Article 250

VIEWING Video FILE requires Real Media Player [ FREE ]

Grounding and Bonding 250.64 Grounding Electrode Conductor Installation The revisions clarify when the raceway for the grounding electrode conductor must be bonded at each raceway/cable termination between the service equipment and the grounding electrode. And additional text clarifies how to size the bonding jumper for the grounding electrode conductor raceway. (E) Enclosures for Grounding Electrode Conductor. Ferrous (iron/steel) raceways, boxes, and enclosures containing the grounding electrode conductors must be made electrically continuous by bonding each end of the ferrous metal raceway, box, and enclosure to the grounding electrode conductor [250.92(A)(3)]. Author’s Comment: “Nonferrous” metal raceways, such as aluminum rigid metal conduit, enclosing the grounding electrode conductor aren’t required to meet the bonding provisions of this section. Where a bonding jumper is used with a bonding bushing or wedge [250.92(B)(4)], the bonding jumper cannot be smaller than the enclosed grounding electrode conductor. Figure 250-22 Author’s Comment: When the ferrous metal raceway (containing the grounding electrode conductor) terminates to an enclosure where no ringed knockout is encountered; the raceway can be bonded by a bonding-type locknut, instead of a bonding bushing/wedge with a bonding jumper [250.94(B)(4)]. CAUTION: The effectiveness of the grounding electrode could be significantly reduced if a ferromagnetic raceway containing a grounding electrode conductor isn’t bonded to the grounding electrode conductor at both ends. This is because a single conductor carrying high-frequency lightning current in a ferrous raceway causes the raceway to act as an inductor, which severely limits the current flow through the grounding electrode conductor. ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) states “an inductive choke can reduce the current flow by 97 percent.” Author’s Comment: To save a lot of time and effort, simply run the grounding electrode conductor exposed if not subject to physical damage [250.64(B)] or enclose it in a nonmetallic conduit that is suitable for the application. Note: The accompanying video clip is in error in regard to the requirement of irreversible compression-type connectors to fasten to a termination busbar [250.64(C)(3)]. This section does not require that the connection be irreversible compression-type connectors, only that connections shall be made by a listed connector or by exothermic welding.

Figure 250-22 (Click on image to enlarge)