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Quizzes on the Code

Nov 1, 2000 12:00 PM, By James Stallcup

Grounding - Art. 250

Choose the best answer:
1. In a paralleled 3-phase, 4-wire service with three 700kcmil THWN copper conductors per phase, what size grounded (neutral) conductor does the NEC require in each run?

(a) No. 6 (b) No. 4 (c) No. 2 (d) No. 1/0

2. In a paralleled 3-phase, 4-wire service with three 700kcmil THWN copper conductors per phase, what size main jumper does the NEC require to ground the bus bar to the service equipment enclosure?

(a) No. 1/0 (b) No. 4/0 (c) 250kcmil (d) 300kcmil

3. In a paralleled 3-phase, 4-wire service with three 700kcmil THWN copper conductors per phase, what size individual equipment-bonding jumper does the NEC require to bond each service raceway?

(a) No. 1/0 (b) No. 2/0 (c) No. 3/0 (d) No. 4/0

4. In a paralleled 3-phase, 4-wire service with three 700kcmil THWN copper conductors per phase, what size equipment-bonding jumper does the NEC require to bond all service raceways?

(a) No. 4/0 (b) 250kcmil (c) 300kcmil (d) 500kcmil

Answers and Discussion

1. (d). You size the grounded conductor no smaller than 12.5% of the area of the largest service-entrance (phase) conductors larger than 1100kcmil copper or 1750kcmil aluminum. Base the grounded conductor on the total circular mil area of the service-entrance conductors, when installed in parallel. When you install grounded conductors in two or more raceways, size them based on the size of the ungrounded service-entrance conductors, but not smaller than No. 1/0.

Step 1: Calculate total circular mil area of service entrance conductors.

700kcmil x 3 = 2100kcmil

Step 2: Calculate minimum size grounded conductor, per Sec. 250-24(b)(2).

2100kcmil x .125 = 262.5kcmil

Step 3: Calculate proper size conductor for each conduit run.

262.5kcmil/3 = 87.5kcmil

Step 4: Select proper size grounded conductor for each conduit run, based on Table 8 in Ch. 9.

87,500 CM requires a No. 1/0 Cu conductor.

Stallcup's Code Loop: Secs. 250-24(b)(1), 250-24(b)(2), 310-4, and Table 8, Ch. 9.

2. (d). If the service-entrance conductors are larger than 1100kcmil copper or 1750kcmil aluminum, size the main bonding jumper (MBJ) to be at least 12.5% greater than the area of the largest ungrounded (phase) conductor.

Step 1: Calculate total circular mil area of service entrance conductors.

700kcmil x 3 = 2100kcmil

Step 2: Calculate minimum size MBJ, per Sec. 250-28(b).

2100kcmil x .125 = 262.5kcmil

Step 3: Select proper size MBJ, per Table 8, Ch. 9.

262.5kcmil requires a 300kcmil conductor.

Stallcup's Code Loop: Sec. 250-28(d) and Table 8, Ch. 9.

3. (b). If the ungrounded service-entrance conductors aren't larger than 1100kcmil copper or 1750kcmil aluminum, size the equipment-bonding jumper (EBJ) on the supply side of the service, per Table 250-66.

Step 1: Select proper size EBJ on the supply side of the service, per Table 250-66.

700kcmil Cu requires a No. 2/0 Cu conductor.

Stallcup's Code Loop: Sec. 250-102(c) and Table 250-66.

4. (c). If the service-entrance conductors are larger than 1100kcmil copper or 1750kcmil aluminum, size the EBJ to be at least 12.5% greater than the area of the largest ungrounded (phase) conductor.

Step 1: Calculate total circular mil area of service entrance conductors.

700kcmil x 3 = 2100kcmil

Step 2: Calculate minimum size EBJ, per Sec. 250-102(c).

2100kcmil x .125 = 262.5kcmil

Step 3: Select proper size EBJ, per Table 8, Ch. 9.

262.5kcmil requires a 300kcmil Cu conductor.

Stallcup's Code Loop: Sec. 250-102(c) and Table 8, Ch. 9.

Next month's "Quizzes on the Code" will contain questions on sizing the bonding jumper and grounding electrode conductor for a separately derived system.