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Grounding current of the distribution box

Fibre interconnection and data room reference Published 24 Apr 2026

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

The grounding current for a distribution box should be sufficient to allow protective devices to operate during a fault, with the grounding conductor sized according to the overcurrent protection device rating and a maximum grounding resistance of 25 ohms.

Key Considerations

Grounding Purpose: The grounding system provides a low-impedance path for fault current, stabilizes voltage to earth, and ensures overcurrent protective devices (OCPDs) operate correctly during faults. Proper grounding prevents equipment damage, electrical fires, and shock hazards. Grounding Conductor Sizing: According to NEC Table 250.

122, the equipment grounding conductor (EGC) must be sized based on the rating of the OCPD protecting the circuit. For example, a 100 A breaker typically requires a #8 AWG copper EGC, while larger breakers require proportionally larger conductors. The grounding electrode conductor (GEC) connects the distribution box to the grounding electrode system (e.g., copper-bonded rods, plates, or concrete-encased electrodes) and must also comply with NEC sizing rules.

Grounding Current: For low-voltage distribution systems, the grounding current should be sufficient to trip protective devices without causing equipment damage. Solidly grounded systems allow full fault current to flow, while high-resistance grounded (HRG) systems limit current to 5–10 A to prevent maloperation. Low-resistance grounded (LRG) systems typically allow 100–600 A, enough to operate protective relays safely.

Grounding Resistance: The NEC and industry best practices recommend a maximum grounding resistance of 25 ohms for distribution boxes. If the measured resistance exceeds this, additional rods or electrodes should be installed and bonded with continuous conductors. Installation Tips: Use a dedicated ground bar in the distribution box; avoid double-tapping neutrals.

Ensure all EGCs run in the same raceway or cable as the circuit conductors to minimize impedance. Test the grounding system after installation to verify resistance and continuity. Summary: The grounding current is determined by the system type and OCPD rating.

For solidly grounded low-voltage systems, the full fault current flows through the grounding path.

For HR

G systems, limit the current to 5–10 A. Always size the grounding conductor per NEC Table 250. 122 and maintain grounding resistance below 25 ohms to ensure safety and compliance.

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Grounding current of the distribution box
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