Sizing the Grounding Electrode Conductor

DOE-HDBK-1092-2004

In grounded systems, the GEC connects to the neutral bar in the service equipment enclosure.

In ungrounded systems, the GEC connects to the grounding terminal bar. It grounds the

following items to the grounding electrode system:

The grounded conductor, if present

The equipment grounding conductor, if present

The metal of conduits, if present

The metal of enclosures, if present

The bonding jumpers bonding together metal enclosures and conduits

The metal enclosure of the service equipment

4.7.1

SIZING THE GROUNDING ELECTRODE CONDUCTOR

NEC 250.66 requires the grounding electrode conductor to be sized by the circular mils rating of

the largest service entrance conductor or conductors and selected from NEC Table 250.66

based on these values.

For example, the size of the service entrance conductors from a delta, three-phase, four-wire

midpoint tap is #250 kcmil, THWN copper for phases A and C, #2/0 for phase B, and #1/0 for

the neutral. What size copper GEC is required to ground this system to a metal water pipe?

Step 1:

Finding the largest phase-NEC 250.66 #250 kcmil is the largest phase

Step 2:

Finding the size GEC-NEC Table 250.66 #250 kcmil requires #2 cu

Answer: The size of grounding electrode conductor (GEC) is at least #2 copper.

Note: NEC Table 250.66 is used to size the grounding electrode conductor for both grounded

and ungrounded systems. The table is used where the grounding electrode conductor is

connected to a metal water pipe or the metal frame of building steel.

4.7.2

EXCEPTIONS TO NEC 250.66

There is an exception to the main rule. It has three parts and pertains to specific types of

grounding electrodes. The exception applies to grounded and ungrounded systems.

Exception (A) applies to made electrodes only, such as rod, pipe, or plate electrodes. The

grounding electrode conductor is not required to be larger than #6 copper or #4 aluminum.

Exception (B) to NEC 250.66 requires at least a #4 copper conductor to be used as a grounding

electrode conductor to ground the electrical system to a concrete-encased electrode.

Exception (C) requires at least a #2 copper conductor to be used as a grounding electrode

conductor to ground the electrical system to a ground ring.

4-8