How to Wire a Sub-Panel Box
A subsidiary electrical panel, or sub-panel, is an additional breaker box that gets power from the main breaker panel. The extra panel can be used to provide more breakers to an expanding home or upgrade the original panel if it cannot accommodate something such as an AFCI breaker switch. Adding a subsidiary breaker box is a safe and simple solution for a building that has outgrown the main breaker panel. Does this Spark an idea?
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Things You'll Need
- Electrical cable
- Conduit
- Fish tape
- Circuit breakers
- Utility knife
- Wire cutters
- Slotted screwdriver
Instructions
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Contact your local building department to check the local codes for electrical work. Homeowners are not permitted to do their own electrical work in some areas. Most codes require an inspection of the layout before the wires are connected to terminals and another inspection after work is completed.
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2
Match the load rating of the electrical cable that will carry power to the sub-panel from the main breaker box and the circuit breaker on the cable between the two breaker boxes. Use a cable such as 6-3G that is capable of carrying 50 amps of power with a 50 amp circuit breaker.
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Push 12 inches of 6-3G cable up into the sub-panel box through a bottom cable port. Slice the outer jacket of the cable lengthwise to make a 12 inch slit and pull the inner wires through the slit. Cut away the empty portion of the jacket. Strip 1/2 inch of the insulation off the ends of the red, black and white wires with wire cutters.
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Loosen one of the green screw heads on the ground bus bar inside the subsidiary panel and slip the bare copper ground wire from the cable into the terminal. Tighten the terminal screw firmly. Loosen the large silver terminal lug on the neutral bus bar that holds the brass colored screw terminals. Push the stripped end of the white wire into the large lug and securely tighten the terminal screw. Loosen the two large terminal lugs at the top of the hot bus above the breaker slots. Insert the red wire into one lug and the black wire into the other. Tighten both terminals firmly.
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Turn off the main power switch on the main breaker panel. That cuts off the electricity to all the circuits in the building. Leave the power turned off until the work is completed.
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Push a fish tape through a conduit that fits between the main breaker box and the subsidiary box. Attach the loose end of the electrical cable to the hook on the fish tape. Pull the fish tape back through the conduit to thread the cable through it. Push 12 inches of the loose cable end into a port of the main breaker panel and attach the threaded conduit connectors to both breaker box ports.
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Strip the black, red and white wires to reveal 1/2 inch of the bare wires. Push the bare copper ground wire into a green ground terminal on the ground bus bar of the main panel and secure it firmly. Connect the white wire to a free terminal on the neutral bus bar. Connect the black and red wire ends to the terminals on a 50 amp breaker switch and tighten the terminal screws. Clip the breaker switch into a section on the hot bar with openings for two breaker switches. The main breaker panel will supply power to the subsidiary electrical panel as soon as the main power switch is turned on.
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Tips & Warnings
Power supplied to the main breaker panel does not shut off when the main power switch is turned off. The large wires bringing power to the separated upper terminals in the box are carrying live electricity. Do not make contact with these wires with metal tools or any part of your body.
Do not attempt electrical work unless you are trained and/or experienced with household wiring.
Do not connect the bonding strap to ground the neutral bus inside the subsidiary panel. Only the main panel has the neutral bus grounded.
References
- Photo Credit electric panel image by DXfoto.com from Fotolia.com
