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Service Maintenance Company, Inc. Electrical Contractors |
| NEC Questions | 4/28/2004 |
| By Mike Holt for EC&M Magazine Q1. When can hospital grade MC cable of the interlocked type be used in a doctor or dentist examining room. A1. There is no such product as "hospital grade Type MC cable," you must be referring to Type AC cable that contains an insulated equipment grounding (bonding) conductor, marked as HFC- Health Care Facility Cable. Type MC of the interlocked type (looks like Type AC cable) is never permitted in the patient care area of a health care facility. Branch circuits serving patient care areas, like an examining room, shall be installed in a metal raceway or listed cable having a metallic armor or sheath that qualifies as an effective ground-fault current path in accordance with 250.118 [517.13(A)]. The metal armored sheath of Type AC cable is listed as a suitable ground-fault current path because it contains an internal bonding strip in direct contact with the metal sheath of the cable [250.118(8)]. However, the outer metal sheath of interlocked Type MC cable is not listed as a ground-fault current path [250.118(10]); therefore, it shall not be used to supply branch circuits in patient care areas of health care facilities. Figure 517-4 Q2. My understanding of Article 645 is that for a room containing data processing equipment to be classified as an Information Technology Equipment Room, all of the provisions of 645.2 must be complied with. This means that if a disconnecting means is not provided to disconnect power to all electronic equipment and HVAC systems serving the computer room, then the room is not an information technology room, and none of the requirements of Article 645 must be complied with. So the way I read it, a disconnecting for the electronic equipment and HAVAC system is an option for a computer room! Am I correct? A2. Yes, you are correct. If a disconnecting means is not installed for the electronic and HVAC equipment in accordance with 645.10, then the electrical installation is not required, as well as not permitted, to be installed in accordance with Article 645. Let me see if I can explain this better, Article 645 relaxes some of the installation requirements that one would typically expect in Chapter 3, but only if the room is classified as an Information Technology Room. For the room to be classified as an information technology room ALL of the following shall be provided: A disconnecting means complying with 645.10 is provided. Dedicated heating/ventilating/air-conditioning (HVAC) system is provided for information technology equipment and it separated from other areas of the occupancy. Only listed information technology equipment is installed in the room. The room is only occupied by those persons needed for the maintenance and operation of information technology equipment. The room is separated from other occupancies by fire-resistant-rated walls, floors and ceilings, with protected openings. Note: An information technology equipment room is an enclosed area specifically designed to comply with the construction and fire protection provisions of NFPA 75 - Standard for the Protection of Electronic Computer/Data-Processing Equipment. Q3. Can 460V branch-circuit conductors be installed in the same raceway with 120V branch-circuit conductors? A3. Yes, as long as the circuit conductors of the different systems are identified by color-coding, marking tape, tagging, or other approved means, and the identification system must be permanently posted at each branch circuit panelboard [210.4(D)]. Also 200.6(D) requires one system grounded (neutral) conductor be identified by a continuous white or gray outer finish, and the other grounded (neutral) conductor shall be identified by a continuous white finish with a readily distinguishable color stripe, other than green, or white or gray outer finish. Traditionally, the 120V grounded (neutral) conductor has an outer finish of white, and the color gray is used to identify the 277V circuit grounded (neutral) conductor. One last point, all circuit conductors shall have an insulation voltage rating not less than the 480V circuit voltage. Since THHN/THWN building wiring is rated 600V, this is not a problem. Q4. Our emergency generator serves a 3000A main lug distribution board (eight sets of 500 kcmil). Are the conductors from the generator to the distribution board considered service or feeder conductors? In other words, are we to use Table 250.66 or Table 250.122 to size the grounding conductor in each raceway? A4. According to the definitions contained in Article 100, "service conductors" originate from the electric utility that delivers electric energy, to the premises served. Whereas "feeders" originate from service equipment, the source of a separately derived system (transformer), or other power-supply source (generator that is not a separately derived system). So, since the conductors from a generator are always a feeder, the equipment grounding (bonding) conductor from the generator to the distribution board shall be sized in accordance with 250.122(F)(1), based on the 3000A protection device size. In your example, a 400 kcmil equipment grounding (bonding) conductor must be installed in each of the eight parallel raceways. Q5. Is it against the Code to GFCI-protect temporary lighting circuits? I have seen exposed temporary lighting wire (wire nuts fell off or were missing) touching metal studs thereby presenting a potentially deadly working environment. A5. The NEC is silent on this issue, so this means, it's okay to GFCI-protect temporary lighting circuit conductors. However, to prevent placing a construction site in the dark by a ground fault, temporary lighting shall not be installed on the branch circuit that supply receptacles [527.4(D)]. Note: Receptacles rated 15A or 20A, 125V used to supply temporary power for construction, remodeling, maintenance, repair, or demolition of buildings, structures, equipment, or similar activities shall be GFCI protected [527.6]. Q6. I am wiring a patio on a home, where three of the walls will be sliding glass doors, with nothing but a 4" post in between them. Does the NEC required receptacle outlets in front of the sliding doors? A6. To ensure that a general-purpose receptacle is conveniently located to reduce the likelihood that an extension cord will travel across openings, such as doorways or fireplaces, a receptacle outlet shall be installed so no point along the dwelling unit wall space will be more than 6 ft, measured horizontally from a receptacle outlet [210.52(A)]. According to 210.52(A)(2), a wall space is: Any space 2 ft or more in width, unbroken along the floor line by doorways, fireplaces and similar openings. The space occupied by fixed panels in exterior walls. The space occupied by fixed room dividers, such as freestanding bar-type counters or railings. So no, a receptacle is not required in front of the sliding door. Q7. If I were to replace a two-wire receptacle in a bathroom outlet that does not have a grounding means, am I required to replace it with a GFCI receptacle? A7. No, the replacement receptacle is not required to be of the GFCI type, however, it is required to be GFCI protected [406.3(D)(2)]. This can be accomplished by the use of a GFCI receptacle, or a two-wire nongrounding type or three-wire grounding type receptacle that is GFCI protected upstream by a GFCI circuit breaker or GFCI receptacle in accordance with 406.3(D)(3). Q8. Is a disconnecting means within sight of an outdoor spa or hot tub for a single-family dwelling? A8. A maintenance disconnecting means shall be installed within sight from spa and hot tub equipment [680.12 and 680.42]. It shall be located not less than 5 ft horizontally from the inside walls of the spa or hot tub, unless separated by a solid fence, wall, or other permanent barrier [680.22(C)]. In other than a "single-family dwelling," a clearly labeled emergency spa or hot tub water recirculation and jet system shutoff shall be supplied. The emergency shutoff shall be readily accessible to the users and located not less than 5 ft away, but adjacent to and within sight of the spa or hot tub [680.41] The maintenance disconnecting means required by 680.12 or a pushbutton controlling a relay located in accordance with this 680.41 could be used to meet the emergency shutoff requirement. The purpose of the emergency shutoff protects users, deaths and injuries have occurred in less than 3 ft of water because individuals became stuck to the water intake opening. This requirement applies to spas and hot tubs installed indoors as well as outdoors. Q9. I don't see where 300.4 limits the number of NM cables through a given opening in wood or metal framing members. I don't want to turn the framing members into Swiss cheese and reduce its structural integrity by boring a hole for every NM cable. So how many NM cables can I install in each bored hole? A9. You are correct, the NEC does not limit the number of cables within an opening, and we must not reduce the structural strength of framing members below the requirements of the building code. However, where multiconductor NM cables are stacked or bundled longer than 24 in., the allowable conductor ampacity, based on 90ºC insulation rating of the conductors as listed in Table 310.16 [334.80], shall be adjusted in accordance with the multiplying factors contained in Table 310.15(B)(2)(a). So what should you do? Well, separate the cables after they pass through the framing members so that they aren't' bundled more than 24 inches. But, you don't have to worry about this if no more than nine current carrying 14 AWG, 12 AWG, or 10 AWG conductors are bundled together for more than 24 inches. New Ampacity of 14 AWG = 25A x 0.70 = 17.5A, okay with 15A protection New Ampacity of 12 AWG = 30A x 0.70 = 21A, okay with 20A protection New Ampacity of 10 AWG = 40A x 0.70 = 28A, okay with 30A protection [240.4(B)] Q10. A motor control center is located in a building where the supply conductors originate from another building. Is a disconnecting means required for the motor control center? If yes, where must it be installed? A10. A disconnecting means is not required for the motor control center, however a disconnecting means is required at the remote building for all conductors that enter a building or structure [225.31]. The building feeder disconnecting means, which disconnects the MCC shall be installed at a readily accessible location, either outside the building or structure or inside the building or structure, nearest the point of entrance of the conductors. Note: Conductors are considered outside of a building or other structure where they are encased or installed under not less than 2 in. of concrete or brick [225.32 and 230.6]. But, where documented safe switching procedures are established and maintained, the building/structure disconnecting means can be located elsewhere on the premises if the disconnect is monitored by qualified persons [225.32 Ex 1]. Note: A qualified person is one who has the skills and knowledge related to the construction and operation of the electrical equipment and installation, and has received safety training on the hazards involved with electrical systems [Article 100]. Q11. When our condo building was built, power-limited fire alarm wiring was installed in raceways and outlet boxes. New power-limited audible alarms cables are now being installed and they are not run in a raceway. At the point where the new cable exits the existing junction box (knock-out), no bushing, clamp or other protection is provided. Does the Code require low-voltage or limited-energy cables that exit an outlet box to have some form of protection at the knockout? A11. I can't find any specific rule in the NEC that specifically states that fire alarm or any other low-voltage or limited-energy cable exiting an outlet box must be provide with a fitting that protects the cable. Maybe because outlet boxes are not required for low-voltage and limited-energy systems. There are rules in 300.16 and 300.17 for building cables and knob-and-tub wiring, but these sections do not apply to low-voltage or limited-energy cables. Q12. A large number of satellite professionals ground the satellite system to the nearest available water hose bib. Is this permitted by the NEC? A12. No. The satellite mast [810.15] and discharge unit (ground block) [810.20(C)] must be grounded (actually bonded) in accordance with 810.20. The electrode that is suitable for this purpose includes the nearest accessible [810.40(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)]. Figure 810-10 Accessible bonding means [250.94]. Metallic service raceway. Service equipment enclosure. Grounding electrode conductor or the grounding electrode conductor metal enclosure. Note: Grounding the lead-in antenna cables and the mast helps in preventing voltage surges from static discharge from reaching the inner conductor of the lead-in cable. Because the satellite sits outdoors, wind creates a static charge on the dish as well as the wire attached to it. This charge can build up on the dish and cable until it jumps across an air space, often through the electronics inside the LNBF or receiver. Grounding the coaxial cable and dish to the building grounding electrode system helps in dissipating this static charge. Nothing can prevent damage from a direct lightning strike, but grounding these systems in accordance with the NEC with proper surge protection (not NEC required) can help reduce damage to satellite equipment as well as other related equipment from nearby lightning strikes. Q13. A third year apprentice came in the office the other day and wanted to know why we mount garage receptacles outlets above 18 inches. I informed him the NEC required this. But his question got me thinking, is there a minimum height for receptacles in a dwelling unit garage? A13. The NEC does not prohibit receptacles within 18 inches of a dwelling unit garage floor. So if you wanted, you could install them in the floor! However in commercial garages [Article 511.1], it would not be cost effective to install a receptacle within 18 inches of the floor. Since this area is classified as a Class 1, Division 2 location, the receptacle would have to be identified as suitable for use in a hazardous Class 1 location. Or in other words, the receptacle and wiring method would have to be explosionproof in accordance with Article 501. Q14. My buddy told me that a 208V motor could be connected to the 208V high-leg conductor derived from a delta-connected transformer bank? I can't believe the load would work if wired this way, but if it does, is this permitted by the Code? A14. The motor will operate perfectly if wired in this configuration. However, a single-pole circuit is rated 120/240V and it is only permitted to serve a circuit where the nominal voltage of any one conductor to ground (actually the metal case) does not exceed the lower of the two values [240.85]. So you can see this would be a violation because the 208V high-leg circuit conductor exceeds the single-pole (120V) voltage rating of the circuit breaker. Q15. I am installing a 3-phase feeder to 480V panel where all of the loads operate 480V phase-to-phase. Is a neutral conductor required in the feeder raceway to the panel? A15. No. However, if the installation was a service raceway, then we have a different ball game. Because electric utilities are not required to provide an equipment grounding (bonding) conductor to service equipment, a grounded (neutral) conductor is required to be from the electric utility transformer to each service disconnecting means. The grounded (neutral) conductor shall be bonded to the enclosure of each disconnecting means as required by 250.24(B) [250.130(A)]. Caution: It is critical that the grounded (neutral) conductor be run to service equipment from the electric utility, even when there are no line-to-neutral loads being supplied at the premises (i.e., a four-wire service drop for a three-phase service). In addition, the metal parts of the service equipment itself must be bonded to the grounded (neutral) conductor to ensure that dangerous voltage from a line-to-case fault will be quickly removed by the operation of the circuit protection device [250.4(A)(3) and 250.4(A)(5)]. Danger: Because of the earth's high resistance and resulting low fault current, the circuit overcurrent protection device will not open and clear the fault. As a result, all metal parts associated with the electrical installation, as well as metal piping and structural steel, will remain energized at a lethal level. Because the grounded (neutral) service conductor is required to serve as the effective ground-fault current path, it shall be sized so that it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the grounded (neutral) conductor in accordance with Table 250.66, based on the total area of the largest ungrounded conductor [250.24(B)(1)]. In addition, the grounded (neutral) conductors shall have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61. Example: What is the minimum size grounded (neutral) service conductor required for a 400A, three-phase, 480V service where the ungrounded service conductors are 500 kcmil and the maximum unbalanced load is 100A? Figure 250-61 (a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG Answer: (d) 1/0 AWG [Table 250.66] The unbalanced load requires the grounded (neutral) conductor to be 3 AWG, which is rated 100A at 75°C in accordance with Table 310.16. However, Table 250.66 requires the grounded (neutral) service conductor to be sized no smaller than 1/0 AWG to accommodate the maximum possible fault current likely to be imposed on it. Copyright © 2004 Mike Holt Enterprises,Inc. |
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