All questions and answers are based on the 2005 NEC.
Q. Are there any restrictions on the length of outside conductor (without overcurrent protection) runs, at the point they receive their supply between the secondary of a customer-owned transformer and a panelboard?
A. Outside secondary conductors can be of unlimited length without overcurrent protection at the point they receive their supply if they are installed as follows [240.21(C)(4)] (Figure):
The conductors must be suitably protected from physical damage in a raceway or manner approved by the authority having jurisdiction (AHJ).
The conductors must terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors.
The overcurrent device for the ungrounded conductors must be an integral part of a disconnecting means, or it must be located immediately adjacent thereto.
The disconnecting means must be located at a readily accessible location that complies with one of the following:
a. Outside of a building or structure.
b. Inside, nearest the point of entrance of the conductors.
c. Where installed in accordance with 230.6, nearest the point of entrance of the conductors.
Q. The electrical inspector said that the NEC requires all machines to be listed by UL or other. I didn't think the NEC applied to machines. Is the inspector correct?
A. I don't think so. To me, the NEC is an “installation standard,” and it does not apply to the internal wiring of electrical equipment [300.1(B)]. However, 90.7 states that factory-installed internal wiring of equipment need not be inspected, except to detect alterations or damage if the equipment has been listed. This does put a heavy emphasis on the listing of equipment, and some states have a specific state law requiring all electrical equipment to be listed.
Q. Is the space beneath a raised floor in a computer room considered an exposed location?
A. Yes. Wiring placed “behind panels designed to allow access” are considered exposed [Art. 100].
Q. Can a power cord be run through the raised floor of a computer room?
A. Not unless the flexible cord is listed as a Type DP cable having adequate fire-resistant characteristics suitable for use under raised floors of an information technology equipment room [645.5(D)(5)].
Q. Does the NEC require fluorescent lay-in luminaires to be supported independent of a suspended ceiling?
A. No, the NEC does not require support of lay-in luminaires independent of the ceiling grid. However, the luminaires must be attached to the suspended-ceiling framing with screws, bolts, rivets, or clips listed and identified for such use [410.16(C)]. Caution: Local building codes might have a specific requirement to independently support the luminaires from the ceiling grid.
Q. I'm confused. Section 210.12(B) requires AFCI protection for all bedroom outlets. I understand that this applies to the smoke alarm outlets too. However, 760.21 and 760.41 state that you can't protect fire alarm circuits with an AFCI protection device. What's the deal?
A. The wiring for smoke alarm outlets located in dwelling unit bedrooms must be AFCI protected as per 210.12(B). The Article 760 GFCI/AFCI limitation only applies to the circuit that supplies a non-power-limited [760.21] or power-limited [760.41] fire alarm system.
Smoke detectors connected to a 15A or 20A, 120V circuit must be AFCI protected if located in the bedroom of a dwelling unit [210.12(B)], because this circuit isn't supplied by a fire alarm panel in accordance with NFPA 72, National Fire Alarm Code.
Q. What size conductor is required for a 3,000A service supplied with eight parallel sets?
A. We must choose individual conductors that are rated at least 375A each (3,000A ÷ 8 sets), as 240.4(C) does not allow the next size up when the overcurrent device is rated above 800A. This ampacity is selected from the 75°C column of Table 310.16 for conductors over 100A in order to accommodate the terminal temperature rating requirements of 110.14(C)(1)(b).
Furthermore, 500-kcmil copper is rated 380A in the 75°C column of Table 310.16 and will be satisfactory as long as no more than three current-carrying conductors are installed per raceway at an ambient temperature not exceeding 85°F.
Any installation making use of parallel conductors per phase must meet all of the requirements of 310.4 for parallel conductors.
Q. How do I calculate the feeder/service load for track lighting in a commercial building?
A. The feeder/service VA load must not be less than 150VA for every 2 feet of track lighting or fraction thereof [220.43(B)]. Where a feeder or service supplies continuous loads, the minimum feeder or service conductor size (before the application of any adjustment and/or correction factors) must have an allowable ampacity of not less than 125% of the continuous load [215.2(A)(1) for feeders and 230.42(A) for services].
This rule doesn't apply to the sizing of branch circuits for track lighting. There is no limit on the length of track on a single branch circuit. Therefore, the maximum number of lampholders permitted on a track lighting system is based on the wattage rating of the lamps and the voltage and ampere rating of the circuit [410.101(B)]. The maximum track lighting load on a branch circuit must not exceed 80% of the circuit rating [210.19(A)(1)].