AFCIs, Cable Trays and More

Article 100 Definitions; Article 210 Branch Circuits; Article 250 Grounding and Bonding; Article 300 Wiring Methods; Article 310 Conductors for General Wiring; Article 334 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS; Article 392 Cable Trays; Article 422 Appliances; Article 440 Air-Conditioning and Refrigeration Equipment; Article 680 Swimming Pools, Fountains, and Similar Installations; Article 695 Fire Pumps

Portions of the 2007 edition of the Guide Information for Electrical Equipment (White Book) published by Underwriters Laboratories Inc. also are mentioned.

Differences in AFCIs

What are the major differences between the branch/feeder-type arc-fault circuit interrupters (AFCI) and the combination-type AFCIs that were required after Jan. 1, 2008?

The 2005 National Electrical Code (NEC) permitted the use of branch/feeder AFCIs until Jan. 1, 2008. Since then, the Code requires combination-type AFCIs.

The main differences between the two types are mentioned in the 2007 edition of the White Book.

The branch/feeder-type protects branch-circuit wiring and feeder wiring from the effects of arcing, and it protects cord sets and power supply cords connected to receptacles. These devices detect series and parallel arcs on circuit wiring and parallel arcs on cord sets (extension cords).

The combination-type AFCI detects the faults that the branch/feeder AFCI detects plus series arcs in branch circuits without a ground and series arcs in cord sets and extension cords.

There are markings on the products that identify the type. They will read, “Branch/Feeder Arc Fault Circuit Interrupter,” “Branch/Feeder AFCI,” “Combination Arc Fault Circuit Interrupter” or “Combination AFCI.”

For more on the various types of AFCIs, see pages 59–61 in the White Book.

Cable tray in fire-rated ceiling

Is cable tray permitted in a fire-rated floor/ceiling assembly? Is metal cable tray acceptable in a fire-rated suspended ceiling?

Metal cable tray may be used in fire-rated floor/ceiling assemblies. However, 392.4 prevents the use in ducts, plenums and other air-handling spaces except as permitted by 300.22.

Section 300.22 deals with wiring in ducts, plenums and other air-handling spaces. These include ducts for loose stock or vapor removal, ducts or plenums used for movement of environmental air and other space used for environmental air.

Where the hollow space created by the suspended ceiling is used for movement of environmental air, solid-bottom metal cable trays with solid metal covers are permitted and mentioned in part (1) of 300.22(C).

All wiring methods entering or leaving the fire-rated floor/ceiling assembly must be firestopped to comply with 300.21. A long fine-print note following this section provides information on fire-stopping materials and building code requirements.

Cord-and-plug-connected appliances

Does the NEC permit the installation of a built-in, cord-and-plug-connected microwave oven and a cord-and-plug-connected range hood exhaust fan on the same 20-ampere branch circuit?

No, these two cord-and-plug appliances cannot be supplied from the same branch circuit because of one of the restrictions in 422.16(B)(4) under the title “Range Hoods,” which requires that the receptacle be supplied by an individual branch circuit. The Code defines an individual branch circuit as “A branch circuit that supplies only one utilization equipment.”

To be allowed to be cord-and-plug connected, the range hood must be acceptable for cord connection in the manufacturer’s instructions. The cord is terminated with a grounding-type attachment plug if the hood is not double insulated. The cord length is not less than 18 inches or more than 36 inches. Also, both appliances must meet the requirements in 422.16 to be eligible for cord-and-plug connection

Minimum size for parallel conductors

A three-phase, 208Y/120-volt service -consists of four 500-kcmil copper conductors in four separate metal raceways. What is the minimum size grounded-circuit conductor for this service?

The answer to this question is in 310.4 and part (2) of 250.24(C). The minimum size parallel conductor is 1/0 AWG aluminum, copper-clad aluminum or copper. This also is the minimum size grounded-service conductor Table 250.66 permits. According to 250.24(C)(2), “Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded service-entrance conductor in the raceway but not smaller than 1/0 AWG.”

Outdoor nonmetallic sheathed cable

Is nonmetallic sheathed cable permitted to supply an outdoor air conditioning unit where installed in liquid-tight flexible metal conduit? The conduit extends from a weatherproof disconnect to the air conditioner and is about 5 feet long.

No, nonmetallic sheathed cable cannot be used as described. The definition in Article 100 classifies unprotected locations exposed to the weather as wet locations, and 334.12(B)(4) prohibits Type NM cable in damp and wet locations.

To clarify the misunderstanding that exists about the interior of raceways installed outdoors, a new 300.9 appears in the 2008 edition of the NEC. Part of this section reads, “Where raceways are installed in wet locations above grade, the interior of these raceways shall be considered to be a wet location.” This addition makes it clear that the interior of a raceway exposed to the weather is a wet location.

Grounding and bonding

A ground rod and grounding-electrode conductor (6 AWG copper) are installed at the meter enclosure to satisfy the electric utility company requirements. Other grounding-electrode conductors from the underground metal water pipe and concrete-encased electrode are terminated at the service disconnecting means. Does the NEC permit the neutral conductor of a 120/240-volt service to act as the bonding conductor between these two different grounding electrodes?

Yes, Article 250.24(A)(1) permits this. Where the neutral is used to connect the two electrodes together, it cannot be smaller than 6 AWG copper. Section 250.142(A) allows the grounded-circuit conductor to ground noncurrent-carrying metal parts of equipment to be grounded on the supply side of the service.

Receptacles in ceilings

Are twist-lock 15- and 20-ampere, 125-volt receptacles required where receptacles are installed face down in a ceiling?

Receptacles are tested to comply with UL 498, which includes a pullout test. An attachment plug with a 3-pound weight must not pull out of the receptacle for one minute and must release with a pull of 15 pounds. If the cord-and-cap weighs less than 3 pounds, it should stay in place when inserted in the receptacle.

Branch-circuit overcurrent protection

What is the largest size overcurrent protection device for three four-wire, 12 AWG copper conductors with Type THWN insulation in electrical metallic tubing? The voltage is 208Y/120, and the load is incandescent -lighting in an office.

Under the conditions specified, the neutral conductors are not considered as current-carrying under 310.15(B)(4)(a), which reads, “(a) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).” This part involves Table 315(B)(2)(a), which includes adjustment factors for more than three current-carrying conductors in a raceway or cable assembly.

The table shows that an adjustment factor of 70 percent must be applied to the ampacity of 12 AWG copper conductor with THWN insulation. The ampacity of this conductor with Type THWN insulation is shown to be 25 amperes in Table 310.16. By multiplying (25 × 0.7), a corrected ampacity of 17.5 amperes is obtained. Therefore, a 20-ampere fuse or circuit breaker is permitted as the protection for these multiwire branch circuits (see 240.4(B)).

The total load on these branch circuits cannot exceed 80 percent of the ampere rating of the overcurrent device where the branch circuits supply a continuous load, such as office building lighting. This results in a connected load of 16 amperes on each branch circuit. The load is limited to 80 percent of the branch-circuit overcurrent device rating unless the assembly, including overcurrent devices, is listed for 100 percent of its rating. This limitation on loading of overcurrent devices supplying continuous loads is in 210.20(A).

Panelboards above stairways

The electrical inspector in my jurisdiction is measuring the height of the ceiling from the top step below the panelboard to determine the headroom. If he measures from the lower step below the panelboard, the height to the ceiling is 6 feet, 7 inches. Is he right in taking this measurement this way?

This is one of the gray areas in the NEC that the 2008 edition has solved.

The requirement for 6.5 feet of headroom in front of the panelboard is in 110.26(E). An exception allows panelboards that do not exceed 200 amperes to have less headroom in existing dwellings. However, a new Part (F) to 240.24 removes the gray area by stating, “(F) Not Located Over Steps. Overcurrent devices shall not be located over steps of a stairway.”

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720.


About the Author

George W. Flach

Code Q&A Columnist
George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical in...

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