Sometimes a service panel does not have enough space for a new circuit breaker. When this is the case, the need arises for a sub-panel. A sub-panel should be wired by a qualified electrician. When purchasing a sub-panel, make sure it comes with a grounding bar and has enough space for your circuit breaker's needs. Sub-panels do not need bonding between ground and neutral bars; this is only done at the main panel. However, you still need a grounding bar on the new sub-panel.
Mounting the Sub-Panel
Find a good, dry location for the new service panel. The panel should be installed between two and six feet from the finished floor level. Service panels cannot be installed in bathrooms or closets.
Level the service panel on the wall and mark the four mounting screw holes on the panel with a marker. These holes will then be used to secure the panel to the wall using the screws that come with the panel. It is best to mount the panel on a wooden surface.
Mount the panel on the wall, then open one of the knock outs on the side of the panel. This is the entry point for the metal conduit and the #6-gauge electric wire as well.
Run 1-inch EMT conduit between the main panel and the sub-panel. To wire a 30-amp sub-panel, you need a 1-inch conduit or larger. Install the conduit fittings, lock nut and the plastic bushing at the end of the run.
Wiring the Sub-Panel
Feed a #6-gauge white wire to the neutral bar on the service panel. The neutral bar should be mounted on the right side of the service panel. The wire that is connected to this bar should be white or have a white marking on it.
Feed a #6-gauge green wire to the ground bar on the service panel. The ground bar should be mounted on the left side of the service panel. The wire that is connected to this bar should be green or have a green marking on it.
Feed two live wires to the hot terminals on the sub-panel. One is #6-gauge red wire, the other is #6-gauge black electric wire. It doesn't matter which wire is connected to which terminal, as long as both are connected to the hot terminals. Your panel is now ready for the new circuit breakers.
Feed the white wire coming from the sub-panel to the neutral bar on the main service panel. On the service panel the neutral bar has a lot of other white wires connected to it. Find an opening in the neutral bar that fits the wire size.
Duplicate Step 4 for the ground wire. Some service panels may not have a separate ground bar for the ground wire. If that's the case, attach both the white and the green wires to the neutral bar. Tighten the terminal screws.
Install the 30-amp 240-volt breaker in the main panel. Connect the two hot wires feeding power to the sub-panel to the circuit breaker's terminal screws. Label the circuit breaker "Sub Panel." Open a place in the main panel cover for the new circuit breaker, then close the main panel cover.
Turn the power on by switching the circuit breaker to the "On" position.