Residential Wiring

Here’s the second part of a series reviewing the most popular questions that have appeared in NECA’s online Code Question of the Day and have generated the most comments from the subscribers. (The first part is available here.) All answers are updated to comply with the 2008 National Electrical Code (NEC). If you are not a subscriber and would like to receive the Code Question of the Day online, go to, and follow the links.

> Here’s a question that comes up repeatedly concerning grounding of the yoke on snap switches.

I replaced some switches in a residence and found they used NM cable without a ground wire. How do I comply with the requirement that switches must be grounded?

NEC 404.9(B) requires that snap switches shall be connected to an equipment-grounding conductor and shall provide a means to connect metal faceplates to the equipment-grounding conductor. There is an exception to this requirement, which provides that where the wiring method does not include an equipment-grounding conductor, a snap switch without a connection to an equipment-grounding conductor shall be permitted for replacement purposes only. A snap switch wired under this exception where located within reach of conducting surfaces must be provided with a faceplate of nonconducting material or shall be protected by a ground-fault circuit interrupter.

Older versions of Type NM cable did not have a grounding conductor, and the NEC, up until the 2005 edition, permitted but did not require a grounding conductor. The 2005 edition of the NEC in 334.108 added a requirement that Type NM cable must have an equipment-grounding conductor.

> On the subject of nonmetallic--sheathed cable, there always seems to be some confusion about the various types or designations of nonmetallic-sheathed cable.

Why are the cartons for NM cable and the cable marked Type NM-B? The NEC only shows Types NM, NMC and NMS.

Prior to the 1984 NEC, Type NM cable was permitted to contain conductors rated 140°F (60°C). Since the 1984 NEC, the conductors used in nonmetallic-sheathed cable are required to be rated 194°F (90°C). However, the permitted ampacity of the conductors used in nonmetallic-sheathed cable must be in accordance with the 60°C (140°F) conductor temperature rating. This change in the construction requirements of nonmetallic-sheathed cable led to a UL designation of this new cable by adding the suffix B.

So, while you still will find older installations with cable marked NM, all installations made after 1984 will be marked as Type NM-B cable. The NEC does not use this designation since all cables now must comply with the requirements of this designation.

Type NMC-B cable has an overall, corrosion-resistant, nonmetallic jacket for use in corrosive areas, and Type NMS-B is not yet listed.

> Running Type NM-B cable outdoors is apparently still a question in the minds of many installers.

If NM cable is run under the shelter of a patio roof or in an overhang where it’s not subject to mechanical damage and not out in the weather, then it should be fine, right? Also, if the NM cable is run in a metal conduit outdoors and isn’t subject to the weather, what would be wrong with that?

Hey man, are you asking or telling? NEC 334.12(B)(4) is very clear. Type NM cable shall not be used in wet or damp locations. Under a patio roof or in an overhang is a damp location. Before you consider running NM cable in a metal conduit, check the raceway articles under 342.22, 344.22 and 358.22. They all say, “Cables shall be permitted to be installed where such use is not prohibited by the cable article.” Type NM cable cannot be run in any raceway, other than using the raceway as a sleeve for protection in accordance with 334.15(B) and 300.154(C).

When installing nonmetallic-sheathed cable, is an outlet box required where an outdoor fixture is being installed? I see a lot of installations where the NM cable is run through the outside wall, and a fixture strap is mounted on the outside wall to hang the fixture using the canopy to cover the splices.

According to NEC 300.15, an outlet box is required at each conductor splice point unless permitted in 300.15(A) through (M). The installation you are describing is not covered in (A) through (M). NEC 314.27 requires that the boxes used at lighting outlets be designed for the purpose. The box must be installed so that a lighting fixture can be installed.

I ran a 1-inch PVC conduit from the house to a detached garage, stubbing up in the garage and into the panel in the basement. I pulled in an NM cable consisting of 3-12 AWG with an equipment-grounding conductor. I installed a bushing where the PVC ended in the garage and ran the cable up to an outlet box from which I intend to switch and feed one circuit for lighting and use the other circuit to feed receptacle outlets. I ran the other end into the panel in the house. Is there something wrong with this installation?

Nothing a properly trained electrician couldn’t fix. First, let’s look at some rules for feeding this type of separate structure.

• NEC 225.30 requires that a separate structure be supplied by only one feeder or a single branch circuit. A multiwire branch circuit is considered a single circuit. (So far, so good.)

• NEC 225.31 requires that means must be provided for disconnecting all ungrounded conductors that supply the building. This can be accomplished by feeding a two-pole snap switch in the garage. (Still good.)

• NEC 225.38 has an exception that permits snap switches as the disconnecting means. (Still good.)

• NEC 225.39(B) requires for a two-circuit installation that the disconnecting means be rated not less than 30 amperes. Now comes our first problem. You are using 12 AWG conductors in your feeder, and the two-circuit installation requires a 30-ampere disconnect. (Strike one!)

• NEC 250.32(A) requires that separate structures have a grounding electrode or grounding-electrode system; however, the exception tells us that a grounding electrode shall not be required where only a single branch circuit, including a multiwire circuit, supplies the structure and the branch circuit includes an equipment-grounding conductor. (So far, so good.)

• NEC 334.12(B)(4) does not permit Type NM cable to be used in wet or damp locations. NEC 300.5(B) tells us that the interior of raceways installed underground shall be considered to be a wet location. (Strike two!)

Before we strike out, let’s make the necessary changes. Since apparently you don’t want to complete your raceway system and pull single conductors with a “W” rating for use in a wet location, let’s pull a 10-3 AWG with ground Type UF cable. Terminate this cable on a 30-ampere snap switch in the garage and on a two-pole circuit breaker in the residence panel. Remember, 210.4(B) requires multiwire circuits to be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.

> And now that we have the use of NM cable with respect to wiring outdoors all figured out, let’s look at a question on how to connect Type NM cable to boxes and enclosures.

Are cable connectors required where Type NM cables enter a box or a panel?

Yes, a general rule. NEC 314.17(A) requires that where cable is used with metal boxes, the cable must be secured to the box. Where nonmetallic boxes are used, NEC 314.17(C) requires that, in all instances, the cable must be secured to the box. Securing to the box may be done by use of cable clamps, which are a part of the listed box, or by separate cable connectors. There is one exception to this rule where nonmetallic-sheathed cable that is used with single-gang boxes (2¼ x 4) securing the cable to the box is not required. Where single-gang boxes are used, multiple cable entries are permitted in a single cable knockout opening.

NEC 312.5(C) requires that where cable is used, each cable must be secured to the panel enclosure. Because installers repeatedly violated this rule and jammed in multiple nonmetallic cables through a large PVC connector, a new requirement was added to the 2002 NEC that appears now in NEC 312.5(C) Exception. This exception permits cables with entirely nonmetallic sheaths to enter the top of a surface-mounted enclosure through one or more nonflexible raceways not less than 18 in. and not more than 10 ft. in length, provided the following conditions are met:

(a) Each cable is fastened within 12 in.—measured along the sheath—of the outer end of the raceway.

(b) The raceway extends directly above the enclosure and does not penetrate a structural ceiling.

(c) A fitting is provided on each end of the raceway to protect the cables from abrasion, and the fittings remain accessible after installation.

(d) The raceway is sealed at the outer end using approved means in order to prevent access to the enclosure through the raceway.

(e) The cable sheath is continuous through the raceway and extends into the enclosure beyond the fitting not less than ¼ in.

(f) The raceway is fastened at its outer end and at other points in accordance with the applicable article.

(g) Where installed as conduit or tubing, the allowable cable fill does not exceed that permitted for complete conduit systems by Table 1 of Chapter 9 of the NEC and all applicable notes thereto.

Remember, this exception can be used only with surface-mounted enclosures and using nonflexible raceways.

Submitting a question to the Code Question of the Day:

If you have a question relating to the NEC, or if you wish to comment, you may send it to

TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at 352.527.7035.


About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.