How to Wire an Electrical Circuit Breaker Panel

Circuit breaker wires
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Overview
  • Working Time: 8 hrs
  • Total Time: 8 hrs
  • Yield: One 15- to 20-circuit breaker box
  • Skill Level: Advanced
  • Estimated Cost: $700 to $950 (labor not included)

This overview describes how a professional electrician connects a residential electrical circuit breaker panel to the main service wires coming into the home, and to the individual branch circuits in your home. This is not a DIY project for most homeowners. Connecting a breaker panel is very dangerous work if you are not an expert, and in most communities, the building codes may not even allow you to do this kind of major electrical work yourself. It requires coordination with the electrical utility company. 

Our example assumes that the electrical panel is being installed on the wall of a utility area near the point where the main feeder wires come into the home, and that all branch circuit cables and conduit runs are already installed. In a new home construction or a major rewiring project, the last step will be connecting all the wires to the circuit breaker box, as described here. 

What You'll Need

Equipment / Tools

  • Voltage tester
  • Screwdrivers
  • Hammer
  • Electrician's fish tape
  • Level
  • Cable ripper
  • Wire strippers

Materials

  • Circuit breaker panel
  • Wall anchors (if needed)
  • 120-volt and 240-volt circuit breakers
  • Electrical tape

Instructions

  1. Check Feeder Wires for Power

    The electrician first makes sure the electricity to the feeder wires has been shut off by the power utility company. They will check the feeder wires with a voltage tester to ensure that this is the case.

  2. Open Knockouts in Box

    The electrician then opens metal knockouts for the main service conduit as well as for each branch circuit that will enter the breaker box. Yet another knockout may need to be opened for the main grounding wire. The knockouts are the round metal discs visible in the top, bottom, sides, and back of the box, of various sizes to match different sized conduits that may need to connect to the box. The electrician uses a chisel or old screwdriver to open the knockouts. They may do all of these at once, or start with only the knockout for the main service conduit, then open the knockouts for the branch circuits later, as individual circuit wires are run. 

    Once the knockouts are removed, the electrician installs the conduit or cable connectors into the circuit breaker panel, securing them with the knurled locknuts threaded onto the tailpieces of the connectors from the inside of the panel. 

    Mount an Electrical Panel
    Mount an Electrical Panel. Timothy Thiele
  3. Mount the Circuit Breaker Panel

    Next, the electrician positions the circuit breaker panel, levels it, and anchors it to the wall with screws or anchors driven through the back of the panel and into the wall. If the holes don't align with studs behind the panel, they may drill additional drill holes in the back of the panel to accommodate the installation or mount a piece of plywood to accommodate the panel.

    If the main service wires are entering the home through a conduit, the panel will be carefully positioned at this time so the conduit slides into the conduit fitting already mounted on the service panel. 

  4. Install the Main Service Wires

    The electrician will now feed a fish tape through the main service conduit opening from inside the panel to the entry point outside the home where the main service wires are available. There are normally three main service wires: two black hot wires and a white neutral wire.

    The main service wires are secured to the end of the fish tape with electrical tape and pulled back down into the breaker panel. It may require a helper to assist pushing the wires from outside while the fish tape is pulled from the inside, as these are large, stiff wires. Plenty of excess wire is pulled into the panel to allow for connections to be made anywhere in the box.

  5. Connect the Main Ground Wire

    The main grounding wire—usually a fairly large bare copper wire—is fed into the panel and is connected to the main grounding connection. Usually, this is a metal lug on the back of the metal panel or at the end of the ground bus bar.

    This main ground wire is usually connected to a grounding rod. In some systems, there may be an additional grounding wire used to ground metal water pipes; both can be connected to the same grounding lug in the panel.

  6. Connect the Main Service Neutral

    The main service neutral wire is now connected to the neutral bus bar. The location of the neutral bus bar varies depending on the panel manufacturer, but it is always located well away from the two hot bus bars. It is a silver-colored bar with many smaller screws and connection points, with one larger hole for the main service neutral wire. 

  7. Connect the Main Breaker

    The electrician now bends the two black service wires for easy connection to the main breaker. In most panels, the main breaker is a large 240-volt circuit breaker that is located at the top of the panel. It will control all the power entering the home and connects to both hot bus bars running down vertically through the panel.

    The ends of the service wires are stripped of just enough insulation to make the connections to the main circuit breaker terminal lugs. The electrician takes care not to leave the excess bare wire because this creates a safety hazard if the wires can come in contact with other wires. 

    The bare ends of the main service wires are inserted into the lugs on the main breaker, and the screws are tightened securely. 

    Breaker Panel Feed Wiring
    Breaker Panel Feed Wiring. Timothy Thiele
  8. Pull Wires for Branch Circuits

    The electrician will now pull the wires for the various branch circuits into the panel. If they arrive at the panel through metal conduit, a fish tape is used to pull them into the panel. If the branch circuits use NM wiring, NM connectors are installed to panel and the cables are held tightly with screws that tighten down onto a yolk. Plenty of wire is pulled into the panel to provide a margin for error; the excess wire will be looped along the inside edges of the panel.

    On NM cable, the outer plastic sheathing of the cable is cut away so that just a small portion remains where the cable enters the panel. For each cable, the electrician will strip away a short length of insulation from each individual conducting wire The use of a special stripping tool assures no damage to the metal wires.

  9. Connect Branch Circuit Ground Wires

    The electrician now connects all of green and bare copper wires for the branch circuits to the ground bus bar inside the panel. the ends of the wires are inserted into openings in the bus bar, and the setscrews are tightened down.

  10. Connect the Circuit Breakers

    The circuit breaker for each branch circuit is now connected, one at a time. The breakers have been carefully chosen to match the required amperage and voltage of the circuits and to meet code requirements. Electricians are carefully trained in selecting the type of breaker appropriate to the circuit.

    For example, some breakers will be GFCI-style breakers, others will be AFCI-style, while others may be combination style breakers. These breakers will have a coiled pigtail wire. There will also be standard-style breakers that have no pigtail wires, only lugs for hot wire connections.

    Begin by connecting the circuit's neutral wire. On standard-style breakers, the circuit's neutral wire will be attached to the neutral bus bar in the panel. But on AFCI, GFCE, or combination AFCI breakers, the circuit's neutral wire will be connected directly to the breaker, using a screw terminal labeled "Panel Neutral" or "White." (Note: 240-volt circuits have no white neutral wires since both circuit wires are hot wires.)

    Next, the electrician attached the circuit's hot wire, which will be black or red. On AFCI or GFCI breakers, this circuit wire will be attached to the terminal marked "Load Power" or "Black."

    Finally, for AFCI or GFCI breakers, attach the breaker's coiled white pigtail wire to the neutral bus bar in the panel.

    With all circuit wires connected, the circuit breaker can be snapped into place into its panel slot. The process may vary slightly depending on the manufacturer, but it generally involves hooking the front edge of the breaker over clips in the panel, then pushing down until the breaker snaps into place.

    Excess wire is neatly looped along the inside edges of the panel. Leaving plenty of excess will make possible repairs or replacements easier in the future. 

  11. Complete the Installation

    The cover of the circuit breaker panel is installed, and the electrician has the utility company turn on the power. The electrician now turns on the circuit breakers and makes sure all circuits in the house are operating correctly.

    The electrician finishes by labeling the panel to identify the function of each circuit. This will make it easier for the homeowner or a future electrician to identify and shut off individual circuits when necessary.