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The requirement for sizing a single equipment grounding conductor for multiple circuits is based on the largest overcurrent device that is part of the installation. This approach ensures that the grounding conductor is capable of safely carrying the fault current that might occur during a fault condition, like a short circuit.

When the grounding conductor is sized according to the largest overcurrent protective device, it ensures that the conductor can effectively protect against overload and fault conditions. The grounding system must be able to carry enough fault current to facilitate operation of overcurrent devices so they can clear faults quickly, thus minimizing the risk of shock and fire hazards.

Choosing the size based on the largest circuit rather than averaging or using the smallest circuit ensures liability and safety across the entire range of connected equipment, as larger devices may present greater fault currents that smaller circuits cannot handle.