Grounding Regulations for Primary Distribution Boxes

Primary distribution boxes must use properly sized copper conductors, ground rods, and bonded connections to ensure system safety and maintain ground resistance below specified limits.

Grounding Conductors

• Conductor Material and Size: Copper is the preferred material. For US markets, a 10 AWG (5.26 mm²) ground wire is standard, while in other regions, a 6 mm² conductor is used for general grounding of distribution boxes and controllers . For connections between mounting plates and equipment like spindles, larger conductors such as 16 mm² (or 6 AWG / 13.29 mm² in the US) are recommended to maintain low resistance .
• Insulated Ground Conductors: Each feeder and branch circuit should include a separate insulated green ground conductor, connected from the panel ground bus to all connected equipment .

Ground Rods and Electrodes

• Rod Specifications: Use 16 mm diameter, 2400 mm long copper-weld rods, or two rods of 1200 mm each, depending on system voltage and grounding requirements .
• Connection to Conductors: Ground rods must be connected with bare copper conductors sized appropriately: 70 mm² for MV grounding and 35 mm² for LV grounding .
• Supplemental Grounding: In addition to rods, building steel, rebar in slabs, and cold water piping can serve as grounding electrodes where available .

Bonding and Connections

• Equipment Bonding: All metal enclosures, panelboards, disconnect switches, starters, pull boxes, and conduits must be bonded to the grounding system using bolted connectors or grounding lugs .
• Conduit and Panelboard Grounding: Conduits over 1-1/4” diameter must have bonded grounding bushings connected to the equipment enclosure. Panel feeders should connect to the ground bus to ensure continuity .
• Raised Floors and Expansion Fittings: Bond raised floor systems to building steel with No. 6 AWG bare copper conductors at each corner, and provide bonding jumpers across expansion or deflection fittings .

Resistance Requirements

• Ground Resistance: The total resistance between system parts should be less than 0.1 Ohm for distribution boxes and connected equipment .
• Testing: Each installation must be tested to ensure compliance with resistance limits, typically maximum 25 Ohms for general grounding systems .

Additional Considerations

• Mechanical Protection: Grounding conductors and rods should be protected against mechanical damage and corrosion, influencing material choice .
• System Coordination: Grounding must safely carry expected fault currents for the duration of fault clearing times, considering soil resistivity and exposure .
• Compliance: Follow applicable standards such as NEC, local utility requirements, and manufacturer guidelines for grounding and bonding . By adhering to these requirements, primary distribution boxes achieve safe dissipation of fault currents, protection against electrical shock, and reliable operation of the electrical system.