Stumped by the Code?
Q. I'm working on a job where a meter enclosure and a main breaker panel are connected by PVC and the neutral is bonded to the meter enclosure and the panel enclosure. The service conductors enter the meter enclosure through a hub with rigid metal conduit. Is any more bonding required for the service?
A. Additional bonding isn't required for this service. The metal riser is bonded to the meter enclosure by the threaded hub, provided the connection is made wrench-tight as required in 250.92(B)(2). The meter enclosure is bonded to the grounded (neutral) conductor because in this case the neutral terminal is bonded to the meter enclosure [250.92(B)(1)] (Figure at right). The service disconnect is bonded by the main bonding jumper that's located in the panel enclosure [250.28 and 250.92(B)(1)].
While the Code-required bonding has been provided (250.92), the service must be grounded to a suitable electrode in accordance with 250.24(A) and 250.50.
Q. I installed a 4-wire, 3-phase, 480V, 600A feeder (parallel 350 AWG THHN in two conduits) to serve a 600A panel that supplies nonlinear loads. If the ambient temperature in the plant is 100°F, does this installation meet Code?
A. The installation is fine as long as the calculated load doesn't exceed 510A. Let's go through the steps:
Step 1. Determine the ampacity of the conductor in accordance with the conductor insulation temperature rating, per Table 310.16. A quick check of the table reveals 350 AWG THHN is rated 350A at 90°C. Note: The conductor ampacity is based on the insulation rating of the conductor, not on the terminal rating of the equipment.
Step 2. Since this feeder supplies nonlinear loads, the neutral is considered a current-carrying conductor. Determine the adjusted ampacity based on four current-carrying conductors [310.15(B)(2)(a)] located in an ambient temperature of 100°F [Table 310.16].
Conductor bundle adjustment factor: 0.80
Ambient temperature correction factor: 0.91
Therefore, the conductor adjusted ampacity: 350A×0.80×0.91=255A×2=510A
Q. I'm wiring 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)].
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)].
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.
Q. 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?
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. Does the NEC permit a transfer switch between the meter and the service disconnecting means? The transfer switch isn't marked “suitable for use as service equipment” as required by 230.66.
A. No. A transfer switch that isn't rated “suitable for use as service equipment” isn't permitted ahead of the service disconnecting means (230.82). However, a transfer switch that's listed as “suitable for use as service equipment,” which means it has a main with overcurrent protection, and a main bonding jumper, is permitted.
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. Both 300.16 and 300.17 have rules for building cables and knob-and-tube wiring, but these sections don't apply to low-voltage or limited-energy cables.
Q. Does the NEC state the number of NM cables permitted under a single staple?
A. No. NM cable must be secured and supported in accordance with 334.30, which states that nonmetallic-sheathed cable must be secured by staples, cable ties, straps, hangers, or similar fittings designed and installed so as not to damage it. However, this section doesn't specify the maximum number of cables permitted under a single staple. As always, the installation must be approved by the AHJ (90.4), so basically it's up to the inspector.
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 doesn't 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. Is GFCI protection required for a 230V, 20A hard-wired pool pump motor?
A. No, GFCI protection is only required if the pump motor is cord-and-plug connected [680.22(A)(5)].
Q. Does the NEC require lighting fixtures in a factory to have a cover to protect against breaking bulbs?
A. No. However, lamps used for general illumination of temporary installations must be protected from accidental contact or breakage by a suitable fixture or lampholder with a guard [527.4(F)].
Clashing with your inspector? E-mail your question to Mike at firstname.lastname@example.org.