Grounding connection between conduit and distribution box

A distribution box must be securely grounded using conduit grounding bushings, bolted ground connectors, and properly sized ground conductors to ensure safety and low-resistance fault paths.

Grounding Conduits and Bushings

All incoming and outgoing conduits connected to a distribution box should be equipped with conduit grounding bushings that are bonded to the equipment enclosure. This ensures that any metallic conduit acts as a safe path for fault currents. For conduits over 1-1/4” diameter, grounding bushings are mandatory at panelboards, pull boxes, and equipment enclosures.

Bonding jumpers should be installed across expansion or deflection fittings to maintain electrical continuity.

Ground Conductors

A separate, insulated green ground conductor must run with each feeder and branch circuit. This conductor connects the panel ground bus to all items connected to the conduits or raceways, including disconnect switches, starters, pull boxes, and transfer switches.

For U

S installations, a 10 AWG (5. 26 mm²) ground wire is typically used, while other markets may require a 6 mm² wire. Larger connections, such as between mounting plates and spindle plates, may require 6 AWG or 16 mm² wires depending on the application.

Bonding and Connections

All metal enclosures, piping systems, and structural steel must be bonded to the service equipment ground bus. Bolted ground connectors or compression lugs are recommended for exposed connections, while exothermic welds (Cadweld) can be used for underground or concrete-encased connections. The ground conductor should be connected to the ground bus within the panel and to all conduit bushings, ensuring a continuous low-resistance path.

Resistance and Testing

The total ground resistance between all system parts should be less than 0. 1 ohm for distribution boxes and controllers. Continuity tests and grounding electrode system resistance tests must be performed to verify proper installation.

Using high-quality copper or copper-bonded steel conductors is critical to prevent corrosion and maintain long-term reliability.

Summary

Install conduit grounding bushings on all incoming/outgoing conduits. Use a separate, insulated ground conductor from the panel ground bus to all connected equipment. Bond all metal enclosures, piping, and structural steel to the grounding system.

Use bolted connectors, compression lugs, or exothermic welds for secure connections. Test for continuity and ensure total ground resistance is below 0. 1 ohm.

Following these steps ensures that the distribution box is safely grounded, protecting equipment and personnel from electrical faults and surges.