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Sunday, December 13, 2009

Stumped by the Code?(Column).

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We recently bid a project that involved a stackable washer/dryer in each dwelling unit of an apartment building supplied by a single 30A, 240V receptacle circuit. The inspector says we must install a 20A, 120V laundry receptacle circuit in addition to the 30A, 240V receptacle circuit. This makes no sense to me.

A.

The Code doesn't provide for an exemption to the rule in 210.11(C)(2) and 210.52(F) that requires a receptacle outlet served by a 20A branch circuit to be provided in the laundry area of a dwelling unit. This circuit would be in addition to the 30A branch circuit required for the combination washer/dryer (Figure at right).

Q.

My understanding of Art. 645 is that for a room containing data processing equipment to be classified as an information technology equipment (ITE) room, all of the provisions of 645.2 must be complied with. This means that if a disconnecting means isn't provided to disconnect power to all electronic equipment and HVAC systems serving the computer room, then the room isn't an ITE room and none of the requirements of Art. 645 must be complied with. So the way I read it, a disconnect for the

electronic equipment and HVAC system is an option for a computer room. Am I correct?

A.

Yes. If a disconnecting means isn't installed for the electronic and HVAC equipment in accordance with 645.10, then the electrical installation is neither required nor permitted to be installed in accordance with the requirements of Art. 645. Art. 645 relaxes some of the installation requirements that one would typically expect in Chapter 3, but only if the room is classified as an ITE room. For the room to be classified as an ITE room, all of the following must be met:

*

A disconnecting means that complies with 645.10 is provided.

*

A dedicated HVAC system is provided for ITE and is separated from other areas of the occupancy.

*

Only listed ITE is in the room.

*

Only those persons needed for the maintenance and operation of the ITE should occupy the room.

*

The room is separated from other occupancies by fire-resistant walls, floors, and ceilings, with protected openings.

Note: An ITE 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.

Q.

Can 480V branch-circuit conductors be installed in the same raceway with 120V branch-circuit conductors?

A.

Yes, as long as all circuit conductors have an insulation voltage rating not less than the 480V circuit voltage. Since THHN/THWN building wiring is rated 600V, this isn't a problem.

Q.

Our emergency generator serves a 3,000A 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?

A.

According to the definitions contained in Art. 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 (most transformers and some generators, where the neutral is switched), or other power-supply source, such as a generator where the neutral isn't switched.

So since the conductors from a generator are always feeders, 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 3,000A protection device size. In your example, a 400-kcmil equipment-grounding (bonding) conductor must be installed in each of the eight parallel raceways.

Q.

I'm wiring a 20-foot by 20-foot enclosed patio that will be connected to a home. Three of the walls will be sliding glass doors, with nothing but a 4-inch post in between them. Does the NEC require receptacle outlets in front of the sliding doors?

A.

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 feet, measured horizontally from a receptacle outlet [210.52(A)].

According to 210.52(A)(2), a wall space is:

*

Any space 2 feet or more in width, unbroken along the floor line by doorways, fireplaces, or 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.

Since all of the glass doors slide in this example, there are no fixed panels in the exterior walls [210.52(A)(2)(2)]. Therefore a receptacle isn't required in front of any of the sliding glass doors on the exterior walls.

However, if any of the exterior panels were fixed - typically one glass panel is fixed and one glass panel slides - a receptacle would be required in front of each fixed panel.

Q.

Is an antishort bushing required for Type MC cable of the interlocked type?

A.

No, the NEC doesn't require the use of antishort bushings for the terminations of MC cable, but 320.40 requires the termination fittings to be listed and identified for the terminations of this type of cable. However, many manufacturers of MC cable provide antishort bushings with the cable, and they do provide protection of the conductors.

Q.

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 their structural integrity by boring a hole for every NM cable. So how many NM cables can I install in each bored hole?

A.

You're correct, the NEC doesn't limit the number of cables within an opening, and you 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 inches, the allowable conductor ampacity, based on 90[degrees]C 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? You can 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, OK with 15A protection

New Ampacity of 12 AWG = 30A x 0.70 = 21A, OK with 20A protection

New Ampacity of 10 AWG = 40A x 0.70 = 28A, OK with 30A protection [240.4(B)]

Q.

Amotor 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?

A.

A disconnecting means isn't required for the motor control center, but 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're encased or installed under not less than 2 inches 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].

Remember, 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 [Art. 100].

Q.

When our condo was built, power-limited fire alarm wiring was installed in raceways and outlet boxes. New power-limited audible alarm cables are now being installed, and they're 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?

A.

I can't find any 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 provided with a fitting that protects the cable. This may be because outlet boxes aren't required for low-voltage or limited-energy systems. There are rules in 300.16 and 300.17 for building cables and knob-and-tub wiring, but these sections don't apply to low-voltage or limited-energy cables.

Q.

A third-year apprentice came in the office the other day and wanted to know why we mount garage receptacle outlets above 18 inches. I informed him that this is an NEC requirement, but his question got me thinking. Is there really a minimum height requirement for receptacles in a dwelling unit garage?

A.

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 wouldn't 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 Art. 501.

Q.

I'm installing a 3-phase feeder to a 480V panel where all of the loads operate 480V phase-to-phase. Is a neutral conductor required in the feeder raceway to the panel?

A.

No. However, if the installation were a service raceway, then you'd have a different situation. Because electric utilities aren't required to provide an equipment-grounding (bonding) conductor to service equipment, a grounded (neutral) conductor is required 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's critical that the grounded (neutral) conductor run to service equipment from the electric utility, even when there are no line-to-neutral loads being supplied at the premises, such as a four-wire service drop for a 3-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 won't 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 220.22.

Trying to settle a bet with your co-worker? Send your question to mike@mikeholt.com

Source Citation
Holt, Mike. "Stumped by the Code?" EC&M Electrical Construction & Maintenance 103.8 (2004). General OneFile. Web. 14 Dec. 2009. .


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