Can the grounding of the distribution box be calculated

Yes, the grounding of a distribution box is typically calculated separately, especially when it is part of a separately derived system.

Understanding Separately Derived Systems

A distribution box connected to a transformer or generator may be part of a separately derived system, meaning it has no direct electrical connection to another source except through grounding and bonding connections. In such cases, the neutral of the distribution box must be bonded to a grounding electrode conductor, and this connection is considered the primary grounding point for that system.

Grounding Calculation

The grounding conductor for a distribution box is sized based on the largest ungrounded conductor in the system, using methods such as the adiabatic formula, which considers prospective ground-fault current and thermal constants. This ensures that the grounding conductor can safely carry fault currents until protective devices operate. The calculation is independent of the upstream service grounding, meaning the distribution box requires its own grounding assessment.

Key Considerations

Single Point Bonding: The neutral-to-ground bond should occur at only one location in the system, typically at the first disconnecting means or at the transformer, to prevent circulating currents. Equipment Grounding: All metallic parts of the distribution box and connected equipment must be bonded to the grounding system to maintain safety. Grounding Electrode: The distribution box may require a separate grounding electrode, especially if it is isolated from the main service or transformer ground.

Compliance: All calculations and installations must comply with NEC Article 250 and local electrical codes.

Practical Implication

Even if the distribution box is downstream of a main service, its grounding is not automatically covered by the upstream system. Properly sizing and connecting the grounding conductor ensures safety, limits touch potential, and allows overcurrent devices to operate correctly during faults. In summary, each distribution box, particularly in separately derived systems, requires its own grounding calculation and proper bonding to ensure code compliance and electrical safety.