Grounding—Art. 250

Choose the best answer:

1. What size copper grounding electrode conductor (GEC) is required for a metal water pipe when the service conductors are No. 400 kcmil THWN copper?

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

2. What size copper GEC is required for a concrete-encased electrode when the service conductors are No. 400 kcmil THWN copper?

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

3. What size copper GEC is required for a driven rod when the service conductors are No. 400 kcmil THWN copper?

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

4. What size copper GEC is required for a ground ring when the service conductors are No. 400 kcmil THWN copper?

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

5. The GEC shall be connected to the grounded service conductor:

(a) Within 5 ft of the service equipment on the load side

(b) At any point on the load side of the service disconnecting means

(c) At a subpanel outside the building

(d) At any accessible point on the load side of the service drop or service lateral.

Answers and discussion

1. (d). The procedure for selecting the GEC to ground the service to a metal water pipe is determined by the size of the service-entrance conductors.

Find appropriate size GEC using Table 250-66

No. 400 kcmil cu = No. 1/0 cu.

Stallcup’s Code Loop: Secs. 250-50 and 250-50(a), Table 250-66, and Sec. 250-104(a)(1).

2. (b). You can use lengths of rebar 1/2 in. in diameter and at least 20 ft in length as a grounding electrode to ground the service equipment. The size of the GEC must be a No. 4 or larger copper conductor per Secs. 250-66(b) and 250-50(c). The rebar system can also consist of a number of pieces of rebar that are spliced together to form a 20-ft length. A concrete-encased electrode can also be a No. 4 copper conductor instead of 1/2-in.220-ft rebar.

Find appropriate size GEC using Secs. 250-66(b) and 250-50(c)

No. 400 kcmil cu = No. 4 cu.

Stallcup’s Code Loop: Secs. 250-66(b) and 250-50(c).

3. (a). You can ground the service equipment with a driven rod or plate in cases where there are no other electrodes available as per Sec. 250-50. A driven rod with a resistance of 25 ohms or less is considered acceptable to allow the grounded system to operate safely and function properly. If you use the driven rod as a supplementary grounding electrode to the metal water system, connect it to the grounded terminal bar in the service equipment panelboard. The Code does not require the grounding electrode conductor to be larger than No. 6 copper or No. 4 aluminum, where connected to made electrodes such as driven rods.

Find size GEC using Sec. 250-66(a)

No. 400 kcmil cu = No. 6 cu.

Stallcup’s Code Loop: Secs. 250-50(a)(2), 250-52(c), and 250-66(a).

4. (c). A bare copper conductor, no smaller than No. 2 and at least 20 ft long, shall be installed to create a ground ring that will encircle a building or structure. You can use this ring as the main grounding electrode, if necessary.

Find appropriate size GEC using Sec. 250-50(d) and 250-66(c)

No. 400 kcmil cu = No. 2 cu.

Stallcup’s Code Loop: Secs. 250-50(d) and 250-66(c).

5. (d). AC grounded systems are required to be grounded at each service and at the outside transformer. A GEC is usually installed from the neutral bus terminal to a grounding electrode system, which could be a cold water pipe, driven rod, other electrode, or any combination per Secs. 250-50(a) through (d). The GEC is installed and connected to the grounded service conductor at any accessible point on the load side of the service drop or service lateral. A terminal or bus is used to connect the grounded service conductor at the service disconnection means.

Stallcup’s Code Loop: Sec. 250-24(a)(1) and 250-50(a) through (d).