Type NM Cable and More

Here's the fourth 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 subscribers. 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 www.neca-neis.org, and follow the links.

> In a recent Code Question of the Day, I was asked about the limitation on the use of Type NM cable run exposed above a suspended ceiling. Part of the problem is the definition of “exposed” (as applied to wiring methods), which includes behind panels designed to allow access. The wording implies that NM cable can be run above a suspended ceiling in a manner that would not be exposed.

I am a municipal building official in Connecticut, where we enforce the 2005 NEC. Section 334.12(A)(2) “Uses Not Permitted” states that nonmetallic cable cannot be installed above a suspended ceiling other than in dwellings. What is the reasoning behind this requirement?

This requirement resulted from the work of a Task Group on Nonmetallic-Sheathed Cable for the 2002 NEC. Basically, I believe it reflects the concern relating to physical damage. Wiring in cavities often is subjected to abuse by other trades, and the cable also could be damaged in steel truss roof construction. The task force felt these types of damage would be more common in types of buildings other than one-family, two-family and multifamily dwellings. See section 334.12(A)(2).

> Here’s a question typical of the confusion 333.12(A)(2) created when the requirement for grounding of snap switches was accepted. The requirement is clear enough, but it appears that the exception requires approval of the authority having jurisdiction (AHJ) to determine whether the switch is located within reach of conducting surfaces if a nonconducting faceplate is not used.

I replaced some switches in a residence and found that 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 effectively grounded and must provide a means to ground metal faceplates. There is an exception to this requirement that where the wiring method does not provide an equipment ground, a snap switch without a grounding connection is permitted for replacement purposes only. If you use this exception where the snap switch is located within reach of earth, grade, conducting floors, or other conducting surfaces, a nonconducting faceplate must be used. Prior to the 2005 NEC, an equipment-grounding conductor was permitted in Type NM cable, but a proposal accepted for the 2005 NEC now requires that Type NM cable must have, in addition to the insulated conductors, an insulated or bare conductor for equipment-grounding purposes only. See sections 404.9(B) and 334.108.

> The following question brings us right back into determining the classification of buildings by types of construction.

Can Type NM cable be used as a wiring method for 277V high-bay fixtures in a warehouse?

The first thing you must do is determine the classification for the type of construction. Type NM cable is permitted to be installed in occupancies that are of Types III, IV, and V construction other than dwelling type. However, it must be concealed within walls, floors or ceilings that provide a thermal barrier, per NEC 334.10(3). The voltage rating is not the problem. Type NM cable is rated for use at 600 volts or less. See section 334.10(3).

> This next question relates to a commonly misunderstood requirement for circuits supplying continuous loads.

Let me break down my interpretation of a NEC section: 210.11 Branch Circuits Required. “(A) Number of Branch Circuits. The minimum number of branch circuits shall be determined from the total calculated load and the size or rating of the circuits used. In all installations, the number of circuits shall be sufficient to supply the load served. In no case shall the load on any circuit exceed the maximum specified by 220.18.” When you look at the parts of the section that are bold, my interpretation is different than what is traditionally taught. First let’s review the first part. What establishes the rating of the circuit? Section 210.3 informs us that the breaker does. A breaker can hold only 80 percent of its rating past three hours, a UL standard for inverse trip.

Well, not exactly. You seem to be hung up on this 80 percent loading. Yes, the UL White Book says, “Unless otherwise marked, circuit breakers should not be loaded to exceed 80 percent of their current rating, where in normal operation the load will continue for three hours or more.” Notice it says, “where in normal operation the load will continue for three hours or more.” Notice also that it says “should,” which is not a mandatory comment, although it’s a good one. The circuit breaker “may” trip if loaded to more than 80 percent of its rating depending on a lot of other factors, e.g., ambient temperature, aging of the breaker, integrity of the terminations on the line and load sides of the breaker. The NEC provides in 210.20(A) for continuous loads. All loads, of course, are not continuous, and if I understand correctly what you are saying, you are treating all loads as continuous. However, what you are doing is not a bad idea. The NEC in the FPN to 220.3(A) tells us that the “unit values herein are based on minimum load conditions and 100 percent power factor and may not provide sufficient capacity for the installation contemplated.” See section 210.20(A).

> Again, running NM cable in a raceway system appears to be confusing and questionable.

A question on the issue of installing NM cable outdoors. We used to run NM cable in residential garages, which had exposed studs, and it was considered legal. So if NM 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, running NM in pipe from a residential panel to enter into the home is protecting it from mechanical damage, and it isn’t subject to the weather. What would be wrong with this?

I believe the answers to your questions are that 334.12(B)(4) does not permit type NM cable to be used in damp or wet locations. Type NM cable cannot be run outdoors even if it is run in a raceway because this is considered to be a wet location. Type NM cable cannot be run in any raceway other than using it as a sleeve in accordance with 334.15(B) and 300.15(C) or in accordance with 334.12(A)(1) Exception. This exception, which was accepted for the 2008 NEC, permits Type NM cable to be run in Type I and Type II construction where it is installed in a raceway permitted for that type of construction. See sections 334.12(B)(4), 334.15(B), 300.15(C) and 334.12(A)(1).

> Here we go again: Up against that definition of “exposed” as it relates to dropped or suspended ceilings.

What is the maximum length of NM cable that can be used to connect a recessed fluorescent fixture within an accessible ceiling?

The NEC does not specify a maximum length. The NM cable must be supported within not more than 4½ feet of the fixture, according to 334.30(B). However, if this accessible ceiling is an occupancy other than one and two family and multifamily dwellings, 334.12(A)(2) does not permit the use of exposed Type NM cable.

If the accessible ceiling is a space used for environmental air (plenum), Type NM cable cannot be used. See sections 334.30(B), 334.12(A)(2) and 300.22(C)(1).

> When referring to Type NM cable, there always is the question relating to securing the cable to boxes and cabinets.

I use NEC 314.17 to enforce the requirement that Types NM and UF cable be attached to boxes they enter. Are there any other sections of which you are aware that would reinforce that section?

NEC 314.17 covers the requirements for securing NM cables to outlet boxes thoroughly, and I can only add 312.5(C), which requires cables to be secured to the cabinet, cutout box or meter socket enclosure. Be sure you note the Exception to 314.17(C), where securing cable to a single-gang box may not be required. See section 312.5(C).

> Now here’s a question showing the frustration relating to running Type NM cable in a circular raceway.

I am getting more confused each time the question of running NM cable in a raceway is brought up. Why don’t the powers that be say yes or no in 334.10 and 334.12? What’s your take on this?

In my opinion, there is no reason to run NM cable in a raceway other than using it as a sleeve to protect the cable from physical damage. If a raceway system is run complete between outlet points as shown in 300.18, I believe that conductors for general wiring as shown in Article 310 should be used. NEC 334.80 goes to great lengths to determine the permitted ampacities of NM cable where bundled or stacked or in contact with thermal insulation but is silent on the heating effects of NM cable when enclosed in a raceway. Note 9 to Table 1 in Chapter 9 permits a multiconductor cable to be treated as a single conductor when figuring conduit fill and explains how to treat an elliptical cross section but offers no tables showing the approximate diameters.

The only place in the Code where it specifically permits installing NM cable in a raceway system is the new exception to 334.12(A)(1) where Type NM cable shall be permitted in Type I and II construction where installed in raceways permitted to be installed in Type I and II construction.

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...

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