How to Install a Circuit Breaker

Often the most intimidating part of electrical work, installation of a circuit breaker into most residential type electrical panels does not have to be dangerous. Here are some simple, step by step procedures to do it safely.


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    Turn off power supply to the electrical panel. Locate the Service Disconnect or Main circuit breaker in the panel, and set to the "Off" position. This circuit breaker is likely to be the largest value and located either at the top or bottom of the panel. If no such circuit breaker is identified in the panel, it is likely in another panel in the building, or in the meter socket enclosure. Look for other panel(s) to find the circuit breaker needed.
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    Inspect the circuit breaker arrangement for unused locations. Pay particular attention to unused spaces at the top and bottom of the cover. Some manufacturers of electrical panels have removable knock outs or plates at these locations, but the panel itself lacks provisions to mount a circuit breaker. Simply stated: Do not rely on the cover alone to determine the ability to add circuits to the panel - check the bus bar.
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    Remove the electrical panel cover. Remove fasteners while a helper supports the cover, then pull straight away from the panel.
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    Test the panel for power. Use a tester or meter set to the highest AC voltage range (120 Volts minimum) available, and check for the presence of power by touching one probe to ground or neutral (the bar that has bare or green and white wires connected - or the bar that has just white or just bare or green wires connected). Touch the other probe to the screw terminal of a circuit breaker that has a black, red or blue insulated wire connected. If 120 (or more) volts is indicated, the panel is still powered. If the Service Disconnect or Main circuit breaker is in this panel, it will always indicate power on the terminals that have cables connected. The output of the Main or Service Disconnect when located in the panel, connects to the bus bar. The bus bar should have no power present when this breaker is OFF. Testing at the Service Disconnect or Main circuit breaker is not recommended due to this "seemingly conflicting" information. Do not continue if power is present on a circuit breaker other than Service Disconnect or Main circuit breaker, until the power source has been shut off.
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    Locate an unused space above, below or between existing circuit breakers. A single pole or single width circuit breaker will provide a a single 120 volt circuit (or in the case of a "tandem breaker" or "half width breaker" it will provide two 120 volt circuits - not a 240 volt circuit), while a double pole or double width circuit breaker will provide a 208 or 240 volt circuit. Carefully compare this location to the cover that was removed earlier. It is very important that the cover has provisions for exposing the new circuit breaker by removal of the metal twist-out(s). If there is no twist-out(s), the circuit breaker must be located to a different place in the panel.
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    Select the correct circuit breaker. The panel label will list all approved circuit breakers for installation. Deviating from the list is a code violation and voids any UL, FM or other listing service's approval. Typically, the only breakers allowed to be installed are from the same manufacturer of the panel - even if other brand breakers are labeled as "fits (brand name here) panels". In the USA, Canada and other countries, a single pole breaker would be used for a 120 volt circuit and a double pole breaker for a 240 volt circuit. The breaker should be of an ampacity that does not exceed the circuit conductor's rating. This is typically 15 amps for #14 copper, 20 amps for #12 copper and 30 amps for #10 copper conductors or wires. The code book should be consulted to determine sizes for other circuits. The breaker's terminals should be rated to match the conductor material; CU for copper and AL for aluminum. The terminal size should be large enough for the wire to fit. The need to remove strands of wire to fit the terminal is an indication of an error somewhere along the line.
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    Locate the circuit breaker mounting points. The circuit breaker will have two mounting points. Both are mechanical contacts; one of which also serves as the electrical input contact. Both contact points are on or very near the bottom of the circuit breaker.
    • The non-electrical contact point is at the screw terminal end of the circuit breaker. This end is engaged first when installing. The contact point will consist of a clip, bracket or tang and works with a support structure on the outboard part of the panel designed specifically for this use. The clip is pressed on and snaps to a raised rail for the Square D QO style (and others) breakers, The Square D HOM, Murray, GE and many others use a bracket molded in the case to land a clip from the panel. Finally, the tang type used by some other circuit breakers simply aligns with a slot in the rail.
    • The electrical contact is at the opposite end of the circuit breaker. This end is engaged last when installing. The contact mechanism is often inside the circuit breaker; partially visible through a slot or opening in the case. It is closely aligned with the bus bar (metal ribs or tabs in the center of the panel) after the mechanical contact point has been engaged. The electrical contact is established when the circuit breaker has fully engaged the bus bar - simple downward pressure is all the is required to fully seat.
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    Set the circuit breaker handle to the OFF position. The circuit breaker has 3 possible positions; ON and OFF and a mid position when TRIPPED. Push the handle towards the OFF position unless OFF already.
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    Install the circuit breaker by aligning with the unused space in the panel. Tilt the circuit breaker so that the mechanical contact point engages the support structure (rail, slot or clip). Once engaged, pivot the circuit breaker on the mechanical contact and roll towards the center of the panel - making sure the bus bar of the panel is still aligned with the slot or opening on the circuit breaker case. Firmly press on the circuit breaker surface to ensure it has been fully seated. While it will require firm even pressure to be seated, it should not have to be forced. Compare it to other previously installed breakers.
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    Connect the circuit. After making sure the circuit breaker is still in the OFF position, connect the circuit conductors or wires to the circuit breaker terminal(s), ground and neutral bar terminal screws. Use appropriate oxide inhibitor when connecting aluminum conductors.
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    Remove all foreign objects. Remove, tools, wire scraps, etc. from the interior of the panel. Look for and remove accidental conductors in the panel that could cause a short circuit when power is restored.
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    Install cover. Bring the cover up to the panel to compare the new circuit breaker location to the cover openings. Remove the metal twist-out(s) at the appropriate location. Place the cover on the panel to check that the circuit breaker has fully seated at both contact points. The circuit breaker should not be a high point under the cover. Remove the cover and press the circuit breaker into the panel as needed to provide proper cover fit. Secure the cover to the electrical panel with the fasteners.
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    Test. Standing to the side of the panel, restore power to the panel by setting the Service Disconnect or Main to "On" and then set the new circuit breaker to "On" Clear any short circuit before attempting a reset if circuit breaker instantly trips. Check for correct operation of the new circuit (light, outlet, etc.) with a test light or meter.
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    Identify the circuit. Locate the panel's circuit directory. It will often be found on the inside of the panel door. Determine the circuit breaker location (or "circuit number") and write a description of the circuit (load type such as "refrigerator" or a location such as "living room") in the space provided. Be sure to edit the directory if any circuits were moved to install the new circuit.


  • Voltage levels as little as 50 volts can be lethal under the right conditions. Most residential electrical systems are 2 to 5 times this value. Shut off power whenever working on circuits and never proceed if unsure or uncomfortable performing the steps above.
  • Prior to using any tester or meter, be sure it works by first checking on a known live circuit. If the device fails to indicate properly, do not use until repaired or replaced.
  • Always stand to the side of an electrical panel when turning circuit breakers "on". The amount of energy being transferred is directly proportional to the amperage value rating on the circuit breaker. While a single pole 15 or 20 amp short circuit would probably not cause catastrophic damage, a double pole 100 or 200 amp short circuit certainly could. Standing off to the side takes a person largely (though not completely) "out of harms way".
  • In the US, 120/240 systems (the type most often found in a residence or dwelling) use a color code for wiring; Black, Red, Blue "hot wires" and White neutral wires. In addition to 120/240 volt systems that appear in residences, many commercial and industrial settings also have 277/480 systems. These higher voltage systems use a different color scheme to immediately alert the electrician of the higher voltage panel. The color scheme used for these systems is Brown, Orange, Yellow (think "BOY") "hot wires" and Gray neutral wires. Be sure new wiring is installed in the correct panel.
  • Never install a circuit breaker into a panel for which it was not designed. Many circuit breakers will physically fit into different panels, however only those identified on the panel's label are approved for use. Using improper circuit breaker causes forfeiture of UL, Factory Mutual and all other "listings". Use of unlisted devices is a code violation and in extreme cases, may result in denial of a claim to the insurance company in the event of a loss.

Things You'll Need

  • Pliers
  • Screwdrivers
  • Flashlight
  • Voltage Tester or Meter
  • Helper

Article Info

Categories: Electrical Maintenance