Most modern house wiring is plastic-sheathed cable (Romex is one brand), but you may find any—or all—of the wiring types described here in older houses. Inside cables or conduits are individual wires, or conductors, that vary in thickness (gauge) according to the current (amps) they carry. More about that in a bit.
Nonmetallic sheathed cable (NM or Romex) is by far the most common type of cable. Covered with a flexible thermoplastic sheathing, Romex is easy to route, cut, and attach. Cable designations printed on the sheathing and the sheathing color indicate the gauge and the number of individual wires inside.
Typically, Romex cable has two insulated wires inside and a ground—which may be insulated or, more often, bare wire. Thus, the Romex used for a standard 15-amp lights-and-outlets circuit will be stamped 14/2 w/grd. For a typical 20-amp circuit, 12/2 w/grd is required. Three-way switches are wired with 14/3 or 12/3 cable, which has an additional insulated wire. Again, wire gauge is rated for the current it can carry, so although you can wire 15-amp circuits with 12-gauge wire, you can’t use 14-gauge wire anywhere in a 20-amp circuit.
Knob-and-tube wiring is comprised of individual wires, each with a protective sheathing of cloth or rubber. The wires are supported by ceramic knobs and tubes that act as insulators to keep the wires from contacting the framing or other surfaces in a house. This type of wiring is no longer installed, but there’s still plenty of it in older houses. If its insulation is intact and not cracked, it may still be serviceable. You may even be able to extend it, but have an electrician do the work.
Metal-clad (MC) cable or armored cable (AC) is often specified where wiring is exposed and could be damaged. In AC cable, the metal covering of the cable acts as the ground; in MC cable, there is a separate insulated green wire that serves as a ground. To strip either type of metal cable, use a Roto-Split cable stripper; it’s vastly superior to the old method of using a hacksaw and diagonal cutters.
MCAP (or MCI) cable has been replacing MC cable in commercial production work because it’s faster to terminate. The “AP” in the name stands for “All-Purpose”, and the cable is essentially an upgraded version of the old MC cable where the armor cladding is designed to be a safe and reliable ground path. You don’t have to bring the cable ground wire into the panel box to terminate it. Instead, the bare ground wire is pulled back around the aluminum jacket and the quick connectors just snap on. Snap the connector into a panel knockout and you’re done. Quick connectors have spring-loaded clips that connect snugly to the panel, so there are no screws to tighten as there are with locknut clamps. When knockouts are limited, using a double-barrel connector (see photo) allows you to quickly put two cables into a panel box.
Conduit may be specified to protect exposed wiring indoors or outdoors. It is commonly thin-wall steel (electrical metallic tubing, or EMT), or PVC plastic. Metal conduit serves as its own ground. Apart from service entrances, conduit is seldom used in home wiring. When connected with weathertight fittings, conduit can be installed outdoors—and PVC conduit even underground.
Cables provide a lot of information in the abbreviations stamped into the sheathing. For example, NM indicates nonmetallic sheathing, and UF, underground feeder which can be buried. The size and number of individual conductors inside a cable are also noted: 12/2 w/grd or 12-2 W/G, for example, indicates two insulated 12AWG wires plus a ground wire. Cable stamped 14/3 W/G has three 14AWG wires plus a ground wire. (The higher the number, the smaller the wire diameter.) The maximum voltage, as in 600V, may also be indicated.
Individual wires within cable have codes, too. T (thermoplastic) wire is intended for dry, indoor use, and W means wet; thus TW wire can be used in dry and wet locations. H stands for heat resistant. N, for nylon jacketed, indicates a tough wire that can be drawn through conduit without being damaged.
Finally, make sure the cable is marked NM-B. Cable without the final “-B” has an old-style insulation that is not as heat resistant as NM-B cable.
Excerpted from Wiring Complete, 3rd Edition (The Taunton Press, 2017) by Michael Litchfield and Michael McAlister
Available in the Taunton Store and at Amazon.com.
Wire and cable need to weave their way through a wide range of environments. From the walls of a home or office building to heavy-duty industrial or underground installations, conduits can protect wire and cable from nearly any threat. The right materials keep cables and wires safe from corrosion, impacts, sunlight, moisture and many other hazards that could impact performance or safety.
Electrical conduit is a plastic or metal tube that holds electrical wires or cables. It can be flexible or rigid and protects wires in a range of settings, including exposed or unfinished indoor areas or outdoor settings. To fit into diverse configurations and demands — including within concrete or underground — electrical conduit comes in many styles and sizes.
Wire conduit must terminate at an electrical box or another piece of equipment that serves the same purpose. Along with the electrical box and connectors, conduit is an important part of an electrical raceway. If it’s made of metal, the conduit can even take on the role of a grounding wire.
You can use conduit for different purposes, including:
For many installations, conduit offers great all-around protection and versatility.
Electrical conduit comes in numerous styles, built for specific installations and demands. You’ll find an array of sizes and materials in both flexible and rigid designs. Metal conduit might use aluminum or steel, while plastic conduit could be made of polyvinyl chloride (PVC) or more advanced materials like polyamide systems. Some types of conduit, like electrical metal conduit, are technically considered tubes, so you’ll see a “T” in their acronyms.
Below are some popular styles of electrical conduit:
EMT is typically made of galvanized steel or aluminum, so it’s a rigid style of conduit. Still, the metal walls are thin, making electrical metallic conduit lightweight and easy to bend. It attaches to other components of the electrical raceway via compression fittings.
As a more attractive style of conduit and one that bends easily, EMT is often used for exposed indoor wiring. While it can be used outdoors, these installations require watertight fittings, and the conduit might not last very long. Other types of conduit usually offer more protection in these instances.
ENT uses flexible plastic tubing to offer moisture and fire resistance, but like EMT, it can’t be used outdoors or in other exposed locations. It’s common in residential or commercial walls and within concrete structures. Electrical non-metallic tubing is flexible and uses glued fittings.
As the name implies, flexible metal conduit bends easily. This quality comes from its spiral design. FMT is a heavy-duty option, typically made of galvanized steel with threaded fittings. It’s commonly used for wire runs with significant bending or those placed in exposed areas, such as wiring for water heaters or in attics.
Liquid-tight flexible metal conduit is similar to FMT but with an added plastic sheath to make it watertight. It’s often used for outdoor wiring, like connecting AC units and other outdoor equipment. Indoor settings that have high moisture levels can also benefit from LFMC.
While often used for similar applications, LFMC and FMT typically outperform PVC conduit. PVC conduit is affordable and watertight, making it suitable for above and underground applications. However, it isn’t very durable and doesn’t bend easily, so it works best on straight runs.
Although PVC’s low cost is a big selling point, FMT conduit can reduce costs in other ways, such as significantly reduced assembly times, application flexibility and extended product life span. Thanks to these qualities, many electricians now opt for FMT and LFMC in place of PVC electrical wire equipment.
LFNC-B — a commonly used type of non-metallic flexible conduit — has a flexible PVC wall embedded with rigid PVC enforcements. It can bend but still offers structure and a watertight build. LFNC-B provides moderate protection in wet, dry or oily settings. It’s also flame-resistant. This durable conduit is popular indoors and outdoors in industrial or moisture-heavy applications, like pools and heating, ventilation and air conditioning (HVAC) systems.
You can also find liquid-tight flexible electrical conduit made of polyamide rather than PVC, adding resistance against oil, benzine, acid and solvents and flames. Polyamide conduit can even self-extinguish, making it ideal for industrial machines, robotics and other demanding applications.
Rigid metal conduit is one of the most heavy-duty conduit options. It has a thick wall, often made of galvanized steel, and uses threaded fittings. You’ll find it used to run cables through walls and support overhead utility service line connections. With its thick metal walls, RMC offers considerable impact and EMI protection.
RMC can be made of different metals and corrosion treatments. Galvanized steel is a popular choice that works well in industrial buildings, but other configurations can offer corrosion resistance in specific environments, such as those with chemical fumes or in coastal areas.
IMC is similar to RMC but thinner and lighter by about a third of the weight. It offers some middle ground between EMT and RMC. Made of rigid steel, IMC has corrosion-resistant coatings and can be threaded or non-threaded. It has a larger diameter on the inside and a smoother coating that makes it a bit easier to pull wires through compared to RMC. The NEC allows IMC to be used in the same applications as RMC, so it has mostly replaced RMC in both commercial and residential projects.
Fiber optic conduit protects and surrounds the individual strands of fiber optic cable. Fiber optic conduit offers greater protection for critical network communications in a wide range of tough environments, including underground installations that involve corrosive soil, temperature changes and accidental excavation.
This conduit is usually made of steel or another rugged metal and may incorporate PVC or fiberglass braiding. Because fiber optic cables transmit digital binary signals and not electrical currents, the conduit can be made of metal without creating shock hazards.
The food and beverage industry must meet strict sanitation requirements to avoid contamination. Without the right care, conduits and their fittings can house bacteria and other contaminants that could reach the processing line or otherwise taint the sterile environment. Sterilization processes can involve high-powered water jets, high heat and corrosive cleaning materials, any of which can wear away at coatings on the wiring or conduit.
The Food and Drug Administration (FDA) has guidelines and third-party testing requirements to help facilities avoid contamination from these sources. Conduits must be made of sanitary materials that are easy to clean and, in some places, safe for food contact. These standards come from organizations like the NEC, the Occupational Safety and Health Administration (OSHA), UL or EcoLab.
Food and beverage conduit and fittings are designed to meet these specifications and provide liquid-tight protection.
Conduit can support many types of wires and cables, but they typically use THHN or THWN wires. THHN refers to wires with thermoplastic insulation, high-heat resistance and a nylon coating, while THWN wires have thermoplastic insulation and heat and water resistance.
You’ll find THHN and THWN wires used on their own and within various cables, like Romex cables. While not common, you can run these cables through conduit. More commonly, individual THHN or THWN wires run through conduit.
If you’re using non-metallic cable, you’ll need a grounding wire, but metal conduit can also serve that purpose. Modern installations typically include a ground wire, but older constructions might use the conduit to ground the raceway.
Pulling wire or cable through conduit can be challenging if the wiring gets caught somewhere along the path. If you’re working with conduit that already has wire inside it, you can attach the new wire to the old one and pull it through. For new installations or old ones with damaged wiring, this approach might not be an option. Instead, you’ll probably need to try one of the wire-pulling methods we’ve covered on our blog:
Be sure to read our guide on running conduit in exterior walls or underground if you’ll be working in those settings. Both of these installations are more complex than others, often requiring a contractor and specialized tools.
The right type of conduit for your project depends on various factors, such as:
Conduit is a necessary part of many electrical raceways, but in some cases, you might be better off with braided sleeves. Braided sleeves offer another type of protection and can help with cable organization. They contain braided fibers typically made of plastic or metal that protect against high heat, corrosion, abrasion, moisture, chemicals and EMI.
The open weave of the sleeve supports heat dissipation, moisture evaporation and flexibility while providing accessibility. Braided sleeves can even be designed to resist specific threats, like fire and damage from rodents.
Choosing the right kind of protection for your cables can be complex. If you’re not sure which type of conduit or cable sleeve is best for your application, the pros at AerosUSA can help.
At AerosUSA, we know conduit is often the first layer of defense for wires and cables. That’s why we offer expert support and an array of conduits to fit whatever project comes your way. We’re committed to innovation and quality across our selection. From cost-cutting installation processes to longer-lasting materials, our cable and wire conduit and braided sleeves can help you keep wires and cables safe in even the toughest or most specialized settings.
Explore our wide selection of conduit, or reach out to us to learn more or get assistance with your next project.
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