How to Wire a Circuit Breaker

To wire a new circuit, identify an open slot in your breaker box for a new circuit breaker. Work during the daytime as you will need to shut off all power in your home during most of the project.

• Measure the room or rooms where you’ll be installing the new branch circuit, and draw a detailed, accurate floor plan. Circuit breakers are typically located in the garage, basement, a storage room or a hallway.
• On the floor plan, mark the position of every receptacle, switch, fixture and major appliance you plan to add to the circuit.
• Draw lines representing the electrical cable for the circuit. The electrical cable should begin in the breaker box and run to every device in the circuit.
• Add up the wattage for all the devices you expect to connect to the new circuit. This will give you the new circuit’s total load.
  (Remember that receptacles and lighting can't be installed on more than a 15- or 20-amp breaker. Make sure your total wattage does not exceed 1800 watts for a 15-amp branch circuit or 2400 watts for a 20-amp circuit). All new 15-amp or 20-amp breakers installed in a home must be arc-fault type breakers, except for the bathroom and garage, which must be GFCI type breakers.
• Determine what type of breaker box you need to install, based on the total circuit load.
• Verify that your breaker box can handle a new circuit. Your breaker box’s amperage rating will be listed on the main breaker. Newer homes will have a 100-amp, 150-amp or 200-amp rating, while some older homes may have a 60-amp rating. The rating tells you how much total current each of two vertical columns of individual circuit breakers can deliver at one time. Any long lengths of wire over 100-feet can experience voltage drop. Consult an electrical manual or electrician for more information.

Safety Tip: All circuit breakers have a safe load capacity rating. For example, a 15-amp circuit breaker has a safe load capacity of 1,440-watts, and a 20-amp circuit has a safe capacity of 1,920-watts. If the total load you want to put on that circuit is greater than 1,920-watts, you must install two separate circuits. Permanently installed appliances, such as A/C units, washing machines and ovens, require their own dedicated circuit.

• Select the electrical boxes you need for every receptacle, switch and fixture in your new circuit.
• When wiring electrical panels and boxes, always shut off the main power on your breaker box before you begin installation work. Doing so protects you from electrical shock in case you accidentally come into contact with existing electrical cable behind your walls.
• Install the appropriate electrical boxes for every device in your circuit, as laid out in your circuit plan.
• If you are adding a circuit to a room with unfinished framing, you can mount new-work boxes directly to wall studs or ceiling joists.
• If you are adding wiring behind existing drywall, you will need to cut openings in the wall or ceiling and install remodeling boxes, also known as old-work boxes.

• Select cable that can handle the load on your new circuit. Use number 14 AWG wire for 15-amp circuits and 12 AWG for 20-amp circuits. Some rooms require that the branch circuit suppling power be a 20-amp circuit. Refer to the NEC code book or consult an electrician to verify.
• Run cable between electrical boxes, as laid out in your circuit plan.
• If you are adding a circuit to a room with unfinished framing, drill 5/8-inch holes through wall studs, and pass the cable through the holes.
• Typically, cable should run in a straight line, one foot above your receptacles. Secure cable to framing members using cable staples. Screw protective metal nailing plates to the framing members at each point where cable passes through. These plates will prevent someone from accidentally drilling into the electrical cable in the future.
• If you are adding a circuit in a room with finished walls, you will need to use fish tape to run cable behind the walls, from electrical box to electrical box. The simplest approach may be to run cable up into an attic or down into a basement.
• Run cable from the first electrical box in the circuit to your breaker panel location, as laid out in your circuit diagram. Do not wire the cable to your breaker panel yet.
• At each electrical box, wire the cable to the receptacle, switch, fixture or appliance specified in the circuit plan.

Once you’ve run the cable to all the devices in your circuit, you can learn how to install a circuit breaker and how to wire a breaker box for a new breaker.

Inside the breaker box, there is a grounding bus bar and a neutral bus bar on opposite sides of the panel. Each has multiple setscrew terminals for wires. Two hot bus bars, which provide electrical power to all household circuits, run down the middle of the panel, with breakers on either side of them.

• Make sure the main breaker switch is in the off position. Before you begin work on the breaker box, all the wires and circuit breakers in the panel must be de-energized. If you have a separate main disconnect, switch it off as well. This may be in a panel outside your house, near the electrical meter.
• Remove the screws fastening the breaker box cover and pull the cover off.
• Confirm that power is shut off using a voltage tester. Hold one probe against the neutral bus bar while touching the other probe to both setscrews on a double-pole breaker. Do not touch the two black cables running to the main circuit breaker, as they are always energized.
• Remove one knockout slug from the side of the breaker box. Use a hammer to tap the knockout slug with a screwdriver, then use a pair of pliers to twist off the slug.
• Insert a cable clamp into the open knockout.
• Use a cable ripper to strip about 12 inches of sheathing from the end of the cable. Cut off the excess sheathing with a utility knife.
• Feed the cable through the cable clamp until there is a 1/2-inch of sheathed cable inside the breaker box. Tighten the cable clamp to secure the cable.

Here’s how to install a circuit breaker for 120-volt and 120/240-volt circuits:

• Use a wire stripper to remove a 1/2-inch of insulation from the white neutral wire.
• Insert the end of the neutral wire into an open terminal on the neutral bus bar and tighten the screw.
  Make sure you put only one wire under each open terminal.
• Strip away a 1/2-inch of insulation from the black hot wire. If you’re adding a 120/240-volt circuit, strip away a 1/2-inch of insulation from the red hot wire as well.
• Insert the end of the hot wire into the terminal on the new circuit breaker and tighten the setscrew. In a 120/240-volt circuit, connect the red hot wire to the second circuit breaker terminal. To install the circuit breaker into the panel, place the breaker where your wire will terminate into the notch or slot inside the panel that is away from the bus bars. Then, snap the other end of the breaker onto the bus of the panel. Make sure the breaker is flush with the other breaks to make good contact. New arc fault breakers may come with a white wire already attached to the breakers. Make sure you read the instructions on how to connect to your panel before proceeding.
• Position the excess neutral and hot wire around the inside edge of the panel, away from the bus bars.

To install a circuit breaker for straight 240-volt circuits:

• Use a wire stripper to remove a 1/2-inch of insulation from the ends of both hot wires.
• Insert the end of each hot wire, which will typically be black and red, into a terminal on the new circuit breaker and tighten the setscrews. There is no neutral wire in this type of circuit breaker.
• Position the excess hot wires around the inside edge of the panel, away from the bus bars.
• Insert the end of the bare, copper-colored grounding wire into an open terminal in the grounding bus bar. Tighten the setscrew to secure the wire. If this is the main service panel and there is no separate grounding bus bar, connect the grounding wire to the neutral bus bar instead.
• Position the excess grounding wire so it runs along the inside edge of the panel, away from the bus bars.

• Use a pair of pliers to remove the breaker knockout on the panel cover that corresponds to the position of the new circuit breaker. With a double-pole circuit breaker, you will need to remove two knockouts.
• Replace the panel cover and tighten the screws to reattach it to the panel housing.
• To avoid a power surge, switch off all the individual circuit breakers before switching on the main breaker.
• Switch on the main breaker. Then switch on the individual circuit breakers, one by one.
• Label the new circuit breaker on the breaker panel cover. This will help in case of an emergency. If the power needs to be shut down immediately, you can identify the correct appliance or room.
• Test each of the receptacles, switches, fixtures or appliances on the circuit to ensure the installation was successful.

• When working with an electrical circuit, always shut off the power at the main breaker panel. Leave a warning note so that no one switches the power back on while you’re working.
• When wiring electrical panels, wear rubber-soled shoes, only use tools with rubber-insulated handles and never stand directly on a damp floor.
• Always consult local building wiring authorities before working on your household electrical circuit. New circuit wiring must meet local electrical code standards, and you may need a permit. Additionally, both your plan and the finished work may need to pass an official inspection. Local authorities may require that a professional electrician handle certain parts of the job.
• For simple additions, such as installing an extra receptacle, you may be able to extend an existing circuit. As long as adding the load doesn’t exceed the circuit breaker’s amperage rating, you can splice wires leading to the new receptacle to the circuit’s existing wire. But if you’re wiring a new room or installing a high-wattage device like a major appliance, you need to add at least one new circuit with its own circuit breaker in the breaker box.
• Select a circuit breaker designed to handle the load on your new circuit and designed to fit your breaker box model. There are three types of circuit breakers: single pole, double pole and tandem.
• If a certain appliance isn't working, it may be because the breaker for that item has tripped. This means too much electricity is flowing and the item cannot handle the excess current load. Resolve this by turning off every circuit, flipping the breaker switch and then testing the circuit.
• Some homes have a GFCI (ground fault circuit interrupter) outlet that will detect dangerous ground faults and will turn off the power to stop the shocks. They are typically found near moisture sources, in locations such as kitchens and bathrooms.