If the service panel does not have room for new circuit breakers and you cannot use tandem breakers, a subpanel may be the answer. A subpanel connects to the main service panel with a thick three-wire cable. The feeder breaker in the main panel acts as the main disconnect for the subpanel. Before installing one, consult with an inspector to make sure you do not overload your overall system.
A subpanel has separate bus bars for neutral and ground wires and typically has no main breaker. It may not be labeled "subpanel" but instead be labeled "lugs only." It may be a different brand than the main panel.
Have the inspector approve the subpanel, the feeder cable, and the feeder breaker.
Shut off the main breaker in the service panel before you begin.
Choosing a Feeder Breaker and Cable
Buy a subpanel larger than you need so you're ready for future electrical improvements. The three parts—subpanel, feeder breaker, and cable—should be compatible in capacity.
To figure sizes, add up the wattages of all the new electrical loads and add 20 percent to allow a safety margin. Then divide by 230 to find the amperage you need.
If the new service totals 4,000 watts, for example, add 20 percent (multiply by 1.2) to get 4,800; then divide 4,800 by 230 to get 20.9 amps. A subpanel supplying 30 amps of service will work.
To supply up to 5,700 watts of new power, install a 30-amp subpanel, a 30-amp feeder breaker, and a 10/3 feeder cable. To supply up to 7,500 watts, install a 40-amp subpanel, a 40-amp feeder breaker, and 8/3 cable.
Cable with 8-gauge wires may be difficult to find. If a home center does not carry it, call an electrical supply dealer.
What You Need
- Voltage tester
- Lineman's pliers
- Long-nose pliers
- Mounting screws
- Approved feeder cable
- Staples or cable clamps
- Approved feeder breaker
- Breakers for the new circuits
Step 1: Mount Subpanel
Mount the subpanel about a foot away from the main service panel. Determine how far the wires will have to travel in the subpanel and pull wires (shown) or add cable and strip sheathing accordingly. Remove a knockout slug, slide the wires through, and clamp the cable.
Step 2: Plan Route
At the main service panel, plan the routes for the four wires: ground, neutral, and two hot wires (black and red). Strip the sheathing, remove a knockout slug, and clamp the cable. Route the neutral and ground wires carefully and connect them to their bus bar(s).
Keep Cable Out of Harm's Way
Professional electricians take pride in the way they run wires into a service panel or subpanel. In a well-wired panel, wires route in neat paths around the perimeter, making it easy to tell which wire goes to which breaker. More important, orderly wires are less likely to brush against hot bus bars, which would create a serious fire hazard.
Step 3: Strip Wires
Route, cut, and strip the red and black wires. Connect them to the feeder breaker. Snap the breaker into place.
Step 4: Connect Wires
In the subpanel, route the feeder wires, cut and strip them, and connect to terminals. Connect the black and red wires to the hot bus bars, the neutral wire to the main neutral terminal, and the ground wire to the ground bus bar.
Step 5: Finish Connecting Wires
Run cable for new circuits into the subpanel and clamp the cable. For each circuit, route wires around the perimeter, connect the ground wire to the ground bus bar, the white wire to the neutral bus bar, and the hot wire to a circuit breaker.