109 explicitly permits metal boxes to be part of the ground-fault current path: Metal enclosures shall be permitted to be used to connect bonding jumpers or equipment grounding conductors, or both, together to become a part of an effective ground-fault current path. Bonding is connecting metallic components to ensure the electrical continuity needed to return the ground-fault current to its source. Figure 1 shows a single-phase, 3-wire overhead service to a building supplying a. An equipment grounding conductor passing through the box without a splice is not required to be joined inside the box to others that are spliced in the box.
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