What is the smallest GEC that can be installed to building steel when a service panel is fed with four 3/0 AWG copper conductors?

To determine the smallest grounding electrode conductor (GEC) that can be installed to building steel when a service panel is fed with four 3/0 AWG copper conductors, one must refer to the National Electrical Code (NEC) requirements that govern the sizing of grounding conductors.

According to NEC Table 250.66, the minimum size of the GEC for a service that has a given size of conductors is specified. The size of the grounding electrode conductor is directly related to the size of the service entrance conductors. When service conductors are 3/0 AWG copper, the NEC specifies that the minimum grounding electrode conductor size that can be used is 4 AWG copper for grounding purposes, but when connecting to building steel, which acts as a structural grounding point, the code allows for a more qualified sizing, which in this instance would allow for the 4 AWG size.

The solid reasoning behind using a 4 AWG as the smallest GEC when feeding a service with larger conductors is due to ensuring adequate grounding capabilities to safely handle any potential fault currents that might occur. This helps protect both equipment and personnel from electrical hazards.

Understanding this context allows one to see why the smallest GEC in this scenario