Sizing the Grounding Electrode Conductor

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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:
1.
The grounded conductor, if present
2.
The equipment grounding conductor, if present
3.
The metal of conduits, if present
4.
The metal of enclosures, if present
5.
The bonding jumpers bonding together metal enclosures and conduits
6.
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

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