The function of branch circuit wiring is to safely convey the electric current from the source to the destination. Branch circuit wires (conductors) carry electricity from the main distribution panel or subpanels to the points of use. These include receptacles, lights, switches and appliances such as stoves, dishwashers, garbage disposals, furnaces and air conditioners.
In this article, we use the words wire and cable rather than conductor because clients understand these words. We also refer to fuses and breakers rather than overcurrent protection devices for the same reason. Similarly, ground wire is the client-friendly term for equipment grounding conductor.
The most common household wiring material is copper. Copper combines excellent conductivity (low electrical resistance) with very good malleability (it deforms under compressive stress) and ductility (it stretches under tensile stress), making it easy to work with. Copper also has a reasonably low coefficient of thermal expansion, so it does not creep with temperature changes. Copper oxide is also a good conductor, so, even if the copper corrodes, it is relatively safe. (Aluminum oxide is not a good conductor and corroded aluminum connections may be unsafe.)
Aluminum, which has a dull silver color, was a popular alternative from the mid-1960s until the mid-1970s. Although aluminum is rarely used for solid conductor branch circuits in new work, it is present in many existing homes. Aluminum wires still are commonly used for service entrance conductors,and some 240-volt circuits use aluminum wire (stoves and clothes dryers, for example).
Aluminum at neutral bar.
There are a couple of other wire types that can confuse you. One is a copper-clad aluminum wire that is rare, but can easily be mistaken for copper. The best way to identify it is to look at the sheathing on the outside of the wire, which will describe it. Alternatively, if you can look at a cut end of the wire, you can see the aluminum core with a thin coating of copper.
Copper-clad aluminum NM wire (photo credit: Roger Hankey).
Ends of Cu clad Al on neutral bus (photo credit: Roger Hankey).
Another type of wire that can fool you on older installations is solder-dipped copper wire. This wire looks like aluminum at first glance, but if you can look at a cut end, you will see a copper-colored core with only a thin silver-colored outer layer. These cables do not have plastic sheathing or uninsulated ground wire typical of aluminum wiring.
Due to the inherent issues with some of these wiring materials, it is important that a home inspector be able to correctly identify the wiring material. In the rest of this article, we will focus on copper wiring.
There are different types of cable for different uses; I will describe a couple of the most common types.
The most common distribution wiring in houses is non-metallic sheathed cable, known as NM. This type is also referred to as Romex or Loomex, which are brand names for this type of wire. This cable has a paper, cloth or plastic (PVC) sheathing. Note: Sheathing may be called jacket or covering. This type of wire is widely used, but is not permitted in some areas. Check your rules!
NMWU or UF (underground feeder) cable is rated for underground use and for wet locations.
In some areas, household branch circuit wiring is run in rigid conduit or electrical metallic tubing (EMT). EMT is not conduit, having thinner walls than conduit. In fact, it is sometimes called “thin wall.” Conduit and EMT are good quality, but more expensive to install than NM cable. They are common in commercial and multi-family residential work. Some areas require the use of EMT in single-family homes. The metal covering may act as the ground wire (equipment grounding conductor). EMT is suitable for use in wet locations.
Armoured cable (AC) has a flexible metal sheathing. It is often referred to as BX, which is a brand name of AC. It is suitable for dry locations only. The metal covering may act as the ground wire (equipment grounding conductor). It was often used with furnaces, boilers and water heaters, although most modern codes don’t require that.
BX (AC) cable.
Wire Insulation and Sheathing
The insulation refers to the material wrapped around the individual wires. On old wire, it was rubber (knob-and-tube, for example). Modern wire has plastic insulation.
The sheathing forms the cable, wrapping around the insulated black, white or red (or a combination of these) wires and the uninsulated ground wire (equipment grounding conductor), if there is one. The sheathing is what you can see and touch. Older wires have cloth or paper sheathing. Newer wires have plastic sheathing.
The function of the insulation is to separate the wires within the cable. The function of the sheathing is to provide mechanical protection for the conductors and their insulation.
Certain types of wires are typical of certain time periods. The box shows which wires were used in which time periods. Keep in mind that these time periods are approximate, allowing for regional differences and the understanding that changes do not happen at one time.
Number of Conductors
Modern wires are typically two or three conductors (black, white and sometimes red) and, after the mid-1960s, they also included an uninsulated ground wire (equipment grounding conductor).
The two-conductor-plus-ground cables are used for most normal household circuits. The three-conductor-plus-ground cables are typically used for 240-volt appliances and multi-wire branch circuits, including split receptacles.
Number of conductors.
Knob-and-tube wires, used before 1945 (and up to 1950 in some areas), are single-wire cables. A discussion of knob-and-tube wiring is beyond the scope of this article.
The most common wire in residential branch circuits is 14-gauge copper. In the United States (but not in Canada), 12-gauge wire is currently required with 20-amp breakers or fuses for at least two kitchen countertop circuits.
The 12-gauge wire is used for kitchen receptacles, some electric baseboard heaters, water heaters, dishwashers, garbage disposals, private well pumps and small air-conditioners, for example.
Solid 10-gauge wire is typically used with air conditioners, electric clothes dryers, some electric water heaters, some cooktops and ovens, and saunas, for example.
Stranded 8-gauge wire is used for stoves, large air conditioners, electric furnaces and boilers, and electric car-charging stations, for example.
Note: Aluminum wires are usually one size larger since aluminum does not conduct electricity as well as copper.
Common household wire and fuse sizes
Solid Wire and Stranded Wire
Common household wires are solid for 14-, 12- and 10-gauge wire. Number 8-gauge and larger wires are typically stranded. Extension cords (often 16- or 18-gauge) are also typically stranded. A note on wire gauges: The larger the number, the smaller the wire diameter. For example, 12-gauge wire is larger than 14-gauge wire. The larger the wire, the more electricity (current flow measured in amperes or amps) the wire is rated to carry.
Stranded wires are more flexible and less likely to suffer strain than solid wires when stretched. Stranded wires are more workable and won’t break with repeated bending. Solid wires aren’t intended to be coiled and uncoiled regularly and, therefore, are not suitable for extension cords.
Solid wire is not suitable for overhead runs from the house to the garage, for example. Overhead wires should be stranded because they flex regularly. Solid wires can only span up to 4½ or 5 feet between supports.
Stranded wire for overhead runs.
Support and Clearances
In the United States, NM and AC cable should be supported every 4½ feet (or every 5 feet in Canada) and should be supported within 12 inches of boxes, cabinets and fittings, often referred to collectively as terminations. (Exception: Cable must be supported within 8 inches of some plastic boxes.) EMT should be supported every 10 feet and within 3 feet of terminations.
Cables are typically stapled in place where required. The cable should lay flat under staples, not on its edge.
Where cables are run in frame walls, they should be set back at least 1¼ inch from the stud face to help prevent nails or screws being driven into them.
Cable support inside walls.
Edge clearance for wire in studs and joists.
Wiring run on the underside of the floor joists in unfinished basements should be consistent with the illustration below, labeled “Wire installations below floors.”
Wire installations below floors.
To avoid overheating, wiring should not be in contact with heating ductwork or hot-water piping.
Extension Cords (Flexible Cords)
As mentioned, extension cords are flexible and stranded. Solid branch circuit cable should not be used as an extension cord.
Conversely, extension cords should not be used as permanent wiring. Extension cords are not designed to be stapled into place and the wire gauge is usually smaller than conventional household wiring, typically 16- or 18-gauge. Cords may cause fires if they are stapled to baseboards or run through floors, walls, windows and doors, or under carpets.
What to Look For
Electrical inspections are challenging because there are so many potentially adverse electrical conditions to watch for. Code requirements change regularly and it seems there are as many exceptions as there are rules. It’s important that home inspectors make it clear to their clients that they are not performing a code inspection.
In this article, I address only the wiring itself, yet there are several adverse conditions to identify. Watch for adverse conditions related to both original installation and performance.
This list highlights some of the most common conditions to watch for:
- Wire not protected by properly sized fuses or breakers (overcurrent protection devices).
- Wire not properly secured (for example, every 4½ feet and within 12 inches of a termination, no cable connectors where entering a box or panel, improper stapling).
- Wire not properly protected from mechanical damage. (There are lots of code rules, but common sense is a good guide.)
- Wire is damaged or has temporary repairs (for example, the use of electrical tape).
- Extension cords used as permanent wiring.
- Solid-strand wiring used as an extension cord.
- Outdoor wiring not rated for outdoor use.
- Poor connections (for example, not in a junction box).
Most of the wiring in a home is not visible, of course. We typically can only inspect the wiring at the electrical panel and in unfinished basements, crawlspaces, attics and garages. Home inspectors should remind their customers that many components of a house are not visible for inspection and are not included in their findings.
Caution: Inspecting electrical systems is potentially dangerous. One moment of carelessness can be catastrophic. Avoid distractions when the cover is off of any energized panel and promptly cover the panel after completing the inspection of that panel.
Alan Carson is a Past-President of ASHI and President and co-founder of Carson Dunlop 800-268-7070.
Thank you to Roger Hankey for his thoughtful review and insights.