Home Wiring and the Grounding System

The Ground Rod

This ½-inch-diameter copper- clad rod is driven 8 feet straight into the ground at a point out side the house, usually near where the service conductors enter the house. The rod can be driven at an angle not to exceed 45 degrees from the vertical, or laid in a trench 2½ feet deep, if solid rock is encountered less than 8 feet down. No part of the rod can be above ground level once the installation is complete. In place of ground rods, ¾-inch galvanized pipe, rigid metal conduit, or plate electrodes can be used; aluminum grounding electrodes can't .

The Grounding: Electrode / Conductor

A wire called the grounding-electrode conductor connects the ground rod to the neutral bus in the panel. It connects to one of the larger terminals in the neutral bus, and a ground clamp connects it to the ground rod. Typically, this conductor is bare copper and is sized according to the service: No. 8 copper for 100-amp services, No. 6 for 150, and No. 4 for 200. The grounding-electrode conductor must never be spliced.

An additional grounding- electrode conductor connects the interior metal water piping of the house to the neutral bus. This connection can be made to a cold-water pipe at any convenient point, again using a ground clamp at the pipe and one of the larger terminals in the neutral bus. Aluminum wire is often used here because the wire doesn’t contact the earth. Use No. 6, No. 4, and No. 2 or smaller

1 or 1/0

2/0 or 3/0

Over 3/0 through 350 MCM Over 350 MCM through 600 MCM Over 600 MCM through 1100 MCM Over 1100 MCM

2 aluminum wire, respectively, with 100-, 150-, and 200-amp services.

There may be additional lo cal requirements, for example, shielding the ground wire in conduit where it would other- wise be exposed to damage, such as in a garage or driveway where a car could back into it.

Bonding

All the metal conduits and en closures that are part of the service must be bonded (electrically connected) together. Threaded conduit connections and hubs on enclosures and conduit bodies provide adequate bonding. Where conduit is connected to an enclosure with locknuts and bushings, the bushing should be a metal grounding bushing with an insulating insert. This type of bushing has an attached lug that allows a bonding jumper to connect to it. The other end of the bonding jumper attaches to a terminal inside the enclosure.

1/0 or smaller

2/0 or 3/0

4/0 or 250 MCM

Over 250 MCM through 500 MCM Over 500 MCM through 900 MCM; Over 900 MCM through 1750 MCM; Over 1750 MCM

A bonding jumper must be installed around the water meter. Ground clamps and a jumper that’s long enough to permit removal of the meter without having to remove the jumper are used to meet this requirement. Some municipalities also require bonding jumpers around water heaters and spas. The required bonding jumpers must be the same size as the grounding-electrode conductor of the system.

Grounding at a subpanel is a bit different than grounding at the service. The neutral bus isn't bonded to the subpanel enclosure. A separate grounding bus is used to connect the grounding conductors of the cables to the ground wire of the branch feed.

All the equipment grounding in the house is connected to the neutral bus in the service panel. The neutral service conductor which is grounded at the distribution system, connects to the neutral bus along with the grounding- electrode conductors that connect the bus to a driven ground rod and the metal water piping of the house.

Sizes of Grounding Electrode Conductors

Size of Largest Service-Entrance Conductor

Aluminum or Copper-Clad

Copper; Aluminum

Size of Grounding electrode Conductor

Aluminum or Copper-Clad

1/0 3/0

2/0 4/0

3/0 250 MCM

Reprinted with permission from NFPA 70 2011 the National Electrical Code Copyright 2011 National Fire Protection Association Quincy MA 02269 This reprinted material isn't the complete and official position of the National Fire Protection Association on the referenced subject, which is represented only by the standard in its entirety.