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Equipment grounding conductors are sized based on the rating of the overcurrent device protecting the circuit. The EGC must be large enough to carry fault current back to the source long enough for the protection to trip. For aluminum conductors used as the EGC with a circuit protected by 125-amp fuses, the table specifies 4 AWG aluminum. This size provides the necessary fault-current path while considering aluminum’s lower conductivity compared to copper. So in a raceway carrying feeder supply conductors protected at 125 A, the required aluminum EGC is 4 AWG. Smaller sizes would not meet the rule, while larger sizes would be more than necessary.