How to Add a New Electrical Circuit to House Wiring

Written by chris deziel Google
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How to Add a New Electrical Circuit to House Wiring
It's better to add a new circuit than it is to overload an existing one. (Thinkstock Images/Comstock/Getty Images)

If one of your circuit breakers is constantly tripping, you could be overloading the circuit and might need a new one to handle your power needs. Except for the logistics of running wires behind existing walls, the procedure is straightforward. Start with a plan so you'll know where to put the devices and run the cables, daisy-chain all the devices in the circuit together and finish up by making connections at the panel. Use a circuit breaker and electrical cable rated for the load you expect on the circuit.

Skill level:

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Things you need

  • Electrical boxes
  • Electrical cable
  • Electrical devices (outlets, switches, light fixtures)
  • Screwdriver
  • Cable clamp
  • Utility knife
  • Wire stripper

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  1. 1

    Draw a diagram of the circuit, indicating all the outlet, lights and switches on it. Estimate the maximum current draw of the circuit if all the lights and appliances on it are working at the same time and buy a circuit breaker and electrical cable that will handle the draw. For a moderately sized circuit mostly servicing lights, a 15-amp breaker is usually sufficient. Use a 20-amp breaker if you will be running larger appliances or power tools on the circuit. Fourteen-gauge wire is sufficient for 15-amp circuits, but for 20-amp ones, use 12-gauge wire.

  2. 2

    Install an electrical box at the location of each switch, light or outlet. Run a length of electrical cable from the panel to the first box, then run it from there to the other boxes on the circuit as needed. The exact wiring configuration depends on many factors, so if you're confused, get advice from a knowledgeable friend or a professional. Leave enough cable at the panel to make connections inside it.

  3. 3

    Wire all the devices on the circuit before you make connections at the panel. The general rule for wiring electrical devices is that their brass terminals are hot and connect to black wires, the silver terminals are neutral and connect to white wires and the green terminals are ground and connect to bare or green wires. Switches have only hot terminals. Join white wires in a switch box to each other to bypass the switch.

  4. 4

    Open the panel door and turn off the main breaker. Unscrew and remove the cover, then look for a place to run the new circuit cable through the side of the panel. If several wires are clamped into a hole, you'll often have enough room to put one more by loosening the clamp with a screwdriver, running the cable through, and tightening the clamp. If all the holes are full, knock out a new one with a screwdriver, screw on a wire clamp, run the cable through it and tighten the clamp.

  5. 5

    Strip the end of the cable with a utility knife and pull the wires apart. You usually have to remove 12 inches of sheathing or more to separate the wires enough to reach their connection points. Strip insulation from the ends of the black and white wires with the knife, or use a wire stripper.

  6. 6

    Loosen the lug in an available slot on the silver and ground bus bars with a screwdriver. Feed the white wire into the silver bus and the bare ground wire into the ground bus and tighten the lugs.

  7. 7

    Connect the black wire to a circuit breaker by unscrewing the screw on the bottom of the breaker, feeding in the wire, then tightening the screw. Find an available slot on the front of the panel and snap the breaker into place, where it will make contact with the hot bus and complete the circuit.

  8. 8

    Turn on the main breaker panel, then turn on the breaker for the new circuit. It should stay on. If it trips, it's probably an indication of a loose connection somewhere along the circuit. Leave the breaker off and check your wiring.

Tips and warnings

  • After you make a diagram of your new circuit, check it with a licensed electrician to ensure the wiring you are proposing will meet code.
  • Use the same gauge wire for the entire circuit. The electrical code prohibits changing wire size in the middle of a circuit.
  • Wear rubber-soled shoes and use insulated tools when working in the panel. Even though you have shut off the main breaker, the brass bus bar is still live, and you can get a severe shock by touching it.

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