Wiring Residences

Nearly all new homes today are wired with non-metallic sheathed cable, but that may not be what you find when you work on an older house. Depending on its age, you may have some surprises.

Knob and Tube Wiring

Homes built before about 1930 usually had knob and tube (K&T) wiring. Wires were strung between porcelain insulators driven into studs and joists. If wire had to pass through framing, a hollow porcelain tube was inserted in a hole drilled through the stud or joist. Conductors were usually single strands covered with cloth insulation. You won’t find a ground wire on K&T. In those days, only lightning rods were grounded.

K&T that’s given trouble-free service for nearly a century could probably do the same for another century, if no further demands were made on the system. But that’s not likely. Because it isn’t grounded, doesn’t have enough capacity, and its insulation isn’t worthy of the name, some insurance policies exclude coverage for homes with K&T wiring. For that reason alone, many of these older homes have already been upgraded.

Aluminum Wiring

Aluminum wire is another type that’s no longer used for interior home electrical systems. Aluminum is a good and durable conductor and is usually less expensive than the more popular copper. However, late in the 1970s, electricians and code officials began to recognize a problem developing in homes with aluminum wiring. When aluminum wire carries current, it warms up and expands, just like copper. When it cools, the aluminum contracts, just like copper. But unlike copper, aluminum connections oxidize during the cool-down phase, creating resistance where conductors join. With time, the resistance grows into arcing – a spark that passes through the corrosion (gap) between the wire and the connector. Given the right conditions, that spark can ignite a fire.

Aluminum Wiring

The aluminum wire itself isn’t the problem. It’s the wire connections that are to blame. You don’t need to rip out all the aluminum wire you find. But it’s prudent to check connections in a home wired with aluminum. Electrical devices used with aluminum wire should be rated specifically for aluminum (usually stamped CO/ALR or Al/Cu). Look for signs of overheating, such as blackened connections or melted insulation. If you elect to extend an aluminum circuit using copper wire, your electrician will need to use a special crimping tool made just for the purpose. When crimped, the wire connection must be covered with anti-oxidant grease.

Aluminum wire is still widely used for residential service entrance, though not for concealed wiring in walls and ceilings. Wires run between the house and the public utility grid are larger in diameter and require very few connections. That makes aluminum a good choice. For interior wiring, the price advantage of aluminum over copper usually isn’t worth the risk or the extra trouble, though aluminum wire with a copper coating is used in some communities.

Non-Metallic Sheathed Cable

Non-metallic sheathed cable, called Romex or "rope" by electricians, is the most common wire type used in homes today. The NEC classifies it as Type NM cable. (Type NM-B cable is identical but has a slightly-better temperature rating.) Romex has two or more insulated conductors and a ground wire, all covered in a plastic sheath. It’s popular because it’s inexpensive and easy to install. You can use NM cable in wall cavities where the wire is protected from physical damage and unlikely to get either wet or hot. When it’s run through 2" x 4" stud walls, protect the cable at each stud with a metal plate to prevent damage from nails. Romex cable can be stapled to studs, rather than attached with nail-on hangers or supports. Most electrical codes permit the use of plastic (rather than metal) outlet boxes with NM cable.

You can’t use Romex for exposed wiring on walls if it’s within 5' of the floor. But most inspectors will approve Type AC (armored cable) for that purpose. AC is like Romex but includes a flexible aluminum cover that protects it against physical damage. If the inspector won’t accept AC cable for exposed runs, we may have to install conduit. Unlike electric cable, conduit includes no wire. It’s a protective tube through which wire is pulled. Conduit is used in most commercial buildings and occasionally in residences, such as in the service entrance mast where overhead wires terminate at the entrance cap. Flexible (flex), EMT (electric metallic tube), GRS (galvanized rigid steel) and IMC (intermediate rigid conduit) are the most common types of conduit. Flex is a hard metallic tube with enough flexibility to snake through studs. EMT is lightweight but not flexible. GRS conduit is heavier. IMC falls between EMT and GRS. Each of these types has specialized uses.

Ground Wire

Electrical wire size is measured in American wire gauge (AWG) and usually abbreviated with the pound sign. For example, #14-3 indicates a 14-gauge wire with three conductors (and probably a separate bare ground wire). The smaller the gauge number, the bigger the wire and the greater its current-carrying capacity. Most circuits in a home are rated at 15 amps and use 14-gauge copper wire. Circuits for kitchen appliances should be rated at 20 amps and use 12-gauge copper wire. Circuits for an electric water heater, air conditioner or electric clothes dryer should be 30 amps and use 10-gauge copper wire. An electric range requires 6-gauge copper wire and a 50-amp breaker. All of these cables should include a copper ground wire.

Usually Type THHN wire is used with conduit because it has a heat-resistant thermoplastic cover. THHN is available in many colors to simplify the identification of conductors after the wire is pulled in the conduit. Small gauges are solid wire. Larger gauges are stranded wire, usually 19 strands per conductor. Stranded wire is a slightly better conductor than solid wire. It’s also not as stiff, so it’s easier to pull stranded wire in conduit.

Use Type UF cable for underground runs or in damp locations. It’s not required by all codes, but common sense dictates that buried UF cable be trenched deep enough to make accidental damage unlikely. Either enclose the UF in PVC conduit or lay a warning tape in the trench over the wire before it’s backfilled.

Protect cable with conduit wherever it’s exposed, particularly where it exits the residence. For outdoor wiring, junction boxes and outlet boxes must be rated waterproof, and all receptacles must be GFCI-protected.