Grounding - Art. 250

Choose the best answer: 1. What size copper bonding jumper (BJ) should you use to bond/ground the secondary of a separately derived system to building steel? The circuit has four No. 3/0 THWN copper conductors.

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

2. What size copper grounding electrode conductor (GEC) should you use to bond and ground the secondary of a separately derived system to building steel? The circuit consists of four No. 3/0 THWN copper conductors.

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

3. In a paralleled 3-phase, 4-wire service with four 600kcmil THWN copper conductors per phase, what size BJ should you use to bond and ground the separately derived system to building steel?

(a) No. 3/0 (b) No. 4/0 (c) 250kcmil (d) 300kcmil

4. In a paralleled 3-phase, 4-wire service with four 600kcmil THWN copper conductors per phase, what size grounding electrode conductor should you use to bond and ground the separately derived system to building steel?

(a) No. 3/0 (b) No. 4/0 (c) 250kcmil (d) 300kcmil

5. What size grounding electrode conductor should you use to bond and ground the secondary of a separately derived system to a driven rod? The circuit consists of four No. 3/0 THWN copper conductors.

(a) No. 8 (b) No. 6 (c) No. 4 (d) No. 2

Answers and Discussion

1. (b). You must size the BJ based on the type and size phase conductors supplying the panel, switch, or other equipment connected to the secondary side of the transformer, per Secs. 250-28(d) and 250-102(c). The BJ shall not be smaller than the sizes shown in Table 250-66 for grounding electrode conductors.

Solution: Find BJ, per Table 250-66.

No. 3/0 THWN Cu requires a No. 4 Cu jumper.

Stallcup's Code Loop: Secs. 250-28(d), 250-30(a)(1), and 250-102(c), and Table 250-66.

2. (a). You must size the GEC based on the type and size phase conductors supplying the panel, switch, or other equipment connected to the secondary side of the transformer.

Solution: Find GEC size, per Table 250-66.

No. 3/0 THWN Cu requires a No. 4 Cu conductor.

Stallcup's Code Loop: Sec. 250 30(a)(2) and Table 250-66.

3. (d). In this case, since the total circular mil area of the paralleled copper service conductors is greater than 1100kcmil, you must size the BJ 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 conductors.

600kcmil x 4 = 2400kcmil.

Step 2: Calculate minimum size BJ, per Sec. 250-28(d) and 250-30(a)(1).

2400kcmil x .125 = 300kcmil.

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

300kcmil requires a 300kcmil jumper.

Stallcup's Code Loop: Secs. 250-28(d), 250-30(a)(1), and 250-102(c), and Table 250-66.

4. (a). Size the GEC, per Table 250-66. If the total circular mil area of the service conductors is greater than 1100kcmil copper and 1750kcmil aluminum, the GEC should be smaller than the BJ.

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

600kcmil x 4 = 2400kcmil.

Step 2: Select proper size GEC, per Table 250-66.

2400kcmil requires a No. 3/0 Cu conductor.

Stallcup's Code Loop: Secs. 250-28(d) and 250-30(a)(2), and Table 250-66.

5. (b). When connecting the GEC to a driven rod or other made electrodes, you size it, per Sec. 250-66(a). It isn't required to be larger than No. 6 copper wire.

Stallcup's Code Loop: Secs. 250-30(a) (3)(c), 250-52, 250-56, and 250-66(a).