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Grounding vs Bonding: 250.66 Grounding Electrode Conductor—Size

February 10, 2006

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PART III. GROUNDING ELECTRODE SYSTEM AND GROUNDING ELECTRODE CONDUCTOR

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Page 12 of 14

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250.66 Grounding Electrode Conductor—Size.

Except for a ground rod electrode [250.66(A)], a concrete-encased electrode [250.66(B)], or a ground ring electrode [250.66(C)], the grounding electrode conductor must be sized based on the largest service-entrance conductor or equivalent area for parallel conductors in accordance with Table 250.66

Question: What size grounding electrode conductor is required for a 1,200A service that is supplied with three parallel sets of 600 kcmil conductors per phase? Figure 250–116

(a) 1 AWG (b) 1/0 AWG (c) 2/0 AWG (d) 3/0 AWG

Answer: (d) 3/0 AWG

The equivalent area of three parallel 600 kcmil conductors is 1,800 kcmil per phase [Table 250.66].

FPN: Because the grounded neutral service conductor is required to serve as the low-impedance ground-fault current path back to the source, it must be sized no smaller than that shown in Table 250.66 [250.24(C)(1)]. Of course, it must be sized to carry the maximum unbalanced load as calculated by 220.61.

(A) Ground Rod. Where the grounding electrode conductor is connected to a ground rod, that portion of the grounding electrode conductor that is the sole connection to the ground rod isn’t required to be larger than 6 AWG copper. Figure 250–117

Author’s Comment: See 250.52(A)(5) for the installation requirements of a ground rod electrode.

(B) Concrete-Encased Grounding Electrode (Ufer Ground). Where the grounding electrode conductor is connected to a concrete-encased electrode, that portion of the grounding electrode conductor that is the sole connection to the concrete-encased electrode isn’t required to be larger than 4 AWG copper. Figure 250–118

Author’s Comment: See 250.52(A)(3) for the installation requirements of a concrete-encased electrode.

(C) Ground Ring. Where the grounding electrode conductor is connected to a ground ring, that portion of the conductor that is the sole connection to the ground ring isn’t required to be larger than the conductor used for the ground ring.

Author’s Comment: A ground ring encircling the building or structure in direct contact with earth must consist of not less than 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4)].

Figure 250–116
(Click on image to enlarge)

Figure 250–117
(Click on image to enlarge)

Figure 250–118
(Click on image to enlarge)

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Comments

We have an network equipment with 660A, what size of ground-rod should be used, 6AWG? If it is OK to use 6AWG, can it be grounded into smaller size (12AWG) by using parallel?

Best Regard, Hung Ton

Home email: hung.ton@embarqmail.com

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