The basis of grounding and bonding starts at the connection of a bonding jumper or an equipment-grounding conductor to a box, an enclosure or other electrical equipment and ends at the point of connection to the service-grounded conductor in a grounded system or at the service-equipment enclosure for an ungrounded system.
Section 250.8 requires grounding conductors and bonding jumpers to be connected by exothermic welding, listed pressure connectors, listed clamps or some other listed means of connection. Solder, a very common connection method in the early part of the last century, may not be used as the sole connection method since the heating from a high-energy ground-fault could cause the solder to melt resulting in loss of the grounding path.
Exothermic welding uses a chemical reaction to release heat and results in the welding of two pieces of metal. It is often used for connecting larger grounding conductors together or for connecting grounding conductors to metal boxes, metal beams or metal busbars. A specific chemical shot or load is required for each different conductor size. These loads are inserted into a carbon mold and then attached to both ends of the copper conductors or to a copper conductor and a piece of metal. When the load is ignited, the heat melts the two metals together, creating the bond.
Sheet metal screws must not be used to connect grounding or bonding conductors to metal enclosures since threads on sheet metal screws are too far apart to provide a low-impedance path from the screw to the enclosure. Most metal boxes have at least one hole in the back of the box that is tapped for a 10-32 grounding screw. These holes may have an identifying grounding mark adjacent to the hole or the box may be unmarked. However, Section 250.148 does not permit the grounding screw to be used for any purpose other than grounding. Most grounding screws are manufactured with a shoulder around the screw head to help capture the conductor and many have a self-tapping feature to ensure a good grounding connection to the enclosure.
Wrapping the equipment-grounding conductor around an NM cable clamp screw in an NM box or around the 8/32 screw for the plaster ring or box cover are common methods used in the field, but are not acceptable in accordance with either Sections 250.8 or 250.148(A). A grounding screw or some other listed grounding device must be used.
Copper crimp sleeves are listed pressure connectors designed for crimping specific sizes and specific numbers of copper conductors together. Crimp sleeves may only be used with copper conductors. Copper and aluminum conductors may not be mixed since aluminum is extremely soft and the crimping process may damage the aluminum conductor. These devices require a special crimping tool that will ensure a good, irreversible connection will be made. Crimping with a pair of side cutters or lineman’s pliers is not an acceptable method since these devices may damage the wire and result in a poor connection. Remember, these connections must be able to carry fault currents long enough for the overcurrent protective device to operate during a fault condition.
Where a 10-32 grounding hole is not already drilled and tapped in a box, a listed grounding clip may be used. Grounding clips are designed for termination of the conductor only, so installing a conductor through the clip and then back into the box to connect to a device or a wire nut is not acceptable.
The proper method of installing a ground clip is to feed the equipment-grounding conductor into the grounding clip with the shorter part of the clip placed inside the box, and then use a screwdriver to force the clip onto the metal box, ensuring good contact to the enclosure. Since ground clips are forced onto a box, care should be taken when reusing a clip to ensure a good electrical connection to the box. Also, ground clips should never be forced onto the rounded portion of a plaster ring since a poor connection may result.
Listed green grounding pigtails and listed green wire connectors are relatively new and provide an option for grounding of enclosures and equipment. A green wire connector may have a pigtail at the end or a hole in the end of the wire connector permitting one of multiple bare equipment-grounding conductors to be fed through the connector for termination on a device while still connecting the grounding conductors together. These connection devices are specifically listed for grounding terminations.
The key issue here is to provide a permanent and positive connection for the grounding system to ensure an effective, low-impedance ground-fault current path. EC
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at email@example.com.