House Electrical Wiring: What You Need to Know

Electrical Wire vs. Electrical Cable

Homeowners and electricians alike casually use the terms “wire” and “cable” interchangeably, but there is a significant difference.

The electrical code officially refers to wires as “conductors,” but let’s just call them wires to make it easy. Wires can be bare, such as for grounding and bonding electrical equipment and circuits. Or they can be covered with a material that only provides protection from corrosion and does not officially qualify as electrical insulation.

Most of you are probably familiar with the colorful wires encased in a code-recognized plastic insulating material. In the early years of electrical wiring, wires were insulated with black rubber. Today they’re insulated various high-tech materials that can withstand harsh environments and conditions. These can be underground; submerged in water; or exposed to sunlight, vibration, chemicals or high temperatures.

Individual electrical wires are generally required to be installed in metal or plastic conduit. The combination of the wires and conduit results in a complete wiring system.

On the other hand, a cable is a factory assembly of two or more bare, covered or insulated wires, enclosed in an overall nonmetallic or metallic sheath for physical protection. Unlike individual wires, cables are a standalone wiring system and are generally not required to be installed in conduit.

Please, Read the Fine Print

We’re all guilty of skipping the fine print in the User Agreement when we download a new app to our smartphones. But in effect, the information on a wire or cable is an important User Agreement.

It’s critical to use the wire or cable in accordance with the electrical code and the manufacturer’s instructions. Understanding this information can mean the difference between a safe or unsafe electrical installation. You’ll also get nice compliments from the local electrical inspector for doing your homework and paying attention to the details.

Individual wires are embossed with a lot of important information. Here are some examples of what to focus on:

• Look for a UL, ETL, CSA or other mark indicating the wire has been evaluated in accordance with national safety standards.
• The size, in American Wire Gauge (a national standard for sizing wires), such as “AWG 12,” or in millimeters.
• The type of metal used for the wire: CU for copper and AL for aluminum.
• Look for the type of insulating material, often multi-rated, such as:
  • Type MTW (machine tool wire);
  • Type THWN (thermoplastic/heat-resistant, approved for wet locations, with a nylon jacket);
  • Type THHN (thermoplastic/heat-resistant, approved only for dry or damp locations, with a nylon jacket).

With cables, the outer sheath contains much of the same information: manufacturer’s name, UL or other safety certification mark, voltage rating, size of the wires inside the cable, the type of cable and even the date of manufacture.

Wire Insulation Color and Sizes

To ensure safety and uniformity, the electrical code requires certain wires to have a specific color. The color green is strictly reserved for grounding and bonding wires. Equipment grounding wires make sure fuses and circuit breakers will trip open and shut off the flow of electricity if a ground fault occurs.

Bonding wires connect the electrical system to other metal systems in your home, like plumbing pipes, gas piping, cable TV and telephone. This ensures there’s no opportunity for a difference in voltage between any of these systems if there was an abnormal surge on the utility power line or a nearby lightning strike. A difference in voltage between any two systems could be a shock hazard.

The color white is reserved for neutral wires. Hot wires can be a variety of colors, but manufacturers have established common color schemes to keep things uniform across the industry.

The colors of wires you’re most likely to find in your home are:

• Black or red = HOT: Hot wires carry electrical current from the electrical panel to a switch, receptacle, light fixture, appliance or other piece of equipment.
• White = NEUTRAL: Neutral wires carry the electrical current back to the panel, completing the circuit.
• Bare or green = GROUND: In the event of a ground fault, the ground wire provides a low-resistance and effective path for the fault current to return to the electrical panel. This trips the circuit breaker or blows the fuse, cutting off the flow of electricity to help prevent electrical shock or fire hazards. A ground fault happens when a hot wire accidentally comes in contact with a grounded object. If you’re removing a faceplate at a wall switch and the screwdriver slips and makes contact between a hot screw terminal and the metal switch box, that’s a ground fault. That’s why it’s important to always turn off the power before working on circuits, and using a tester to make sure it’s off.

Cable Sheath Color and Sizes

The electrical manufacturing industry has a uniform color scheme for cable sheaths to make life easier for electricians and homeowners.

Except for certain types of underground wiring, individually insulated wires must be installed in metal or plastic conduit to have a complete system. On the other hand, the nonmetallic-sheathed cable prevalent in our homes is a complete, standalone wiring method with the insulated wires enclosed within a protective outer jacket.

Running cable is much easier and less expensive than installing conduit and wire. The color of a cable’s outer sheath is a handy way to remember the gauge and amperage rating of the wire inside the cable.

• Gray: Generally used for Type UF-B Underground Feeder and Branch-Circuit Cable and Type SE Service-Entrance cable. Check the embossed information on the cable sheath for details.
• Black: Generally 8- or 6-gauge wire, 40- to 60-amp circuits. Also check the embossed information on the cable sheath for details.
• White: 14-gauge wire rated for 15-amp circuits.
• Yellow: 12-gauge wire rated for 20-amp circuits.
• Orange: 10-gauge wire rated for 30-amp circuits.

Type NM-B Cable

This is the most common type of electrical cable in homes. As the name implies, this cable has an outer nonmetallic (NM) sheath made from plastic material.

The “-B” suffix indicates a nonmetallic-sheathed cable with individual wires that have an insulation rating of 90 C. This compares to outdated Type NM cable from the early 1980s that contained wires with insulation only rated at 60 C.

Type NM-B cable is permitted to be installed in one- and two-family dwellings and associated garages and accessory buildings, such as a backyard storage shed. It may be concealed or exposed in normally dry locations. The outer sheath is flame retardant. It’s also moisture resistant, so it can endure the nominal amounts of moisture that might occur during construction until the building’s walls, windows and roof are in place.

Typically, NM-B cable has two or three wires along with a ground wire. To translate the embossed information on the cable sheath, the “14” is the wire gauge, “2” is the number of insulated wires, and “with Ground” refers to the bare equipment grounding wire.

The hot and neutral wires are individually insulated. The ground wire is either bare copper or green insulated. The group of wires is wrapped in a paper covering. And the entire assembly is sheathed in a plastic outer jacket.

Common sizes and uses for Type NM-B cable:

• 14-2 (or 14-3) with Ground (rated for 15 amps); general purpose lighting and receptacle circuits.
• 12-2 (or 12-3) with Ground (rated for 20 amps); kitchen, laundry, bathroom and garage circuits.
• 10-2 (or 10-3) with Ground (rated for 30 amps); clothes dryers and A/C units.
• 8-2 (or 8-3) with Ground (rated for 40 amps); electrical ranges, cooktops, A/C units and large appliances.
• 6-2 (or 6-3) with Ground (rated for 55 amps); electrical ranges, cooktops, A/C units and large appliances.

Type UF Cable

Type UF indicates underground feeder (UF) and branch-circuit cable. It’s permitted to be installed underground in direct contact with the earth, and in other wet, damp, dry or corrosive locations.

Type UF primarily brings power to detached garages, outbuildings or for outdoor lighting. Unlike Type NM-B cable, the insulated wires and the bare ground wire in Type UF cable are molded within the outer sheath.

Compared to the ease of removing the outer sheath of Type NM-B cable, removing the outer sheath of Type UF cable often becomes a tug-of-war. However, it’s purposely made to be extra tough to withstand the harsh environment in a direct-burial installation.

Depending on the situation, Type UF is direct buried or run in conduit, such as a short length of metal or PVC conduit used as a sleeve under a sidewalk. It also must be protected from physical damage by metal conduit or Schedule 80 plastic conduit where it exits the ground. That way, you can’t wreck it with your lawn mower.

The insulated wires in regular Type UF are only rated for 60 C. Where Type UF will be installed indoors as a substitute for Type NM-B, look for Type UF-B to be sure the enclosed wires are rated for 90 C.

Type MC Cable 7

Type MC cable has largely replaced Type AC cable (armored cable). The latter was a popular factory assembly of insulated wires with a bare metal bonding strip, enclosed within flexible interlocked metallic armor.

Type MC cable is similar, but it’s manufactured in lots of sizes and configurations for all kinds of residential, commercial and industrial installations.

Type MC cable is a good option for unfinished areas where more robust protection is needed to avoid physical damage, such as a basement or garage workshop. Type NM-B cable would be riskier in that environment.

Common Specialty Cables for Electrical Wiring 8

Here’s a list of other specialty cables and wires you’ll find at home centers and hardware stores:

• Type SE Cable (service-entrance cable)
  • Type SE cable is similar to Type NM-B cable because it’s a factory assembly of insulated wires and a grounding wire. Type SE cables have a flame-retardant and moisture-resistant nonmetallic sheath. It’s often used as a substitute for Type NM-B cable for electric ranges, electric clothes dryers, air conditioning units and similar large appliances and equipment. It’s also a popular option if you need to install a feeder circuit from your main service panel to a sub-panel.

• Type USE Cable (underground service-entrance)
  • Type USE can be individual wires or a multi-conductor cable. It cannot be used for interior wiring because it lacks flame retardant properties. It’s intended for outdoor use. It would be a good option for running power to a large detached shop building. Here are a few ways to run an outdoor electrical wire.

• Mobile home feeder cable
  • This is a convenient assembly of four wires for supplying 100 amps of power to a mobile or manufactured home. It’s labeled as “2-2-2-4 Aluminum Mobile Home Feeder.” Translation: It consists of two #2 AWG hot wires, one #2 AWG Neutral wire and one #4 AWG ground wire. This assembly lacks an overall outer sheath because the wires are approved for direct burial in the earth. The four wires are twisted together at the factory for ease of installation in the field.

• Twisted well pump cable
  • This is another convenient assembly of wires without an overall plastic sheath, approved for direct burial to supply power for submersible well water pumps at residential and farm properties. The individual insulated wires are similar to Type USE wires and can withstand harsh underground conditions. It’s available in lots of sizes depending on the installation requirements.

• URD cable (underground rural distribution)
  • This is a classification of large cables used for distributing electrical power. This would be a good option for distributing electrical power to multiple buildings on a single property, such as a home, a shed, a storage building and so on. It’s often labeled as “2/2/4 Aluminum URD.” Translation: The factory assembly consists of two #2 AWG hot wires and one #4 Neutral wire. This particular example does not have a ground wire. However, this cable assembly is available in many configurations and sizes, with or without a ground wire.

Common Specialty Cables for Low-Voltage Wiring 9

• Landscape lighting cable
  • This is used for low-voltage outdoor lighting for landscaping, accent or security lighting. It’s approved for outdoor use, suitable for direct burial and is sunlight resistant.

• Thermostat cable
  • This is used for thermostat wiring and other similar types of control wiring. It’s available in lots of sizes and configurations. It’s also used for doorbell wiring. This cable is approved for in-wall use. It will often be labeled as Type CL2 (Class 2 Circuits).

• Audio cable
  • This is a power limited circuit and communications cable for whole-house audio systems. It’s approved for in-wall use. It will often be labeled as Type CL2 (Class 2 Circuits) or CM (Communication Circuits).

• Security system cable
  • Similar to an audio cable, it’s a power limited circuit cable intended for whole-house security systems. It’s approved for in-wall use. It will often be labeled as Type CL2 (Class 2 Circuits), Type CL3 (Class 3 Circuits), CMR (Communication Riser), or Type FPLR (Fire Power Limited Riser). It’s a multipurpose low-voltage cable.

• Speaker wire
  • This is the transparent wire we use for connecting speakers and woofers to our audio equipment or audio system wall jacks. Beware: This type of speaker wire is NOT approved for in-wall use.

• Computer network cable
  • CAT 5E and CAT 6 computer network cables are used for high-speed data transmission for computers, modems, routers, printers and similar equipment connected to a data network. These ethernet cables are approved for in-wall use. They do not come from the factory with pre-installed connectors. This type of cable needs to be terminated at wall jacks or crimped connectors. These cables contain the familiar grouping of four twisted pairs of wires. They will often be labeled as Type CM (Communication), Type CMR (Communication Riser), or CMP (Communication Plenum). Lots of types and configurations are available. Beware: Ethernet cables with pre-assembled, factory-installed connectors are usually not rated for in-wall use. They’re intended for interconnecting computer equipment in a room, not permanently installed inside walls, floors, and ceilings.

• Coaxial cable
  • This is often referred to as “RG6″ cable. It’s used for video signals, closed-circuit TV and broadband internet access. Approved for in-wall use, this cable usually has several layers of shielding to eliminate electronic interference from nearby electrical sources. It’s often labeled as Type CM (Communication), CL2 (Class 2 Circuits) or Type CATV.

• Telephone cable
  • Yes, in the modern era of cellphones you can still buy telephone cable for installing landline phones. This type of cable is approved for in-wall use. The round cable consists of four 24 AWG wires. It’s typically labeled as Type CM (Communication) and is not a substitute for ethernet cable.

Stranded Wire vs. Solid Wire

Stranded wire is more flexible than solid. If you’re pulling wire through conduit, stranded wire makes it easier to get around corners and bends. On the other hand, if you’re working alone and don’t have someone to help with the wire pulling effort, solid wire can often be easily pushed through the conduit to the next outlet or junction box.