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
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 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 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
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.
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.
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.