Project Guide

How to Install Circuit Breaker

Plan the circuit  - Install Circuit Breaker

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.   

  • Measure the room or rooms where you’ll be installing the circuit, and draw a  detailed, accurate floor plan.  
  • 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 panel 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.  
  • Determine what type of circuit breaker you need to install, based on the total circuit load.  
  • Verify that your breaker panel can handle a new circuit. Your breaker panel’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 amperage loss. Consult a wiring book or electrician for more information.
Install electrical boxes  - Install Circuit Breaker
  • Select the electrical box you need for every receptacle, switch and fixture in your new circuit. Our Electrical Box Buying Guide explains the functions of the different box designs.  
  • Shut off the main power on your circuit breaker panel before you begin installation work, in case you 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.
Run cable devices  - Install Circuit Breaker
  • Be sure to select cable that can handle the load on your new circuit.  
  • 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. Learn more in our Running Electrical Wire Behind Walls Project Guide. 
  • 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 at this time.  
  • At each electrical box, wire the cable to the receptacle, switch, fixture or appliance specified in the circuit plan.
Run cable breaker panel  - Install Circuit Breaker

Inside the breaker panel, 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 panel, 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 panel cover, and pull the cover off the breaker panel.  
  • Confirm that power is shut off using a neon 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 panel. 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 ½ inch of sheathed cable inside the breaker panel. Tighten the cable clamp to secure the cable.
Wire new circuit breaker - Install Circuit Breaker

Tip: For information on AFCI circuit breakers, see our Installing an AFCI Circuit Breaker project guide. Insert the end of the bare, copper-colored grounding wire into an open terminal in the grounding bus bar and tighten the setscrew to secure the wire. If it 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.  

120-volt and 120/240-volt circuits 

  • Use a wire stripper to remove a ½ 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. 
  • Strip away a ½ inch of insulation from the black hot wire. If you’re adding a 120/240-volt circuit, strip away a ½ 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.  
  • Lock the new circuit breaker into an open slot, by slipping the side with the attached hot wire under the tab beside the hot bus bar. Push the other side in, toward the bus bar, until it snaps into place, flush with the other breakers in the panel.  
  • Position the excess neutral and hot wire around the inside edge of the panel, away from the bus bars.  Straight 240-volt circuits 
  • Use a wire stripper to remove a ½ 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.  
  • Position the excess hot wires around the inside edge of the panel, away from the bus bars.
Replace panel cover test - Install Circuit Breaker
  • 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 and then switch on the individual circuit breakers, one by one.  
  • Label the new circuit breaker on the breaker panel cover.  
  • Test each of the receptacles, switches, fixtures or appliances on the circuit to ensure the installation was successful.