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Code Quandaries

Aug 1, 2000 12:00 PM, By Mike Holt

Here are the latest short answers to questions posted on our Web site. In our discussion, we will cover topics from Secs. 90-2(b)(4), 250-2(a), 250-30(a)(1), 250-66(a), 800-11(a), and 810-18; and Art. 800 of the National Electrical Code (NEC).

Q. The NEC requires you to ground metal parts of the electrical system to the earth to limit high voltage imposed from lightning, line surges, or unintentional contact with higher voltage systems [Sec. 250-2(a)]. To accomplish this, you must install a conductive path from the metal parts of the electrical system to the earth. What is the largest size copper grounding electrode conductor required to bond the metal parts of the electrical system to a "made electrode," such as a ground rod? How does the size of the service affect the sizing of the grounding electrode conductor to a made electrode?

A. The size of the service has no effect on the sizing of the grounding electrode conductor to a made electrode because this conductor is not sized to carry fault current. As a result, the grounding electrode conductor from the grounded conductor to a "made electrode" is not required to be larger than No. 6 copper or No. 4 aluminum wire. You can find this requirement in Sec. 250-66(a).

Q. We place telephone service entrance cable in the same trench with the utility power cables to serve our residential telephone customers. This practice is in accordance with our company telephone engineering specifications. What are the NEC guidelines regarding "joint trench applications?"

A. NEC Sec. 90-2(b)(4) does not cover the installation of communications equipment under the exclusive control of communications utilities located outdoors. Therefore, it's not an NEC violation to install telephone cables in the same trench with service entrance cable. However, if the telephone wiring is not under the exclusive control of the communication utility, then Art. 800 requirements apply.

Unfortunately, there is no specific rule covering this subject! Sec. 800-11(a) requires separation by brick, concrete, tile partitions, or a suitable barrier for underground circuits entering buildings in a "raceway, handhole, or manhole."

Note: Sec. 810-18 requires you to separate underground "radio and television conductors" at least 12 in. from power circuit conductors.

Q. We recently had a contractor bond the neutral and grounds at both the transformer case and at the secondary disconnect. When asked about bonding at two separate locations, he said the transformer is the same as the power company's transformer, and the secondary disconnect is the same as the service entrance equipment. Is this correct?

A. No. You can only bond transformers (separately derived systems) at one location. You can make this connection at any point on the separately derived system from the source to the first system disconnecting means per Sec. 250-30(a)(1).

A new exception was added to this section of the Code that permits you to install a bonding jumper at both the source and the first disconnecting means, which does not establish a parallel path for the grounded circuit conductor. I recommend you use the neutral-to-ground bond at one location, either at the source or at the first system disconnecting means.