In residential wiring, the most commonly utilized type of wire is NM wire, commonly referred to as Romex wire. This wire is typically employed for a wide range of electrical installations, including GFCI outlets, lighting fixtures, and standard wall outlets. Discover the similarities that exist between NM wire and Romex wire.
- NM wire, or non-metallic wire, consists of electrical conductors encased in a PVC outer sheath. This sheath encases multiple individual wires, which are made of copper and are each covered in a layer of plastic.
- Romex is a brand name associated with a specific kind of non-metallic (NM) wire. While some people may use Romex to refer to any NM wire, it is important to note that it specifically pertains to NM wire produced by Southwire Company.
Contents
NM Cable vs. Metallic Sheathed Cable
NM wire differs from metallic sheathed cable, often referred to as BX cable, as it consists of individual wires encased in a metal sheath for protection. This distinction is important: the metal sheath provides a grounding mechanism for devices, while the plastic sheath of NM wire does not serve this grounding purpose.
What is Non-Metallic (NM) Electrical Wiring?
Regardless of the NM cable brand you choose, the cable generally consists of three main components: the outer sheathing, the insulation of the wires, and the individual conductors. Let’s explore these elements in greater detail.
Cable Insulation
The outer covering of NM electrical cable consists of a robust 30 mil-thick PVC jacket designed to safeguard the group of individual wire conductors.
NM cable featuring 14-gauge wires is encased in a white outer sheath, while cable with 12-gauge wires is covered in a yellow sheath, and 10-gauge wire is typically found with an orange or reddish outer layer.
Older NM cables might feature a white outer sheath, irrespective of the gauge of the individual wires inside.
To establish connections with devices, the outer covering of the cable is sliced open sideways using a tool known as a cable ripper. Once the rip is complete, any leftover sheathing is removed using a utility knife, scissors, or the cutting edge of a wire stripper.
Cable Insulation
The individual wires within the sheathing are covered with color-coded PVC (polyvinyl chloride) insulation. Typically, the insulation of these conductors is black, white, or red.
Additionally, there exists a copper grounding wire that is typically left exposed and uncoated, although it may occasionally be covered with green PVC.
Typically, a piece of paper is interlaced among the separate wires inside the cable to act as a divider. This paper can be cut and removed from the damaged section of the cable.
Conductors (Wires)
Every electrical conductor inside the insulation consists of pliable, bare copper.
In the past, certain types of NM cable incorporated aluminum or aluminum wrapped in copper, but current regulations prohibit these materials. Now, only solid copper wires are permitted for residential electrical installations.
NM Wire Sizes
NM cable is available in various wire sizes, but typical residential circuits generally utilize 12-gauge or 14-gauge wire, which can contain either two or three conductors, in addition to a bare copper ground wire. Below are the amp ratings:
- A cable marked as 14-2 with ground consists of two insulated conductors made of 14-gauge wire along with a bare copper wire for grounding. This type of cable is designed for 15-amp circuits and is commonly utilized to supply power to circuits originating from panels and specific devices.
- A cable marked as 12-3 with ground consists of three insulated conductors of 12-gauge (white, black, and red) along with an uninsulated copper grounding wire. This type of 12-gauge cable is suitable for circuits with a 20-amp rating.
- For circuits exceeding 30 amps, like those for 30- or 40-amp appliances, it may be necessary to use even thicker NM cable, such as 10-gauge or 8-gauge.
Tip
To establish a connection with devices, the insulation is removed from the copper wire using a manual wire stripper. This tool features a set of holes that correspond to various wire diameters or gauges. For instance, to strip the insulation from a 12-gauge wire, select the hole marked 12.
Guide to Purchasing Romex and NM Cables
While Romex is a well-known name in NM-sheathed wire, it is not the sole manufacturer of NM wirehowever, it is the leading brand in North America.
A distinctive characteristic of Romex is its inclusion of SIMpull, a smooth coating on the insulation that minimizes friction during the installation of the cable through studs and challenging areas. Other manufacturers have started to adopt a comparable coating as well.
Aside from this, the copper wire in Romex is essentially the same as that in other brands, with no significant differences. In accordance with electrical regulations, the wire gauges and metal composition are consistent across the board.
History of the Romex Wire Brand
The name Romex is derived from Rome Cable Corp., based in Rome, New York, which was the original manufacturer of the cable, although the significance of the x in Romex is still unclear.
The firm was a frontrunner in its sector until it declared bankruptcy in 2003, and its manufacturing facility was mostly torn down in 2010.
Rome Cable Corp. served as a significant supplier of aluminum wire manufactured by its parent company, Alcoa. Aluminum wire is regarded as less effective than copper wire, and its use in residential settings is deemed hazardous. In 1964, a court mandated that Alcoa divest its interests in Rome Cable Corp.
Advantages and Disadvantages of NM Wiring
- Lightweight
- Easy to draw out.
- Simple to slice
- The covering is simple to tear.
- Easy to manage
- Not intended for visible application.
- Primarily unsuitable for outdoor applications.
- Not suitable for use in suspended ceilings.
- Not resistant to fire
- The sheathing can be penetrated.
You can wire a house exclusively with NM wire. Taking all aspects into account, DIY electricians will discover that utilizing NM wiring makes their projects simpler and more cost-effective.
Pros
NM wire can be used in plastic enclosures without the need for additional grounding for the box. It is lighter than metal-sheathed wiring, making it simpler to unwind and straighten.
NM wire can be effortlessly threaded through stud openings due to its smooth outer layer. For Romex, an additional coating is applied to enhance the slickness of the sheathing.
NM cable can be easily trimmed using a pair of side-cutting pliers. After cutting, it can be effortlessly stripped along its length, as the outer layer is made of plastic rather than metal. A utility knife can be used to slice through the sheathing, and it can be pulled away by hand.
NM cable can be more conveniently secured to framing components, needing just simple plastic cable staples that are lightweight.
Cons
NM cable is not suitable for specific scenarios, including outdoor applications (with the exception of UF-type cable, which is designed for direct burial) or when the wiring is visible on the surface of foundation walls. In such cases, the electrical code requires the use of conduit for installation.
In specific applications or when not enclosed within a wall, NM cable requires some form of protection.
NM cable is not suitable for installation above drop ceilings in light commercial settings.
Is it permissible to install Romex inside conduit?
Using Romex inside conduit is not a common practice, although it is permitted in various regions. Typically, it is advisable to run separate THHN or THWN-2 wires through conduit instead of using Romex or NM wire.
Is it permissible to join Romex wires together?
Splicing Romex is permissible, provided it is carried out in accordance with code regulations, typically within an accessible electrical junction box. It is important that splices are not concealed within walls. By using an electrical junction box, you ensure that the splices are secure, visible, and accessible for any future maintenance.
What is the maximum number of Romex cables that can be passed through one opening?
Typically, you can pass two Romex cables through one hole in a stud. If the opening is larger, you can accommodate additional Romex wires.
