How to read NEC Table 250.66 grounding electrode conductor sizing

Sizing the GEC without the guesswork

Look up Table 250.66 in the free NFPA viewer. NFPA offers read-only access to current NEC editions at no cost. This page explains how to use the table; it does not reproduce it.

The grounding electrode conductor (GEC) ties the service — or a separately derived system like a transformer secondary — to the grounding electrode system: building steel, the metal water pipe, the Ufer in the footing, ground rods. Table 250.66 sizes it by the largest ungrounded conductor, with separate columns for copper and aluminum service conductors and for a copper or aluminum GEC. The table also has a top row — once the service is large enough, the GEC stops growing.

The electrode-type limits. Three electrode types get a smaller ceiling when the conductor is the only connection to them: rods, pipes, and plates (250.66(A)); concrete-encased electrodes (250.66(B)); and ground rings (250.66(C)), which never need a conductor larger than the ring itself. That’s why a large commercial service can still land a small jumper on its rods.

Worked example. An 800 A service runs three parallel sets of 350 kcmil copper. One phase totals 1050 kcmil, which falls in the Table 250.66 row that calls for a 2/0 copper GEC — that’s the conductor to the water pipe and building steel. The jumper that only goes to the rods can stop at 6 AWG copper (250.66(A)), and the one that only goes to the Ufer at 4 AWG copper (250.66(B)).

Don’t mix up the two grounding tables: the equipment grounding conductor follows the breaker size — see the equipment grounding conductor chart (NEC 250.122). To size the service conductors themselves, start with the NEC 310.16 ampacity chart and the standard breaker sizes guide. Field PM keeps grounding inspections, photos, and as-builts with the job, so the record is there when the inspector asks.