Working With Nonmetallic-Sheathed Cable (Romex¨) David W. Doty copyright 1992 David W. Doty Nonmetallic-sheathed cable, or Romex¨ as it is often called in the electrical trade, is an excellent choice for residential wiring. It is easy to work with, inexpensive, abundant, and long lasting when installed properly. Construction: Nonmetallic-sheathed cable is a factory assembly of two or more insulated conductors covered by an outer sheath. In addition to the insulated conductors, the cable may (and usually does) include a bare or insulated grounding conductor. Cable is available with copper conductors ranging in size from No. 14 up to No. 2 gauge. Cable with aluminum or copper-clad aluminum conductors is available in the No. 12 to No. 2 sizes. Although aluminum is available, I would avoid using it because of the potential corrosion problem at the connections. Photo #1 shows various types and sizes or nonmetallic-sheathed cable. INSERT PHOTO 1 The most common type of cable is designated Type NM by the National Electrical Code¨ and is covered in Article 336 (NEC¨-336) in the 1990 Code book. This cable has an outer sheath made from a flame-retardant and moisture-resistant material. The insulation on the inner conductors is rated at 90ˇ C. Manufacturers designate their cable as NM-B. The "B" was added to distinguish the cable from earlier cable manufactured before the 90ˇC insulation rating was required. In spite of this 90ˇC rating, the conductor ampacity is based on a 60ˇC insulation rating (NEC¨-336-26). The maximum rating of copper cable at 60ˇC is shown in table #1. INSERT TABLE 1 Applications NM cable may be used in one or two family and multifamily dwellings as well as many other structures. It may be used as both exposed or concealed wiring in areas that are normally dry. It may not be embedded in concrete or used in structures exceeding three floors above grade, commercial garages, theaters, Places of Assembly, or motion picture studios. Type NM cable is also prohibited in storage battery rooms. This should be of special interest to AE users. Article 336-4 of the NEC¨ covers these areas where NM cable may not be used. Installation When NM cable is used as exposed wiring, it should be installed so that it closely follows the building surface (NEC¨ 336-10(a)). We do not want it hanging out and catching on everything that goes by. NM cable should also follow the lines of the building. In other words, do not run it at odd angles across walls or ceilings. Running it at odd angles may save you a couple feet of wire, but it will be a very poor looking installation. Article 110-12 of the NEC¨ requires electrical equipment to be "installed in a neat and workmanlike manner." Physical Protection Cable should be protected against physical abuse where necessary. This can be accomplished with conduit, guard strips, or by other means. It must also be protected where it penetrates floors in exposed applications by conduit extending at least 6 inches above the floor surface (NEC¨ 336-10(b)). When run at angles to joists in unfinished basements, the smaller size cables must be run through bored holes in the joists or on running boards. Cables with 2 No. 6 or 3 No. 8 conductors and larger may be run directly across the edge of the joists. When run parallel to the joists, all sizes of cable will be secured to the side of the joists (NEC¨ 336-12). Holes bored in joists or studs for NM cable must be positioned so that the edge of the hole is at least 1.25" from the edge of the framing member (NEC¨ 300-4). This is required to prevent nails or screws from penetrating the cable. If you are using 2"X4" studs with an actual size of 1.5"X3.5", the largest hole you can bore would be 1" in diameter. A 1" hole would have to be perfectly centered on the stud in order to maintain the 1.25" spacing on each side of the hole. If this spacing can not be maintained, a steel plate at least 1/16" thick must be placed on the edge of the stud or framing member to protect the cable. Support NM cable must be supported at intervals not to exceed 4.5 feet. It must also be secured within 12" of a steel box or cabinet which contains a cable clamp (NEC¨ 336-15). When using plastic boxes, the cable must be secured within 8" of the box (NEC¨ 370-7(c)). The exception to these rules is where the cable is fished into existing finished walls, where it would be impossible to support the cable. The most common method of support is the use of staples which are made specifically for this task. These staples are available in a variety of sizes for different size cables and are relatively inexpensive. Staples should be driven in straight and in such a manner so not to damage the outer sheath of the cable. Bends When bending or handling NM cable, care should be taken to prevent the outer sheath from being damaged. The minimum bending radius is 5 times the diameter of the cable (NEC¨ 336-14). For a #12/2 cable with ground, the minimum bending radius would therefore be approximately 2.5 inches. Less than 50 volts Article 720 of the NEC¨ covers systems operating at less than 50 volts. The minimum size wire allowed for systems operating at less than 50 volts is No. 12 copper or equivalent. For circuits supplying more than one appliance or appliance outlet, the minimum conductor size is No. 10 copper or equivalent (NEC¨-720-4). You must also consider the voltage drop associated with these low voltage circuits. The maximum permissible voltage drop for branch circuits should be 3% or less (NEC¨-210-19(a) (FPN No. 4)). This means that you can loose only .36 volts in your wiring for a 12 volt system, or .72 volts for a 24 volt system. For a system operating at 120 volts, you could loose up to 3.6 volts in your branch circuit wiring. Table #2 shows the maximum one-way distance in feet for #12 through #2 NM cable for systems operating at 12 volts at various current levels. For systems operating at 24 volts, multiply the distance times 2. For 120 volt systems, multiply the distance times 10. INSERT TABLE 2 Miscellaneous Requirements Where NM cable enters a box, the outer sheath must extend at least .25" into the box (NEC¨ 370-7(c)). Also, a minimum of 6" of free wire is required in the box to allow for splices or connection to fixtures or devices (NEC¨ 300-14). Often I run across wiring done by amateur electricians where there isn't enough wire left in the boxes. They cut the wires so short that it is almost impossible to remove the outlet or switch that they are connected to. At the other extreme, if you leave too much wire, you may not be able to stuff it all in the box. Unreeling NM cable in the smaller sizes normally comes coiled up in a box in 250 foot lengths. For jobs that require more than a couple feet of wire, the entire roll should be removed from the box and unrolled so that the cable lies flat and is not twisted. Cable that is twisted is very difficult to pull through bored holes. Photo #2 shows a simple device, made from a piece of plywood, some rope and a swivel, that can be constructed to facilitate unrolling cable that comes in a box. It is hung from an overhead support (rafter or whatever) by a rope and allows the wire to be unrolled without twisting. I do not know who originally came up with this idea, but it is simple and works quite well. INSERT PHOTO 2 Stripping If you are installing much NM cable, it would be worth a trip to the local hardware store to pick-up a tool made just for stripping the outer sheath. These tools, which are designed for use on flat cable, usually cost under two dollars and are well worth the money. The sheath is removed by slitting it down the middle on one side, peeling the sheath back, and trimming it off. Photo #3 illustrates one of these tools manufactured by Gardner Bender, Inc. (Cat.#CR-100). The sheath can be removed in the same manner with a knife, but with the tool you run less risk of cutting off center and damaging the insulation on the conductor. Connections When properly applied, twist on wire connectors (commonly called a Wire-Nut¨) will make reliable long lasting connections. It should be noted that these connectors are rated for use in dry locations only. Do not use them outdoors and expect the connection to last. There are many different brands of connectors on the market. My favorite is the Wing-Nut¨ made by Ideal. Its design gives you a lot of leverage when twisting them on. Buchanan also makes a connector very similar to the Wing-Nut¨ which is also quite good. Sources National Electrical Code¨ -1990 by the National Fire Protection Association Standard Handbook for Electrical Engineers edited by Donald G. Fink and H. Wayne Beaty Access Author: David W. Doty 14702 33rd Ave NW, Gig Harbor, WA. 98332 (206) 851-2208 National Electrical Code¨ and NEC¨ are registered trademarks of the National Fire Protection Association Inc. Romex¨ is a registered trade name of the General Cable Company. Wire-Nut¨ and Wing-Nut¨ are registered trade names of Ideal Industries, Inc.