How to Add a Subpanel

Two Methods:Prepare for InstallationInstall the Sub panel

Adding a sub panel, commonly called a breaker box, to a building can provide additional circuit capacity which is especially helpful if your main panel is already full. Many homeowners who expand or remodel find that adding a sub panel is helpful or even necessary in order to provide electrical power to the new rooms. If aren't sure you can safely add the sub panel yourself, you should hire a professional electrician instead.

Method 1
Prepare for Installation

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    Check with your city's building and codes department to see if you need to acquire a permit before you begin the work. Minor electrical changes generally do not need a permit, but each jurisdiction varies. In some areas, permitting is required for any type of Electrical work, no matter the size in scope or significance. This work however, would by no means ever be considered "minor".
    • When you meet with the building code representative, ask about the inspection schedule so you know when to call for a rough inspection as well as a final inspection. Many localities have part time inspectors, so if you're on a schedule you should discuss it with them first.
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    Find a good light source that can run off a battery or generator. You will need light to work on your project after you shut off the electrical power to the building.
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    Determine where you will mount the sub panel. If space is an issue, you don't need to install it next to the main panel. However, you must be able run wire through to that area. Keep in mind that a closet is not an acceptable location for a panel. The area 3 feet (0.9 m) in front of and above and below the the panel should be clear from the floor to the ceiling.
    • If you install your sub panel closer to the rooms you are adding power to, you will use more of the larger sub-feed cable between the two panels; but fewer smaller wires to the outlets than if you installed the sub panel close to the main panel.
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    Shut off your main power breaker before beginning any actual electrical work.

Method 2
Install the Sub panel

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    Remove all sub panel covers or doors.
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    Measure about 5 feet (1.5 m) from the floor to mark the spot where you will mount the panel. This is a good height that is within comfortable reach of most adults.
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    Remove the lowest amp rated double pole circuit breaker or 2 low rated single pole circuit breakers from the main panel to make room for the new double pole circuit breaker that will supply the new sub panel. The 2 circuits you removed will need to be re-fed from the new sub panel. If the sub panel is the same brand, you might be able to reinstall the original circuit breakers in it to re-feed those 2 circuits.
    • If you have breakers terminal screws that are serving more than one conductor, called double-lugging; consider moving circuits to the sub panel so that none are double-lugged - or combine the them with a short length of the same size wire (called a "pigtail") in a wirenut and use the pigtail to feed them.
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    Route / extend the wires to the sub panel, splicing them with new wire if necessary.
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    Use a 4 wire cable (sized for the breaker protecting it) to supply the sub panel from the main panel. If using a cable with aluminum conductors, be sure to apply an oxide inhibitor on the aluminum before securing in a terminal or other pressure connector.
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    Insert the neutral and ground conductor connections in open terminal holes of the neutral / ground buss bar in the main panel and tighten screws, clamps, etc..
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    Insert the "hot" conductor connections ("Line 1" and "Line 2" or black, red or blue) into the terminal openings of the new double pole circuit breaker in the main panel.
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    Route and secure the sub panel feeder cable into the sub panel.
    • Connect the ground wire, which is bare, to the grounding bar or buss in the sub panel.
    • Connect the neutral wire, which will be white, to the neutral buss in the sub panel.
    • Attach the hot wires (black and red or blue) to the main breaker lugs of the sub panel.
    • Take the time to tidy up the wires and connections so that you can easily trace them back to their breaker if the need arises in the future.
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    Locate and remove any installed neutral bonding jumper, buss bar screw or strap to completely isolate the sub panel from the ground. This will ensure that the neutral and ground are connected at the main panel and not the sub panel.
    • The buss bar bonding jumper can be identified as a long screw or metal strap that effectively bonds the neutral buss bar to the grounded panel enclosure / ground buss bar.
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    Join the branch circuit wires in the same fashion - grounds to the ground bar, neutrals to the bus bar, and hot wires to the breaker lugs. Do not interchange ground and neutral wires in the other's buss bar. Install the breakers.
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    Re-install the panel cover / door assembly.
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    Restore power by first turning on the sub-feed breaker on the main panel, then the main breaker on the main panel, then the main breaker on the sub panel. Check to see if all the circuits are working.
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    Label the panel to identify which circuits feed into which panel. You will not pass an electrical inspection unless you complete this step.

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Categories: Electrical Maintenance