Standard Requirements for Grounding Bushings in Distribution Boxes

Grounding bushings in distribution boxes must be bonded to the enclosure, connected with appropriately sized conductors, and maintain a low-resistance path to ensure safety and compliance with NEC standards.Key RequirementsBonding and Connection: All conduit grounding bushings must be bonded together and connected to the equipment enclosure on incoming and outgoing conduits for distribution switchgear, switchboards, and panelboards. This includes conduits over 1-1/4” diameter at panelboards, pull boxes, and equipment. Bolted ground connectors should be provided on all enclosures for disconnect switches, starters, pull boxes, wiring troughs, and similar equipment, connecting to the conduit grounding bushings . Ground Conductors: Each feeder and branch circuit must include a separate, insulated green ground conductor. This conductor should connect from the panel ground bus or source connector to all items connected to the conduits or raceways, bonding to a ground lug within each panel, box, or equipment . Conductor Sizing: For distribution boxes in the US, a 10 AWG (5.26 mm²) ground wire is standard, while in other markets a 6 mm² wire is used. For connections between mounting plates and equipment like spindles, larger conductors (e.g., 6 AWG or 16 mm²) may be required depending on cable length and number of spindles to maintain low resistance . Resistance Requirements: The ground resistance between all system parts should be less than 0.1 Ohm. For longer tool cables or multiple spindles, parallel grounding conductors may be used to ensure the total resistance remains below the safe threshold . Pull Boxes and Panelboards: Ground connectors must be bolted in all pull boxes and connected to grounding bushings on all incoming and outgoing conduits using copper conductors for boxes containing power feeders. Panelboards with a ground bus must have the panel feeder ground conductor connected to the ground bus terminals . Additional Bonding: Bonding jumpers should be installed across all expansion or deflection fittings, and all metal piping systems must be bonded to the service equipment ground bus. Raised floor systems should be bonded to building steel with a No. 6 AWG bare copper conductor at each corner . Material Considerations: Only copper conductors should be used for buried or critical grounding runs, as aluminum corrodes over time. Conductors should match the overcurrent protection rating to avoid bonding issues . Testing and Verification: After installation, grounding systems should be tested to ensure resistance does not exceed 25 Ohms for the overall system, with multiple rods or continuous conductors used if necessary. Approved conductive backfill like bentonite clay may be used in challenging soil conditions .SummaryProper grounding of bushings in distribution boxes involves bonding all bushings to the enclosure, using correctly sized copper conductors, maintaining low resistance, and following NEC and manufacturer guidelines. Regular testing and adherence to material standards are critical to ensure safety, prevent electrical faults, and maintain system reliability .