Derating of conductors

Q: One of my residential wiring jobs was turned down because I did not derate four two-wire-with-ground NM-B cables where they pass through a single hole in an upper wood plate. The cables leave the attic, pass through a double 2-by-4 wood plate at the ceiling down to a distribution panel in the stud wall. All cables are two 12 AWG with ground and three supply 120-volt lighting and receptacle branch circuits. The other branch circuit is 240 volts. It supplies a 240-volt, single-phase air- conditioning unit with a nameplate current of 20 amperes. The cables are not bundled for more than 6 inches.

A: Although 310.15(B)(2) does not require derating of conductors where bundled for less than 2 feet, a change in 334.80 in the 2005 edition of National Electrical Code (NEC) now requires derating of conductors under some conditions where the bundle is less than 2 feet long. Here is the change: “Where more than two NM cables containing two or more current- carrying conductors are bundled together and pass through wood framing that is to be fire- or draft-stopped using thermal insulation or sealing foam, the allowable ampacity of each conductor shall be adjusted in accordance with Table 310.15(B)(2)(a).”

However, the first paragraph of 334.80 allows derating of the ampacity from the 90 C column in Table 310.16, provided that the adjusted ampacity is not greater than the values shown in the 60 C column of Table 310.16 and 240.4(D). According to this table, the ampacity of 12 AWG copper with 90 C insulation is 30. For eight current-carrying conductors, the derating factor in Table 310.15(B)(2)(a) is 0.7. Therefore, the adjusted ampacity is 21 (30 x 0.7). The lighting and receptacle branch circuits may be protected with 20-ampere overcurrent devices. However, the 12 AWG conductors supplying the air-conditioning load are too small. Minimum branch-circuit conductor ampacity is 25-amperes (20 x 1.25) to comply with Part IV of Article 440.

The cable that supplies the air-conditioning load should either be increased to 10 AWG copper or this cable should be removed from the hole and pulled through a separate hole adjacent to the existing one.

Finally, it should be emphasized that no derating is required if the hole is not fire- or draft-stopped with thermal insulation or sealing foam.

Concrete-encased electrode

Q: I attended a recent seminar on Code changes. The instructor said a concrete-encased electrode must be part of the grounding-electrode system on new construction. I can't find this requirement in the 2005 edition of the NEC. Can you help?

A: There is a change in the language in 250.50. The revised wording reads like this: “Grounding Electrode System. All grounding electrodes as described in 250.52(A)(1) through (A)(6) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.”

The exception does not require the use of reinforcing steel in the foundation of an existing building or structure where it is necessary to break the concrete. The words in the previous edition only required the use of a concrete-encased electrode where it was available. Reinforcing steel in a concrete slab was usually not available when the electrical contractor arrived at the job site.

Now the electrical contractor will have to ask the general contractor to turn up about 2 feet of _-inch or larger steel reinforcing rod that is not less than 20 feet long on both sides and near the front of the proposed building if the location of the electric service is not known.

The following types of grounding electrodes that are present must be bonded together to form the grounding-electrode system: 10 feet or more of buried metal water pipe, the metal frame of a building, a concrete-encased electrode, a ground ring, rod and pipe electrodes, and plate -electrodes.

Securing flexible metal conduit

Q: Am I allowed to install 6 feet of unsupported or unsecured flexible metal conduit as fixture whips for lay-in type fluorescent lighting fixtures?

A: The answer is no if you are working in a jurisdiction that is enforcing the 2002 edition of the NEC. Exception No. 4 to 348.30(A) was added to the 2005 National Electrical Code that allows 6 feet of flexible metal conduit to be installed without being securely fastened where supplying luminaires (lighting fixtures) in an accessible ceiling.

Securing back-fed circuit breakers

Q: I was mailed a violation notice that said the back-fed circuit breaker in a panelboard was not properly secured in place. If this is a requirement in the Code, where is it located?

A: Back-fed circuit breakers that are plugged into the panelboard must be securely fastened in place so that they cannot be accidentally removed. This rule appears in 408.36(F): “Back-Fed Devices. Plug-in-type overcurrent protection devices or plug-in main lug assemblies that are back-fed and used to terminate field-installed ungrounded supply conductors shall be secured in place by an additional fastener that requires other than a pull to release the device from the mounting means on the panel.”

Twist-and-solder wire conductors

Q: Does the NEC permit twisting all of the equipment-grounding conductors together and soldering them in a junction box? Can solder be used to splice branch circuit conductors?

A: Twisting conductors together for splices and taps and then soldering them is still an acceptable method of making connections. In 110.14(B) this sentence appears: “Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered.” Tightly twisting the conductors together makes them mechanically and electrically secure and prepares them for solder. Solder flux used on the splice cannot adversely affect the conductors or equipment.

Because equipment-grounding conductors are being connected together, it is appropriate to look at connection methods permitted in Article 250-Grounding and Bonding. There is a sentence in 250.8 that limits the use of solder, but it does not apply to this question. The sentence that restricts the use of solder reads: “Connection devices or fittings that depend solely on solder shall not be used.”

Branch circuit for microwave oven

Q: The wiring for a new home includes a separate 20-ampere branch circuit for a microwave oven. The outlet for this circuit is located on the underside of a kitchen cabinet above the oven. At trim-out, I installed a 15-ampere duplex receptacle for this branch circuit. The inspector said the 15-ampere receptacle should be replaced with a single 20-ampere receptacle. Is this a Code requirement?

A: No, it is not a Code requirement unless the microwave oven has a nameplate current rating in excess of 12 amperes. According to 210.21(B)(2) and Table 210.21(B)(2), the maximum permissible load for a cord-and-plug connected appliance is 12 amperes on a 15-ampere receptacle and 16 amperes for a 20-ampere receptacle.

A 15-ampere duplex receptacle is two receptacles and is permitted on a 20-ampere branch circuit. For branch circuits that have two or more receptacles, Table 210.24 lists 15- and 20-ampere receptacles as suitable for connection to a 20-ampere branch circuit. Although a 15-ampere duplex receptacle is mounted on a single yoke, it is two receptacles according to 210.24.

Protection for NM-B cable

Q: Does the NEC allow NM-B cable to be run though metal studs that are provided with a threaded bushing that is held in place by a locknut?

A: There are two Code references that apply to this installation. They are 300.4(B)(1) and 334.17. Where nonmetallic-sheathed cable is pulled through slots or holes in metal framing members, the cable must be protected by listed bushings or listed grommets that are securely fastened to the framing members prior to installation of the cable. This requirement is more restrictive in 334.17 because the grommets or bushings must be listed for protection of the cable.

If the threaded bushing that is held in place by a locknut is listed for the purpose of cable protection, it satisfies the requirements in the National Electrical Code.

Wiring through a safety switch

Q: Am I allowed to pull conductors through a fused safety switch to feed another switch?

A: You are allowed to pull conductors through a switch enclosure provided that the wiring space in the enclosure is not reduced by more than 40 percent. Where splices or taps are made in the wiring space, the conductors, splices or taps cannot fill the wiring space to more than 75 percent of the cross-sectional area of that space. These requirements appear in 312.8 and 404.3(B). EC

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