DIY 240-V Extension Cords


Many do-it-yourself enthusiasts equip their home workshops with 240 volt power tools and use extension cords. Resistance caused by current flowing through wire results in a voltage drop. Thin wire produces greater resistance than thicker wire -- the longer the wire, the greater the voltage drop. Since a power drop in excess of 5 volts can reduce the life of electrical motors, it is advisable to use the shortest extension cord possible. Although extension cords are available in different lengths, customize your workshop by making up several cords to cover all your requirements rather than buying expensive finished cords.

Things You'll Need

  • Heavy-duty 8/3 power cord
  • Side cutters
  • Male three-prong 240 volt plug
  • Phillips screwdriver
  • Utility knife
  • Wire stripper
  • Soldering flux or paste
  • Soldering iron
  • Solder
  • Flat screwdriver
  • Three-pin 240 volt female plug
  • Multimeter
  • Cut a piece of cord to the required length from a roll of heavy-duty 8/3 power cord, using a pair of side cutters. Use this wire for all your extension cords; heavy gauge wire causes the least resistance, which results in a lower voltage drop across an extension cord.

  • Undo the three screws attaching the front of a male three-prong 240 volt plug to the back housing with a Phillips screwdriver. Lay the front section holding the three prongs on one side.

  • Loosen the two screws on the wire clamp on the back of the rear housing. Slide the power cord through the clamp and housing so that it protrudes about 6 inches.

  • Slice through one side of the power cord outer insulation with a shallow 1-long cut, using a sharp utility knife. Do not cut through the inner paper filling material while doing so.

  • Cut through the outer insulation around the circumference of the power cord at the base of the lengthwise cut. Peel the inch-long section of outer insulation away. This will reveal the paper filling surrounding the wires.

  • Expose the three wires in the power cord by carefully stripping the inner paper insulation away. Snip off the ends of the paper filling level with the outer insulation with a pair of side cutters. Bend each wire to ensure that you haven't cut through the plastic insulation.

  • Spread the wires apart. Place the inside face of the front section against the wires to ensure that each wire reaches 1/2 inch past its respective terminal.

  • Remove a 1/2 inch of insulation from the ends of each wire with a wire stripper. Twist the copper strands together.

  • Dip the twisted ends into soldering flux or paste. Heat the side of the twisted wire with a soldering iron. Hold the end of coiled solder against the twisted wire opposite the soldering iron until the solder melts and runs freely through the twisted strands.

  • Insert the soldered end of the white wire between the two faces of the clamping device on the silver terminal. Tighten the clamping screw firmly with a flat screwdriver.

  • Connect the remaining two wires the same way; black wire to the gold colored terminal, green wire to the green terminal. Give each wire a sharp tug to ensure that the terminal clamps are tight enough.

  • Slide the rear section up the power cord. Align the square notch on the inside of the back section with the protruding lug on the front half. Push the two halves together. Insert the three screws into the front of the plug and tighten them down evenly with a Phillips screwdriver. Tighten the two screws on the power cord clamping mechanism on the back of the plug.

  • Separate the two halves of the three-pin 240 volt female plug by undoing the three recessed screws on the face of the plug. Loosen the wire clamping mechanism and slide the back half of the plug over the power cord.

  • Strip away 1 inch of outer insulation on the end of the power cord. Remove the paper filling material and prepare the ends of the three wires with solder as described above.

  • Insert the end of the power cord through the loosened wire clamp and through the body of the rear section of the female plug. Separate the wires and clamp the soldered ends into their respective terminals; white wire to the silver terminal, black wire to the brass terminal and green wire to the green terminal. Tighten the clamping screws. Tug each wire to ensure that they are clamped firmly.

  • Tighten the two screws in the power cord clamp at the back of the plug housing. Carefully align the slots in the front face of the plug so that the prongs of the male plug on the other end of the cord penetrate properly. Make sure that the screw holes and the notched key section on the two halves line up. Press the two halves together. Insert the three recessed screws into the front face of the plug. Tighten securely with a Phillips screwdriver.

  • Follow the instruction on a multimeter user manual and use the instrument to test each wire on the power cord for continuity. When satisfied that each end has been connected correctly, plug the male end of the extension cord into a 240 volt wall outlet and the female end onto the male plug of a 240 volt power tool. Test the new extension cord by turning the power tool on.

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