Simultaneous Disconnects, Generator Wiring and More

By Oct 15, 2002
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Article 100 Definitions

Article 210 Branch Circuits

Article 230 Services

Article 250 Grounding

Article 312 Cabinets, Cutout Boxes and Meter Socket Enclosures

Article 445 Generators

Article 518 Places of Assembly

Disconnect for multiwire branch circuit

Q: There was a requirement in the 1999 National Electrical Code (NEC) to simultaneously disconnect the ungrounded conductors of a multiwire branch circuit that supplies more than one device on the same yoke in dwelling units. This rule does not appear in the 2002 Code. Is simultaneous disconnection of a split-wired duplex receptacle connected to a multiwire branch circuit in a dwelling unit still required?

A: The answer is yes. There has been a change in the 2002 edition. The new requirement applies to all occupancies and covers both multiple circuits and multiwire branch circuits.

There are now two places where the requirement for simultaneous switching of the ungrounded conductors on multiwire branch circuits appears. There is no change in the requirement that appeared in the 1999 edition. This rule without change still appears in the 2002 NEC. The addition of 210.7(C) applies to multiwire and also multiple branch circuits (two two-wire branch circuits) that supply duplex receptacles with both break-off tabs removed. Note that this change applies to all occupancies, multiwire, and multiple branch circuits and receptacle circuits only. Multiwire branch circuits are considered to be multiple circuits in Section 210.4(A). under 210.4(B).

When wiring dwelling units, you must comply with 210.4(B) and 210.7(C). For all other occupancies, you must comply with 210.7(C).

Bonding service raceways

Q: On an overhead service, rigid-metal conduit is connected to a meter base. A metal conduit nipple is connected to the meter socket and service disconnecting means. A bonding bushing is used on the nipple at the meter base. Is this a violation of 250.6, which discusses objectionable current flow over grounding conductors?

A: Although the grounded service conductor (could be a neutral) is in parallel with the metal conduit nipple, this arrangement is not considered to be one that causes objectionable current flow.

Bonding is required by Part V of Article 250, and 250.92 specifically covers services. Metal service raceways are required to be bonded in 250.92(A)(1), and part (B)(4) recognizes bonding bushings as one method of satisfying this requirement.

Wiring a standby generator

Q: Does Article 230 apply to the conductors and overcurrent protection from a standby generator located outdoors about 20 feet from the building it serves?

A:No, Article 230 applies to services, and the definition of a service, which appears in Article 100, reads: "Service. The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served." The key words in this definition are "serving utility;" therefore, Article 230 does not apply.

Article 445 covers generators and references Article 695, 700, 701, 702 and 705, which must be complied with depending on the generator's use. Alternating current machines must be protected from overloads by inherent design, fuses, circuit breakers or other acceptable overcurrent protection.

Supply-conductor ampacity from the generator to the overcurrent protection cannot be less than 115 percent of the nameplate current rating of the generator. These requirements appear in 445.12(A) and 445.13.

If the standby generator supplies a fire pump, additional requirements for the generator set are found in Article 695. A generator used to supply emergency power must comply with the rules in Article 700, especially 700.12(B). Requirements for a legally required standby system are similar in some respects to those in Article 700 and are in Article 701. If the generator is connected to optional standby loads, Article 702 applies. Where the generator operates in parallel with the utility system, Article 705,Interconnected Electric Power Production Sources applies. Each Article has different requirements but they all prohibit inadvertent parallel operation of the generator with the utility system.

This short discussion is intended to point out that rules for the installation of a standby generator are dependent on its use.

Temporary wiring in exhibition halls

Q: Is it permissible to use cable trays for temporary wiring of display booths at trade shows in exhibition halls? Are Type SO and SJT flexible cords permitted for temporary wiring, and may they be installed in cable trays?

A: Flexible cords and cables suitable for hard usage or extra-hard usage may be laid on the floor where protected from contact by the general public. They may also be installed in cable trays where placed in a single payer provided that qualified persons will service the installation and the cable tray is marked at least once every 25 feet with a sign that reads: "Cable Tray for Temporary Wiring Only."

According to Table 400.4 Type SO flexible cords and cables are listed for "Extra Hard Usage," and Type SJT flexible cords are suitable for "Hard Usage." Therefore, both types may be used for temporary wiring in large exhibition halls that are within the scope of Article 518,Places of Assembly. The rules for temporary wiring appear in 518.3(B) and the Exception.

Supporting and securing nonmetallic sheathed cable

Q: May nonmetallic sheathed cable be run in a 2-inch metal raceway that is 24 inches long into a panelboard without securing the cables within 12 inches of where they enter the panelboard? Are nonmetallic sheathed cables permitted to enter a panelboard through a 3-inch conduit nipple?

A: The general requirement for cable termination is in 312.5(C). This one-sentence rule says: "Where cable is used, each cable shall be secured to the cabinet, cutout box, or meter socket enclosure." However, there is a long exception for nonmetallic sheathed cable, and it is pretty detailed.

Nonmetallic sheathed cables are permitted to enter the top of a service-mounted panelboard through a nonflexible raceway that is not less than 18 inches long or longer than 10 feet. The cable must be secured within 12 inches of the outer end of the raceway; the raceway must extend directly above the panelboard and cannot penetrate the structural ceiling; a fitting is required at each end of the raceway to protect the cables from abrasion and the fittings must remain accessible after installation; the raceway must be sealed at its outer end; the cable sheath must be continuous in the raceway and extend into the panelboard beyond the fitting for at least 1?4-inch; the raceway is secured in accordance with the requirements of the applicable article for the wiring method; and the wire-fill table in Chapter 9 must be used to prevent cable overcrowding.

To determine the number of cables permitted in the 2-inch conduit, Note 9 to Table 1 in Chapter 9 requires that multiconductor cables be treated as a single conductor. For cables with oval or elliptical cross sections, the cross-sectional area is based on the major diameter of the ellipse to obtain an equivalent circle diameter. A 3-inch nipple does not satisfy these requirements.

Metal conduit connection to an outdoor load center

Q: Are sealing locknuts listed for installation on the top of an outdoor load center? If they are, how many are required at each conduit entry, one or two?

A: Sealing locknuts may be used with threaded rigid-metal conduit or threaded intermediate-metal conduit. They are suitable for use in wet locations.

Only one sealing locknut is required on each conduit entry into the enclosure, and it can be installed on the outside or inside of the load center. This information is obtained from the General Information for Electrical Equipment Directory published by Underwriters Laboratories Inc.

Concrete encased electrode

Q: Does the language in 250.52(A)(3) mean that a 1/2-inch reinforcing rod in a concrete foundation has to be in one piece and not less than 20 feet long to qualify as a grounding electrode? For example, may two 12-foot-long 1/2-inch reinforcing rods be spliced together with steel tie wires to form the grounding electrode?

A: The concrete encased electrode does not have to be a single 1/2-inch or larger reinforcing rod that is at least 20 feet long. However, the total length of the reinforcing rod must be at least 20 feet to qualify as a grounding electrode. It also must be encased in at least 2 inches of concrete and be located near the bottom of the concrete foundation or footing in contact with the earth.

In the example, the two 12-foot rods will probably be more than 22 feet long after they are spliced with steel tie wires. This length and size satisfies the requirements in 250.52(A)(3). The grounding electrode conductor connected to the reinforcing rod cannot be smaller than 4 AWG copper to comply with 250.66(B).

Two buildings supplied by a single service

Q: Am I allowed by the NEC to run a three-wire, 480-volt, three-phase feeder supplied from a 480Y/277-volt service to another building without an equipment-grounding conductor? The feeder supplies three-phase motor loads and a step-down transformer for lighting and receptacles. A grounding electrode and grounding-electrode conductor will be provided at the second building to ground exposed metal parts as required for ungrounded electrical systems.

A: This is not the proper way to ground electrical equipment at the building served. You have to provide a neutral or equipment-grounding conductor in the feeder. These choices are given in 250.32(B)(1) and (2). An equipment-grounding conductor must be included with the feeder conductors and must be used for grounding or bonding of equipment, structures, etc. This conductor is sized according to 250.122.

Where an equipment-grounding conductor is not run with the feeder conductors, and there is no continuous metallic paths between the buildings, and ground-fault protection is not provided at the service, a grounded circuit conductor run with the feeder conductors is permitted to ground and bond equipment, metal frames of buildings, etc. The grounded circuit conductor cannot be smaller than required by 220.22 or 250.122. The equipment-grounding conductor or grounded circuit conductor must comply with Parts (D) and (E) of 250.32.

Since you have a choice of providing an equipment-grounding conductor or grounded circuit conductor, I would choose the equipment-grounding conductor instead of the grounded circuit conductor because there are fewer restrictions on the use of equipment-grounding conductors. EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.254.2132.





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