Main Service

Service Entrance

A modern residential overhead service entrance comprises three cables — two hot and one neutral — that run from the utility lines to a point of attachment at the weather head, and then down an entrance cable or conduit to the meter socket. In this type of system, the utility company’s equipment ends at the point of attachment on the house. Older systems may have different configurations (below).

Figure: Evaluating an Overhead Service Entrance

The service entrance may run overhead or underground. For either type of entrance, check with the local electric utility for its requirements. Be sure to seal the exterior wall penetration at the service entrance with an approved fitting and/or sealing compound. Water leaks at the service entrance can lead to corrosion of electrical components at the main panel.

Overhead Clearance

Overhead service entrance cables must be higher than 10 ft. above walking surfaces, including a deck or porch. As of the 1999 NEC, this 10-ft. clearance must be maintained at least 3 ft. out from a porch or deck in every direction, including around corners (below).

Figure: Overhead Service Clearance

Underground Service

An underground service cable may be either directly buried or encased in conduit. If in conduit, the raceway must be sized adequately for the conductors that it is carrying. With an underground service, the underground equipment typically belongs to the owner and must be connected to the utility company’s pole or pad-mounted transformer.

Temporary Service

For information on running a temporary service during construction, click here.

Main Disconnect and Distribution Panel

Each hot wire terminates at a breaker or fuse in the main disconnect, which is the first means of over-current protection. The main disconnect is housed either in the distribution panel (below) or in its own separate box.

Figure: Termination of Grounds and Neutrals in Panel Containing Main Service Disconnect

This depends in part on the location of the distribution panel. Many electrical inspectors and utility companies will allow no more than 15 ft. of entrance conductor within the structure before terminating in the main disconnect. This limits the amount of unprotected cable within the house. (Note: Check with your local electrical inspector or utility company for allowable entrance conductor length.)

The rating of each fuse or breaker in the main disconnect should equal the amperage of the electric service. Thus, a 100-amp service (the smallest allowable service for a house by code) will have a main disconnect with a two-pole, 100-amp circuit breaker or two 100-amp fuses.The cable that runs from the meter base to the main disconnect and distribution panel must be rated for the particular service or panel ampacity.

Distribution Panel

The distribution panel takes current from the main disconnect and distributes it via breakers or fuses to the individual branch circuits as shown in Figure: Termination of Grounds and Neutrals in Panel Containing Main Service Disconnect, above.

Each of the two hot wires (red and black) entering the panel terminates at its own live busbar. Single-pole, 120-volt breakers or fuses are connected to either one of these two busbars. Double-pole, 240-volt breakers are connected to both live busbars. In fact, these are really two breakers joined together, and each is connected to a different busbar.

The neutral wire (white) from the main disconnect terminates at a neutral busbar. The grounding wire (green or bare) also terminates at the neutral busbar if the distribution panel contains the service disconnect.

It’s important to note, however, that if the service disconnect is located in a separate box before the distribution panel, the grounding wire does not connect to the neutral busbar. Rather, it connects to a separate grounding busbar in the panel.

The NEC requires that panels be mounted on a backboard and that there be an accurate panel schedule designating which loads are serviced by the individual branch circuit breakers.

Panel Placement

For a 200-amp or smaller panel, code requires clear space of 36 in. in front of the panel that is at least 30 in. wide and extends from the floor to a height of 6 ft. 6 in. (below). No pipes or ducts may cross this space. In addition, the door to the panel must be able to open at least 90 degrees.

Figure: Clearances at the Panel

Never recess the panel in a firewall, such as the wall separating an attached garage from the living space, since this violates fire codes.

Subpanels

A new distribution panel must not exceed 80% full, which means that it must include slots for future add-ons. But an older panel may not have room for additional circuits. If that’s the case, you can enlarge an existing system by adding a subpanel. Subpanels are fed from a double-pole breaker in the main panel and have a 4-wire feeder cable that carries a separate ground and neutral conductor.

It’s important to note that, in a subpanel, branch-circuit ground and neutral wires are connected into separate busbars. They should not be connected into the same busbar, the way they are at the main disconnect. This is a common wiring mistake. A subpanel must be equipped with separate ground and neutral bars, but only the ground bar should be bonded to the panel enclosure (below).

Figure: Termination of Grounds and Neutrals in Subpanel