How to Identify the Types of Household Electrical Wires

Identifying the types of electrical wires in your home is an important skill for any homeowner to have. Knowing the specifics of your home's wiring allows you to complete electrical projects safely and choose compatible devices and components. In this article, I will provide an in-depth guide on how to identify common types of household electrical wires.

Non-Metallic Sheathed Cable (NM, NMC)

Non-metallic sheathed cable, commonly abbreviated NM or NMC, is the most common type of electrical wire found in modern homes. Here are some ways to identify it:

Insulation: NM cable has two or three insulated wires bundled together inside a plastic sheath. The insulation on the individual wires is color coded - black for hot, white for neutral, and bare copper or green for ground.
Markings: Printed text on the outer plastic sheath will read "NM-B" or "NMC-B" to indicate it is approved for residential branch circuits. Older versions may simply say "Type NM" or "Type NMC".
Sheath Color: The plastic sheath is typically orange or yellow. The sheath color can vary from white, gray, to black in older homes.
Sheath Material: Modern NM cables use PVC plastic for the sheath. In older homes, it may be a rubberized material.
Wire Size: NM cables are commonly 12, 14, or 16 gauge wire for 15amp and 20amp residential branch circuits. Larger gauge wires may be used for high power circuits.

Metallic Sheathed Cable (MC, AC)

Metallic sheathed cables, or MC and AC, are armored cables protected by an outer metal covering and waterproof coating. They are found in old homes built before the 1960s:

Sheathing: The outer metal armoring is made of flexible steel tape or a spiral/interlocked aluminum sheath. A layer of tar paper or plastic is between the wires and sheathing.
Wires: Older AC has rubber insulated wires while newer MC has modern PVC insulated wires. Both have two or three current carrying conductors and an uninsulated bare bonding wire.
Markings: Printed text will indicate “Type AC” or “Type MC” along with the wire gauge size.
Appearance: MC/AC cables are thicker in diameter and stiffer than NM cables due to the metal sheathing. Rust is visible on older exposed AC wires.

Knob and Tube Wiring

Knob and tube wiring consists of individual rubber insulated wires run through ceramic knobs, tubes, and flexible conduit:

Wires: A black hot wire and white neutral wire are separated by about 1-2 inches. They are anchored by porcelain knobs attached to framing.
Tubes: Where wires enter boxes or cross framing, they are protected by cylindrical porcelain tubes.
No Ground Wire: Knob and tube wiring has no dedicated grounding conductor. The metal boxes and armored cables provide some grounding.
Cloth Insulation: Very old knob and tube can have cloth or braided cotton insulation that cracks easily. Rubber insulation in good condition can be flexible.
Dating: Knob and tube wiring was phased out by the 1940s as NM sheathed cables became prevalent.

Bundled Service Entrance Cables

The service entrance cables that provide power to your home have multiple insulated wires bundled together in a thick protective sheathing:

Number of Wires: Service entrance cables typically contain 4 to 8 individual wires to supply all the electrical needs of the home. More than 3 wires indicates a service cable.
Large Wires: The wires in service entrance cables range from 8 AWG (for 100amp service) up to 2/0 AWG (for 200amp service). Larger than any branch circuit wiring.
Thick Sheathing: A black polyethylene sheathing that is thicker than NM cable encases the bundle of large wires.
Bare Neutral: One wire is an uninsulated bare neutral conductor that branches off to the electric meter and grounding.
Insulation Type: THHN/THWN-2 insulation rated for wet locations is used on the interior wires.

Identifying Wire Size and Type

In addition to the cable jacket, you can examine the individual wire insulation to determine size and type:

Wire Strip: Cut away a sample length of insulation using wire strippers to inspect the bare copper wire underneath.
Gauge Size: Refer to a wire gauge chart to match the diameter to the wire size, typically measured in AWG.
Insulation Color: The insulation color helps identify wire function. Black for hot, white for neutral, green/bare for ground.
Markings: Printed numbers and letters on the wire insulation indicate gauge size and insulation material like THHN.
Conductor Material: Most wire is copper, but aluminum wire was used from the 1960s to mid 70s before being phased out.

When in Doubt, Call an Electrician

Identifying wires is a useful DIY skill. But if you cannot determine the type of wiring in your home, the safe option is to call a licensed electrician to inspect. Professionals have the tools, testing equipment and experience to safely and correctly identify wiring. They can ensure your home's electrical system is up to code.

In summary, the type of cable, jacket markings, sheathing materials, insulation colors, wire count and size all provide clues to identifying common residential electrical wires. Match your home's wires to these descriptions to decipher which is which. Being able to identify electrical systems allows you to better understand your home and make informed upgrades.