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Code Quandaries

Dec 1, 2005 12:00 PM, By Mike Holt, NEC Consultant

Stumped by the Code?

Q. I was told there is a 30-day limit for extension cord use in a commercial building. What's the story?

A. Extension cords cannot be used as a substitute for fixed wiring [400.8(1)]. In addition, extension cords can only be used for temporary wiring if approved by the authority having jurisdiction (AHJ) [590.2(B)].

But temporary wiring is only permitted for construction, remodeling, maintenance, repair, or demolition of buildings, structures, equipment, or similar activities or for emergencies and for tests, experiments, and developmental work. When temporary wiring is permitted by the AHJ, the wiring (cords) must be removed immediately upon completion of construction or purpose for which the wiring was installed [590.3].

Q. If a 480-120/208V 3-phase transformer has primary protection set at no more than 125% of the primary current rating in accordance with 450.3(B), what is the maximum secondary conductor length from the transformer if it's located indoors?

A. The primary protection of a transformer does not provide secondary conductor protection for a 4-wire secondary. Therefore, secondary conductors must not be run more than 25 feet without overcurrent protection, if [240.21(C)(6)]:

1. The secondary conductors have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device that protects the primary of the transformer.

2. Secondary conductors terminate in a single circuit breaker or set of fuses rated no greater than the tap conductor ampacity in accordance with 310.15 [Table 310.16].

3. The secondary conductors must be protected from physical damage by being enclosed in a manner approved by the AHJ, such as within a raceway.

Q. When does the Code require low-voltage or power-limited control or signaling cables to be plenum rated?

A. Plenum rating is required for all low-voltage and power-limited cables that are installed either exposed or within a plenum-rated raceway above a suspended ceiling or below a raised floor used for environmental air movement [725.3(C) and 725.61(A)]. Nonplenum-rated cables are permitted above a suspended ceiling or below a raised floor that is used for environmental air if installed within a metal wiring method in accordance with 300.22(C)(1).

Low-voltage and power-limited cables installed beneath a raised floor in an information technology equipment room (i.e. computer room) aren't required to be plenum rated [300.22(D) and 645.5(D)(5)(c)].

Q. A separate 20A, 120V circuit is required for dwelling unit bathroom receptacles. Are we required to include 1,500VA for this circuit when calculating the service load?

A. When performing dwelling unit load calculations, a 1,500VA load is required for each small-appliance and laundry branch circuit [220.52(A) and (B)], but this does not apply to the bathroom receptacle outlet branch circuit.

Q. 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 feet above the equipment or to the structural ceiling — whichever is lower — must be dedicated for the electrical installation.

As shown in the Figure above, no piping, duct, or other equipment foreign to the electrical installation can be installed in this dedicated footprint space [110.26(F)(1)(a)].

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)].

Q. 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 feet from its point of entrance [250.52(A)(1)],

• Accessible service bonding means [250.94],

• Metallic service raceway,

• Service equipment enclosure, or

• 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.

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