a single-phase motor of the same horsepower rating, the motor can be wired using smaller size conductors. Three-phase motors are three-lead, nine-lead or 12-lead motors. Three-phase motors with three-lead motors are designed to operate on a single voltage. Most three-phase motors are dual-voltage motors and have either nine or 12 T-leads. The operating voltage is selected by how the T-leads are interconnected. The good news here is that you don't have to memorise the connections; there's a connections diagram on the motor. Most three-phase motors are wired using an across-the-line magnetic starter/contactor.

Use a flat blade screwdriver or a Phillips head screwdriver, depending on the type of screw head, to remove the cover to the motor's terminal box and pull all the leads out. If this is a new motor, there may be protective tape over the connectors that you will need to remove.

Use a screwdriver and a nut driver to loosen the nuts and bolts located in motor leads T1, T2 and T3. Referring to the connections diagram, connect the T-leads for the voltage being used. Typically, for a nine-lead motor to be operated on the lower voltage you would connect T-leads 1 and 7, 2 and 8, 3 and 9, and 4, 5 and 6 together for a "Star" or "Wye" wound motor.

Install the flexible metal conduit between the motor controller/motor contactor and the connection box on the motor. Feed four lengths of wire from the controller into the motor terminal box. Select the wire size based on the motor's Full Load Amperes (FLA) and the wire Ampacity table 310-16 of the National Electrical Code (NEC) book. The red, black and blue conductors will be the hot phase conductors. The green wire will be the machine-bonding conductor.

Remove ¾ inches of insulation from the ends of the conductors using wire strippers. Install four ring connectors on the motor end of the conductors using the crimping feature on the stripper tool. Connect these phase wires to the motor's T-leads as indicated on the connection diagram. The bonding conductor connects to the motor housing grounding screw. Tape the connections and replace the connection box cover.

Connect the phase conductors to the T1, T2 and T3 terminals on the motor controller. Connect the green grounding/bonding conductor to the motor controller enclosure.

Turn on the power and check the motor's direction of rotation. Three-phase motors rotate in either direction, clockwise or counterclockwise. If it's rotating in the wrong direction, reverse any two of the phase wires on the controller and you will reverse the motor's direction of rotation. Replace the motor controller cover.

Things You'll Need

  • Screwdrivers
  • Set of nut drivers
  • Flexible conduit
  • Flex connectors
  • Stranded copper wire (red, black, blue, green)
  • Solderless insulated ring connectors
  • Wire strippers/crimper
  • Black plastic electrical tape
  • Diagonal pliers/wire cutters
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About the Author

Based in Colorado Springs, Colo., Jerry Walch has been writing articles for the DIY market since 1974. His work has appeared in “Family Handyman” magazine, “Popular Science,” "Popular Mechanics," “Handy” and other publications. Walch spent 40 years working in the electrical trades and holds an Associate of Applied Science in applied electrical engineering technology from Alvin Junior College.