NEC Grounding
Table 250.66 has separate columns for copper and aluminum service conductors — using the wrong column is a real, easy-to-make mistake.
NEC Table 250.66 sets the minimum size for the Grounding Electrode Conductor (GEC) — the conductor connecting a system's grounded conductor (or the equipment grounding system) to the actual grounding electrode, such as a ground rod. It's a table that looks simple but has one specific trap worth knowing about.
GEC minimum size is determined by the size of the largest ungrounded (service-entrance) conductor or equivalent area for conductors in parallel — not by the load current, and not by a fixed ratio to the service size. It's a lookup, not a calculation.
Table 250.66 has genuinely separate circular-mil breakpoint columns for copper and aluminum service conductors — this isn't a case where you look up copper and then apply a standard conversion factor for aluminum. A service using aluminum conductors requires the size to be read from the aluminum-specific column, which follows different breakpoints than the copper column entirely.
The GEC doesn't carry current under normal operation, which is exactly why undersizing it doesn't cause any visible symptom day to day — there's no nuisance tripping, no dimming, nothing that signals a problem. The consequence only shows up during an actual fault event, when an undersized GEC can fail to safely handle the fault current it's meant to carry to earth.
Before looking up a GEC size, confirm two things: the actual size of the largest service-entrance conductor (or equivalent parallel conductor area), and whether that conductor is copper or aluminum. Both facts determine which row and which column of the table actually apply to your installation.
Get the correct minimum GEC size for your service conductor and material in the NEC Grounding calculator.