A main lug circuit box or panel is similar to a main service panel except it does not have a main circuit breaker. It is served by a double pole breaker in the main service panel. One hundred amp refers to the maximum current the box is allowed to distribute. The breakers or fuses in the main panel connecting to the main lug must not exceed 100 amps.
Run sheathed feeder cable between the main lug circuit box and main service panel. Choose wire feeder cable with the correct wire sizes: 1 gauge for up to 100 amps, 2 gauge for up to 95 amps, 3 gauge for 85 amps, 4 gauge for 70 amps and 6 gauge for 55 amps.
Remove the cover from the main lug circuit box by removing the screws. Remove a knockout plug from the box by placing the blade of a screwdriver against the plug and tapping the screwdriver sharply. Grasp the knockout with pliers and twist it off. Install a cable clamp in the hole -- many insert into the hole and attach with a nut.
Remove enough sheathing from the cable with a utility knife to allow neat routing of the wires inside the box. Insert the cable through the cable clamp and into the main lug box so about 1/4 inch of sheathing protrudes into the box. Tighten the cable clamp snugly with a screwdriver.
Route the two black hot wires to the two large terminal lugs that connect to the panel's copper circuit breaker buses. Trim the wires with the cutter on lineman's pliers. Remove 1 inch of insulation and insert the wire ends into the terminals. Tighten the terminal screws firmly.
Route the white neutral wire to the neutral bus -- the neutral bus is insulated from the metal box. Remove 1 inch of insulation from the wire, insert it into the large terminal and tighten the screw firmly. Repeat this for the ground wire and the ground bus. Connect the ground bus to the metal main lug box with the supplied grounding strap or connector.
Install the box cover and screws. The main lug box is wired and ready for branch circuit wiring.
Turn the main circuit breaker off and remove the cover screws and cover. Do not touch the large terminals or wires supplying the main circuit breaker -- they are always hot. Remove a knockout plug from the box and install a cable clamp in the hole.
Install the new double pole circuit breaker. Hook the breaker onto the retaining bar and push on it until it clicks into place.
Remove enough sheathing from the cable to allow neat routing of the wires inside the main panel -- up to four feet of sheathing for a large panel. Insert the cable through the clamp and tighten the clamp firmly.
Route the ground wire to the ground bus, identified by bare ground wires attached to it. The ground bus may also serve as the neutral bus. Insert the ground wire into a large terminal and tighten the screw firmly. Repeat this for the white neutral wire and the neutral bus, removing 1 inch of insulation from the wire before installing in the terminal.
Route the two black wires to the new circuit breaker. Trim the wires to length and remove 1 inch of insulation. Insert the wires into the terminals on the circuit breaker and tighten the screws firmly.
Remove two knockout plugs from the main panel cover to accommodate the new double-pole circuit
Neatness counts. Follow the wire routing convention used by the electrician who installed the main service panel. The main lug circuit box can be used as the main service panel if it is connected to a fused, external disconnect. Wire it the same way but bond (connect) the ground bus to the neutral bus in the main lug box. The ground bus must also be connected to the home's copper water pipe entrance with 4-gauge copper wire.
According the National Electric Code, the only place the neutral-to-ground connection may be made is in the main service panel. Do not connect the neutral-to-ground in the main lug box when it is used as a sub-panel, such as in this installation. If you're using the main lug circuit box as a main service panel, remember the fuses in the external disconnect must not exceed the main lug circuit box rating -- 100 amps in this case.