NEC Table 250.66 GEC Sizing

How to use this table

Find the largest ungrounded service-entrance conductor in the left column and read the required GEC in copper or aluminum. For parallel service conductors, the printed table works on the equivalent area of the set: add the circular mils of one phase's parallel conductors together and size from that total. The lookup above lists each standard size individually, which is how the JobVolt grounding engine reads the same table.

The electrode caps

Certain electrodes cannot use more conductor than they can deliver. A connection that serves only a rod, pipe or plate electrode never needs a GEC larger than 6 AWG copper or 4 AWG aluminum · 250.66(A). A connection serving only a concrete-encased electrode never needs more than 4 AWG copper · 250.66(B). A ground ring connection never needs to be larger than the ring conductor itself · 250.66(C). An aluminum GEC also cannot terminate within 18 inches of the earth · 250.64(A).

GEC is not EGC

The grounding electrode conductor and the equipment grounding conductor answer different questions. The EGC runs with the circuit conductors and is sized from the overcurrent device per Table 250.122. The JobVolt app's grounding calculator sizes both, applies the electrode caps and the 250.122(B) upsizing rule, and cites each step.