The number and the content of "Code Question of the Day" submissions concerning the installation of nonmetallic sheathed cable suggests vast differences among, or lack of proper inspection of, electrical installations using Type NM cable as the wiring method. This, coupled with some electrical installers' seemingly indifferent attitude toward keeping a "neat and workmanlike manner," suggests a need for improved education relating to the installation requirements for nonmetallic-sheathed cable.
Following are some questions submitted to "Code Question of the Day."
QUESTION: We have a new inspector in our area and he's driving me nuts. "Mr. Gung Ho" says that, in the attic of new homes, he wants the NM cable that is run across the joists to be protected by guard strips. He told me to check the Code, but I can't find anything in Article 336 where he told me to look. What should I do?
ANSWER: Three cheers for Mr. Gung Ho. Take another look in Article 336, specifically Section 336-6(d). This tells us, "The installation of cable in accessible attics shall also comply with Section 333-12."
Article 333 relates to "Armored Cable: Type AC," but Section 333-12 is applicable by reference from Section 336-6(d). Section 333-12 requires, "where run across the top of floor joists (meaning the attic floor joists), the cable shall be protected by substantial guard strips that are at least as high as the cable." This is a "performance" requirement because you are not told how to do it, just that you must do it. It can be readily accomplished by fastening 1 x 2 furring strips on each side of the cable.
You could also protect the cable by not running it across the top-of-floor joists. Instead, the cable could be run through bored holes in the joists. Remember Section 300-4(a)(1) when doing this. Holes shall be bored so that the edge of the hole is not less than 1-1/4 inches from the nearest edge of the wood member. If you can't maintain this distance you must protect the cable with a steel plate at least 1/16 inch thick.
Another point to remember is that Section 333-12 requires this protection in the entire attic space only if it is accessible by permanent stairs or ladders. Where the attic space is accessible through a scuttle hole, the protection is required only within 6 feet of the scuttle hole.
QUESTION: When installing nonmetallic sheathed cable at the main panel in a residence, how is it possible to fasten each cable within 12 inches of the cabinet as required by Section 336-18? If I bring all the cables to the studs to fasten them, then I can't keep the required clearance from the edge of the stud.
ANSWER: Section 300-4(d) requires a steel plate at least 1/16-inch thick to be installed where 1-1/4-inch clearance from the edge of the framing member cannot be maintained. A header between the studs could be installed to secure the cables to.
QUESTION: Table 370-16(a) is titled "Metal Boxes." How do I determine the box fill requirements when using NM cable and nonmetallic boxes?
ANSWER: Section 370-16(a)(2) requires nonmetallic boxes to be durably and legibly marked by the manufacturer with their cubic-inch capacity. Using this information, together with Table370-16(b) requirements, the installer is required to comply with the box conductor fill requirements. Then of course, there is the clamp fill, support fittings fill, device or equipment fill, and the equipment grounding conductor fill. If you're looking for the cubic-inch capacity of a nonmetallic box, you will find the durable and legible marking inside the box at the back.
QUESTION: The building I am wiring has steel studs. The inspector says I have to install grommets or bushings to protect the cable where I run the Type NM cable through holes in the studs. I can't find this anywhere in the Code. Is he right?
ANSWER: Yes, Section 300-4(b)(1) requires that, where nonmetallic cables pass through holes or slots in metal framing members, the cable shall be protected by bushings or grommets covering all metal edges and securely fastened in the opening prior to installation of the cable.
SYNOPSIS: Every effort has been made through the Code-making process to establish requirements for the installation of nonmetallic-sheathed cable to ensure that complying installations will be safe. There's no reason for inspection or installation procedures not to be consistent with these requirements.
TROUT is a technical consultant For Maron Electric Company of Chicago, and he is Chairman of the National Electrical Code-Making Panel 12, representing NECA. He is also the principal author of Electrical Contractor magazine's on-line feature, "Code Question of the Day."