PART III. GROUNDING ELECTRODE SYSTEM AND GROUNDING ELECTRODE CONDUCTOR
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) 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
(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.
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
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.
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)].
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