Know what you're getting into
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An inside look at your main panel
Your main panel might not be exactly like the one here. With any panel, find the large cables and the lugs they're connected to. They're the parts that are always live, even when the main breakers are switched off
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Yours might look like this
The photo shows a different configuration where the large cables and lugs are located on the right-hand side of the circuit breaker box, rather than at the center.
We believe in safe DIY. That's why we've always been reluctant to show readers how to open a breaker box and connect a new circuit. Even with the power shut off, there's a chance you could touch the wrong parts and kill yourself. But then we figured if we didn't show you, you'd just go search the Internet. And that scared us even more. So we're going to walk you through the process, showing you the safest way to open the breaker box, wire a new breaker and test your work.
Opening the main breaker box and installing a new circuit is actually pretty easy. You only have to connect three wires, and each is color-coded. But there are some safety precautions, and if you ignore them, you could kill yourself. Really. If you follow our safety steps in order and to the letter, you'll be fine. But if at any point you're unsure how to proceed or feel uncomfortable with the project, call an electrician.
Stay away from the large wires and lugs. They're always live, even with the main breaker (service disconnect) shut off. If you touch them, you could die. Cover the live areas with a cardboard shield to prevent accidental contact while installing the new circuit. If you have any doubts about which areas stay live, contact an electrician.
Know what's what
A: Main lugs. They're always live—even when the main breaker is off. NEVER TOUCH THEM.
B: Main cables. The black ones are always live. And although they're insulated, avoid touching them.
C: Main breaker. Always switch it off before removing the panel's cover.
D: Breaker. The hot wire (usually red or black) from each circuit connects to a breaker. If you're installing an AFCI breaker (as shown on the following pages), you'll also connect the neutral wire to the breaker.
E: Breaker bus. Distributes power from the main breaker to the individual circuit breakers. Each breaker snaps onto the bus.
F: Neutral bus. All ground and neutral (white) wires connect here. If you're installing a standard breaker, the neutral (white) wire connects here, too. If you're installing an arc-fault circuit interrupter (AFCI) breaker, you'll connect the neutral to the breaker and run a “pigtail” wire to the neutral bus.
G: Breaker space. This panel has room for three more breakers. You can install your new breaker in any open space.
Get the right parts and tools
Before you go shopping, open the door of your breaker box and copy the manufacturer's name, the box model number, and the style numbers of the breakers that are approved for your box. Then buy one of those breakers. If your home center doesn't sell the right model or brand, you'll have to go to an electrical supplier. You cannot install a circuit breaker style that isn't specifically approved for use in your box—even if it fits inside the box. While at the store, pick up a few 1/2-in. plastic snap-in cable clamps to secure the new cable. They're safer than metal clamps because you don't put your hand in the panel to install them.
You have to shut off the power to your whole house, so you'll need a powerful work light. An LED headlamp is also a great idea so you won't have to juggle a flashlight, wire strippers and a screwdriver. Round up a utility knife, wire strippers, electrical tape, a circuit tester (not a voltage sniffer), and a flat-blade screwdriver or No. 2 square-drive tip for your multi-bit driver.
Insert a cardboard safety shield, remove a knockout, and feed in the cable
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Smack it down, twist it out
Jam a needle-nose pliers or a short screwdriver into the knockout to bend it down. Then grab the “slug” with your pliers and twist it back and forth until it breaks off.
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Run the cable through the clamp
Snap in a plastic clamp and then feed in the cable. Tape the wires together so one doesn't stray behind the cardboard.
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Push until the jacket enters the clamp
Keep pushing the cable into the clamp far enough to get at least 1/4 in. of the outer jacket inside the box. Then secure the cable outside the box with an insulated staple within 12 in. of the clamp.
You can insert the new cable into any knockout on the top, bottom or sides of the box. Find the least congested area and remove one small knockout. Then snap in a plastic cable clamp (the screw-style ones shown aren't as easy to use).
Hold the cable up to the box to determine how much of the outer jacket you should strip off. Slice off the jacket and remove the paper insulator. Then wrap the ends of the loose wires with electrical tape to prevent them from touching a live portion of the box.
Route the cable and install the breaker
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Hook and push
Install the breaker by slipping the tab into the hook. Then snap the breaker into place, forcing the slot onto the bus. We're using an AFCI breaker.
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Strip and clamp
Strip off 5/8 in. of insulation from the white and black wires and insert them into
the AFCI terminals. Tighten until snug.
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Ground and pigtail go to the neutral bus
Route the AFCI neutral pigtail and ground wires to empty screws on the neutral bus and tighten. If you're installing a breaker on a subpanel, place the neutral and ground on separate bus bars.
Neatly route the black and white wires to the empty breaker space. Attach the wires to the breaker and then snap it into the box, or install the breaker first and insert the wires last. Just be aware that wiring an AFCI-style breaker is different from wiring ordinary breakers. The neutral (white) from the new cable attaches to the AFCI. On a main panel, you connect the ground wire from the new cable and the neutral (white) pigtail from the AFCI to the neutral bus. If you're installing a breaker on a subpanel, place the neutral and ground on separate bus bars.