Grounding - Art. 250

Choose the best answer:

1. What size copper grounded (neutral) conductor does the National Electrical Code (NEC) require if the service-entrance conductors are rated at No. 250kcmil THWN copper?

(a) No. 6

(b) No. 4

(c) No. 2

(d) No. 1/0

2. What size copper main bonding jumper does the NEC require if the service-entrance conductors are rated at No. 250kcmil THWN copper?

(a) No. 4

(b) No. 2

(c) No. 1/0

(d) No. 2/0

3. What size copper equipment-bonding jumper does the NEC require if the service-entrance conductors are rated at No. 250kcmil THWN copper?

(a) No. 2

(b) No. 1/0

(c) No. 2/0

(d) No. 3/0

4. What size copper grounding electrode conductor does the NEC require to ground a service to a metal water pipe supplied by No. 250kcmil THWN copper conductors?

(a) No. 8

(b) No. 6

(c) No. 4

(d) No. 2

5. What size copper grounding electrode conductor does the NEC require to supplement the metal water pipe in Question 4 to a driven rod?

(a) No. 8

(b) No. 6

(c) No. 4

(d) No. 2

answers and discussion

1. (c). The grounded (neutral) conductor must be routed with the ungrounded (phase) conductors and shall not be smaller than the required grounding electrode conductor (GEC), per Table 250-66. However, it's not required to be larger than the largest ungrounded service-entrance (phase) conductor. Note: The grounded (neutral) conductor must also comply with Secs. 220-22 and 310-15(b)(4)(c).

Step 1: Find the appropriate grounded (neutral) conductor, per Table 250-66.

Solution: No. 250kcmil Cu requires No. 2 Cu.

Stallcup's Code Loop: Secs. 250-24(b)(1), and 250-142(a)(1), and Table 250-66.

2. (b). The size of the service main bonding jumper (MBJ) is based on the size of the service-entrance conductors. You size it, per Table 250-66.

Step 1: Find the MBJ, per Table 250-66.

Solution: No. 250kcmil Cu requires No. 2 Cu.

Stallcup's Code Loop: Sec. 250-28(d) and Table 250-66.

3. (a). The size of the equipment-bonding jumper (EBJ) on the supply side of the service is based on the size of the service-entrance conductors. You size it, per Table 250-66.

Step 1: Size EBJ on supply side, per Table 250-66.

Solution: No. 250kcmil Cu requires No. 2 Cu.

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

4. (d). The procedure for selecting the proper size GEC to ground the service to a metal water pipe is determined based on the size of the service-entrance conductors. Note: There is no OCPD ahead of these conductors.

Step 1: Size the GEC, per Table 250-66.

Solution: No. 250kcmil Cu requires No. 2 Cu.

Stallcup's Code Loop: Secs. 250-66, 250-104(a), 250-50, and 250-50(a), and Table 250-66.

5. (b). You may install the copper tubing or metal water pipe in a new installation in the earth or above grade. Either installation requires an additional electrode to supplement the copper or metal water pipe. This additional grounding electrode must conform with Sec. 250-50(b) through (d) or Sec. 250-52(a) through (c).

Step 1: Size the GEC, per Sec. 250-66(a).

Solution: No. 250kcmil Cu requires No. 6 Cu.

Stallcup's Code Loop: Secs. 250-50(a)(2) and 250-66(a).

Next month's "Quizzes on the Code" will contain questions on sizing the grounded (neutral) conductor, main bonding jumper, and equipment-bonding jumper for a paralleled installation.

Each quiz on the 1999 National Electrical Code is provided with Stallcup's Code Loops to show the direct correlation with other sections of the NEC. This allows the reader to review other sections of the NEC when determining if the correct answer to a problem is in one or multiple sections of the Code.