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Standard for grounding wire of secondary distribution box

Fiber and communications infrastructure knowledge brief Published 01 Jun 2024

Use this reference to prepare product questions for BAGYOU FIBRE. Check interfaces, capacity, protection and installation requirements against the current product documentation.

For a secondary distribution box, the standard grounding wire is typically a bare copper conductor sized according to the overcurrent protection device, commonly 35 mm² (approximately 2 AWG) for low-voltage applications.

Grounding Conductor Requirements

The equipment grounding conductor (EGC) connects all non-current-carrying metal parts of the distribution box to the grounding system, providing a low-impedance path for fault currents to ensure overcurrent devices trip quickly during faults (NEC Article 250. 122). For secondary distribution boxes, the EGC is usually bare copper, though insulated copper may be used in some installations.

Aluminum is generally avoided for buried or exposed runs due to corrosion concerns. Typical Wire SizesLow-voltage secondary distribution (LV, 120/240V or 277/480V): 35 mm² bare copper conductor is standard for grounding. Medium-voltage secondary systems: 70 mm² copper may be used depending on fault current and OCPD rating.

Sizing is based on the rating of the overcurrent protection device protecting the circuit, as per NEC Table 250. 122. If the ungrounded conductors are upsized for voltage drop, the grounding conductor must also be proportionally upsized to maintain effective fault protection.

Grounding Rods and Connections

Standard grounding rods are 16 mm (5/8 inch) diameter, 2. 4 m long, or two rods of 1. 2 m each, connected with the grounding conductor.

The grounding conductor should be directly bonded to all accessible metal parts of the distribution box and connected to the system neutral at the substation or transformer. For difficult soil conditions, horizontal burial of copper wire at 500 mm depth can substitute for vertical rods, ensuring equivalent grounding performance.

Installation Best Practices

Maintain a maximum ground resistance of 25 ohms for effective fault dissipation. Ensure the grounding conductor runs with the circuit conductors in the same raceway or cable to minimize impedance. Use certified copper conductors to avoid corrosion and ensure long-term reliability.

Test the grounding system after installation to verify resistance and continuity.

Summary

For a secondary distribution box, use a bare copper grounding conductor sized according to the OCPD rating, typically 35 mm² for LV systems, connected to standard grounding rods or equivalent buried conductors. Proper installation ensures safety, compliance with NEC standards, and reliable fault current protection.

Standard for grounding wire of secondary distribution box - BAGYOU FIBRE NETWORKS [PDF]

Standard for grounding wire of secondary distribution box

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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