Send to a Friend View / Add Comments  
June NEC Questions 2005 Part 2

By Mike Holt for EC&M Magazine

June Part 2 Questions

11. What are the requirements for running air duct and or water pipe above indoor switchboards, panelboards, distribution boards, and motor control centers?

A. The footprint space (width and depth of the equipment) extending from the floor to a height of 6 ft above the equipment or to the structural ceiling, whichever is lower, must be dedicated for the electrical installation. No piping, duct, or other equipment foreign to the electrical installation can be installed in this dedicated footprint space [110.26(F)(1)(a)]. Figure 110-50

Foreign systems such as air ducts and water piping can be located above the dedicated space if protection is installed to prevent damage to the electrical equipment from condensation, leaks, or breaks in the foreign systems 110.26(F)(1)(b)].

12. What are the grounding requirements for a satellite dish?

A. According to 810.20, each lead-in conductor from an outdoor antenna must be provided with a listed antenna discharge unit located nearest the point of entrance, but not near combustible material. The antenna discharge unit and the mast or metal structure that supports an antenna must be grounded to one of the following [810.21(F)(1)]:

  • Building or structure grounding electrode system [250.50].
  • Interior metal water piping system, within 5 ft from its point of entrance [250.52(A)(1)].
  • Accessible service bonding means [250.94].
  • Metallic service raceway.
  • Service equipment enclosure.
  • Grounding electrode conductor or the grounding electrode conductor metal enclosure.

If a ground rod is installed for the antenna, it must be bonded to the building's power grounding electrode system with a minimum 6 AWG conductor [810.21(J)] to reduce voltage differences that may develop between the building's power and the radio and television equipment grounding electrode system during lightning events. Figure 810-12

13. What is the smallest conductor required for a 9 kW, 240V single-phase electric space heater that contains a 3A blower motor if the terminals are rated for 75°C conductor sizing?

A. According to 424.3(B), the branch-circuit conductors and overcurrent protection device for electric space-heating equipment must be sized no less than 125 percent of the total load.

Step 1: Determine the heating load
I = VA/E
I = 9,000 VA/240V
I = 37.5A

Step 2: Conductor size at 125% of the total heating load.
Conductor Size = (37.5A + 3A) x 1.25
Conductor Size = 50.63A, round up to 51A
Conductor required would be 6 AWG, rated 65A at 75°C is required [Table 310.16].

14. I have a 25' metal light pole on which are mounted speakers along with stage lighting. The pole is hollow and the lighting and audio circuits have been run together inside the pole. I am in the process of pulling these poles for rehab and need to know if I am required to separate the audio cables from the power conductors within the pole?

A. The output wiring for audio circuits must be in accordance with the marking for use with the specific class of wiring method. Typically audio equipment marking indicates that Class 2 wiring methods in accordance with 725.82 are permitted. According to the requirements of 725.55(A), Class 2 conductors and power conductors must not be placed in any enclosure or raceway with power conductors, unless the power conductors or Class 2 circuit conductors are contained in a Chapter 3 wiring method [725.55(J)]. So just be sure the power conductors are contained in a Chapter 3 wiring method such as UF cable, ENT, Flex, etc. and you're good to go.

15. What is the minimum step-back working space required for a 277/480V system, when there are exposed live parts on one side and grounded parts on the other side?

A. Well… if you do the measurement in inches, the minimum distance measured from the enclosure front must not be less 3 1/2 ft (42 inches). However, if you use a metric tape measuring device you need 1 meter, which by the way is about 39 inches [90.9(D) and Table 110.26(A)(1)].

Note: I don't make the rule's, I'm just the messenger…

16. What is the largest size transformer permitted to be mounted above a suspended ceiling used for environmental air?

A. Dry-type transformers, rated not more than 50 kVA, are permitted above suspended ceilings or in hollow spaces of buildings, if not permanently closed in by the structure [450.13(B)].

17. I am entering a building with a 600 pair underground telephone cable and I need to run this cable a few hundred feet above a suspended ceiling. Can I use EMT for this purpose?

A. No. Communications wires and cables installed within buildings must be listed in accordance with 800.113 and 800.179. However, unlisted communications cable (underground) is permitted if the length of the cable within the building from its point of entrance doesn't exceed 50 ft and the cable terminates in an enclosure [800.113 Ex. No. 2].

Note: The point of entrance is defined as the point where the cable emerges from an external wall, from a concrete floor slab, or from a rigid metal conduit or an intermediate metal conduit that is grounded (bonded) to an electrode in accordance with 800.100 [800.2]. This is why EMT can't be used.

18. I recently came across a handhole with a metal cover in the backyard of a house, that is used to splice the underground service conductors for the home. I told the utility mechanic that the cover needed to be grounded (bonded) since there could be accidental contact with live electrical circuits. He said (basically take a hike) that he did not have to ground the metal cover because utility companies comply with the National Electric Safety Code (NESC), not the National Electrical Code (NEC).

A. According to the NEC, metal covers and other exposed conductive surfaces of handholes must be effectively bonded to an effective ground-fault current path in accordance with 250.4(A)(5) that ensures electrical continuity and the capacity to conduct safely any fault current likely to be imposed on them [250.96(A)]. This is accomplished by bonding the metal parts to an equipment grounding (bonding) conductor that is sized to the circuit protection device in accordance with 250.122.

A ground rod cannot be used for this purpose because the earth contact resistance is so high, very little current would return to the electrical supply source via the earth. Plus, since the utility does not provide overcurrent protection for their secondary conductors, the fault would not be able to be cleared anyway. New Graphic.

Bottom line… don't ever touch anything metallic that contains utility wiring!

19. Electricians in our area ground the metal conduit that that runs up a utility pole to a local ground rod. Is this a Code compliant installation?

A. The NEC requires all metal raceways that contain service conductors to be grounded [250.80], but grounding a metal raceway to a ground rod serves no purpose in removing or reducing dangerous voltage on metal parts from a ground fault.

What is required is that all metal raceways and enclosures containing service conductors be bonded to an effective ground-fault current path in accordance with 250.4(A)(5). This can be accomplished by the installation of an equipment bonding jumper, of any length, from the top of the metal pipe to the utility secondary neutral terminal in accordance with 250.102(E).

20. When can I mix the control wiring for air-conditioning equipment with the equipment's power conductors in the same raceway?
A. If the remote control circuit conductors are classified as Class 1 and they are installed in accordance with 600V insulation [725.27(B)], then they can be in the same raceway with the A/C equipment power conductors [725.26(B)(1)]. But, control wiring for most A/C equipment is classified as Class 2, not Class 1 because it is supplied by a Class 2 power supply.

The NEC permits Class 2 circuits to be reclassified as Class 1, if the Class 2 equipment markings on the power supply are removed, but all of the circuit conductors in the raceway must have 600V insulation and they must be within a Chapter 3 wiring method [725.25 and 725.27(B)].

Note: Class 2 circuits reclassified as Class 1, are no longer Class 2, regardless of the continued connection to a Class 2 power source and they are not permitted to be installed with other Class 2 or Class 3 circuits that have not been reclassified as Class 1 [725.55].

[ View More Newsletters ] View / Add Comments  

02a. Understanding the NEC, Volume 2 Article 500-Annex C
Understanding the NEC Volume 2, 2005 Edition was written to provide insight into, and an understanding of many of the technical rules of the NEC. This 368 page textbook, printed in full color, covers Article 500-Annex C, contains 396 clear graphics, examples, and over 200 practice questions. Based on the 2005 NEC.

Subjects include: Hazardous (Classified) Locations, Motor Fuel Dispensing Facilities, Health Care Facilities, Manufactured Wiring Systems, Swimming Pools, Fountains and Similar Installations, Emergency Systems, Optical Fiber Cables and Raceways and more.

If you want to really understand the NEC, this series is for you.

Product Code: 05UND2
ISBN: 1-932685-17-0
Pages: 368
Illustrations: 396

Table of Contents
Sample Pages
Sample Graphic

  Send to a Friend View / Add Comments  

  [ Back to Top ]

Copyright © 2005 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)