DIY Guide: How to Wire a Sub Panel

You might need a sub panel if there aren’t enough circuit breakers in your main electrical service panel. To connect one to your main service panel, you’ll need a thick three-wire cable. While it’s best to consult with an electrical contractor, here are the basic things you need to know about installing an electrical sub panel.

How a Sub Panel Works

This component is suited for homes that can’t accommodate tandem circuit breakers, and don’t have room for new breakers. The main panel’s feeder breaker works as the main disconnect for the subunit. There is usually no main breaker here, and separate bus bars for neutral and ground wires.

What You’ll Need

Tools you’ll need for installation include a screwdriver, hammer, voltage tester, flashlight, wire strippers, lineman’s pliers, and long-nose pliers.

For materials, you’ll need the sub panel, mounting screws, a feeder cable approved for the application, staples or cable clamps, circuit breakers, and an approved feeder breaker.

Cable to Use

Only 600-volt copper wire should be used. Here are the appropriate sizes/amp ratings:

30 amp: 10 gauge
20 amp: 12 gauge (yellow)
15 amp: 14 gauge (white)
15-60 amp: 6 gauge
40-50 amp: 8 gauge

Always use copper for a ground wire (and never as a neutral wire). Nonmetallic flexible cables must be used to carry ground wire, while armored steel cable can be used as a grounded connection. And make sure to use insulated electrical tools to avoid injury.

Sub panels hold two to 12 breakers. For a six-space panel used indoors, a 100-amp wire size is needed, while a two-space, outdoor panel can work with a 60, 100, or 200-amp wire. Always check the product rating to make sure you’re using the correct wires.

Indoor vs. Outdoor Panels

Indoor sub panels are usually mounted between 2 x 4 studs and are not weathertight. They must be attached to the structure and not mounted to drywall. Mounting must be done so that the cover fits flush against the metal box once the drywall is installed.

Outdoor sub panels are weather resistant. They should be placed in a sheltered, shaded area and at least 12 inches above where snow is likely to accumulate. The panel must be mounted to a permanent structure and where equipment cannot impact it.

Wiring the Sub Panel

When mounting the sub panel, make sure it’s about 1 foot away from the main service panel and measure how far each wire will need to travel.
Plan the routes for the ground, neutral and black and red hot wires.
Strip enough wire sheathing to attach the wire; then remove a knockout slug and clamp the cable in place.
Route neutral and ground wires and connect them to their respective bus bars. Then route, cut, and strip the red/black wires and connect them to the feeder breaker.
Cut and strip the feeder wires and connect them to the proper terminals. The red/black wires connect to the hot bus bars, while the neutral (white) wire goes to the main neutral terminal. Connect the ground wire to the ground bus bar.
Once new circuit cables are run into the sub panel, clamp each cable and route the wires for each circuit around the perimeter.

Sub Panel Wiring Tips

Make Sure Wires Are Tight: Wire insulation should be stripped such that there’s only copper wire under the screw.
Use Double-Pole Breakers: 208–240-volt breakers must have connected switches so both legs are either on or off (separate single-pole breakers are a code violation).
Installing a Push-Down Breaker: The breaker notch sits against the rail or a cleat; rotate the breaker down on the Busbar and then push down.
Installing a Hook-On Breaker: Hooks or snaps over the round rod. To secure, rotate the breaker down and snap it over the Busbar.
120 Volt/240 Volt Breakers: A 120-volt breaker circuit can be wired to up to 12 outlets, while a 240-volt circuit is typically wired to a single appliance, such as a water heater or air conditioner.
For a Square D Breaker Box: Replace a 240-volt breaker with a new one or remove two 120-volt breakers to accommodate the new one; add a tandem breaker if there are no extra slots.

Contact Express Electrical Services

Breaker boxes are powerful, and important, components in your home. Adding a sub panel requires a variety of technical steps, but our trained, licensed Los Angeles electricians can safely add one to distribute power to specific rooms or appliances. Call 323-727-7799 to schedule a service appointment today!