Grounding wires in distribution boxes must use copper conductors of appropriate size, securely connected to the enclosure and grounding system, with total resistance kept below specified limits.

Wire Material and Type

• Copper is the standard material for grounding wires, especially for buried or exposed runs, as aluminum corrodes over time and reduces conductivity .
• For stainless steel enclosures, bonding connectors and flexible copper braided tapes are recommended to prevent electrochemical corrosion and maintain low contact resistance .
• Coatings or oxide layers on contact surfaces should be removed, and conductive paste can be applied to prevent oxidation and moisture-related resistance increases .

Wire Size

• In the US market, a 10 AWG (5.26 mm²) copper wire is typically used for grounding distribution boxes .
• For other international markets, a 6 mm² copper wire is standard .
• For connections between mounting plates and equipment with longer cables or multiple spindles, larger wires such as 16 mm² or 6 AWG (13.29 mm²) may be required to maintain low resistance .
• Wire sizing should also match the overcurrent protection to avoid bonding issues with downstream devices .

Connection and Bonding

• Ground wires should be connected to dedicated ground studs or bus bars inside the distribution box .
• Flexible copper braided tapes are recommended for moving parts like doors to maintain consistent contact .
• Use anti-loosening washers and ensure proper torque on threaded connections to prevent loosening over time .
• Bond all metal piping and conduit systems to the panel ground bus using separate insulated green conductors for each feeder and branch circuit .
• Bonding jumpers should be installed across expansion or deflection fittings to maintain continuity .

Resistance Requirements

• The ground resistance between all system parts should be less than 0.1 Ohm for distribution boxes .
• For larger systems, multiple rods or conductors may be required to achieve a maximum 25-ohm resistance for the overall grounding system .
• Continuity tests and grounding electrode system resistance tests should be performed to verify proper installation .

Environmental Considerations

• In outdoor or high-humidity environments, condensation can increase contact resistance; proper surface preparation and conductive paste application are essential .
• Stainless steel enclosures require careful material matching and corrosion prevention to maintain long-term grounding reliability . By following these material specifications, wire sizing, connection methods, and resistance requirements, distribution boxes can achieve safe, reliable grounding that protects both equipment and personnel.