Understanding How Electrical Wiring Is Labeled

An image of electrical wire lettering.
Electrical Wire Lettering. Timothy Thiele

With electrical wire that runs in the familiar plastic or vinyl jacket--called NM (for non-metallic) cable--the labeling is fairly easy to understand. NM cable will be identified by its wire gauge (such as 12-gauge or 14-gauge), and by its designated usage, but there's not much more you need to know when buying NM cable. 

It's a different matter when you're buying individual conducting wires for wiring that runs in metal conduit or in surface-mounted wiremold systems.

Here, it's more important to know something about how the insulation of the wires in categorized and labeled. 

Electrical wiring to be run in conduit comes in individual strands, encased in insulation to protect the wire and insulate it from other wires and the conduit. A typical metal conduit may have two, three or even more individual conducting wires within it, each surrounded by a thin color-coded insulating jacket. The copper conductors within these wires may either be solid come in either solid or stranded, depending on the wire size. Bare conductors without insulating jackets are used for ground wires only.

Insulation Type

The insulation around the individual conductors is made to take on some rather extreme conditions. Heat-, oil-, gasoline- and water-resistant coatings are all available to help your electrical wire survive.

The labeling on a wire's insulating jacket tells the story of the wires ruggedness.

You'll likely see labels like THHN or THWN written on the wire. THHN wire stands for thermoplastic high heat-resistant nylon coated wire. THWN stands for thermoplastic heat- and moisture-resistant nylon coated wire.

The "T" stands for thermoplastic, the type of insulation covering the wire itself. The "H" stands for a heat resistance of up to 167 F.

Likewise, the "HH" stands for a heat resistance, only it increases the rating to 194 F. The "W" stands for moisture resistance. The "N" stands for an additional nylon coating that makes the wire both oil and gasoline resistant. As you can see, these wires are built to take on many different conditions.

Wire Size and Composition

The labeling on the wire also tells the conductor size and what the wire is made of--either aluminum or copper. In order to install any electrical wire installation, the proper wire size for the application is needed. But how do you know what size wire to use? Wire is sized by the American Wire Gauge (AWG) system. Your installation of conductors will depend on several factors:  the gauge of the wire, wire capacity, and what the wire will feed should all be considered.

Wire gauge is the physical size of the wire, rated by gauge diameter. For instance, common sizes include 14-, 12-, 10-, 8-, 6-, and 2-gauge wire, with lower numbers denoted wires of greater diameter. The gauge of the wire dictates the amount of current that can safely pass through the electrical wire, as measured by ampacity. Ampacity is defined as the measurement of how much electrical current can flow through an electrical wire safely.

This ampacity should match the circuit size, meaning the circuit breaker or fuse that protects it. ​

As a guide, #14 wire is designated for 15-amp current, #12 wire for 20-amp current and #10 wire for 30 amps. As the number gets smaller, the size of the wire gets larger and the number of amps it can handle also gets larger. 

Some wire is stranded, while other wire is solid. The solid wire is stiffer and doesn't always pull as easy through conduit, especially when the run has many bends, but it is much easier to place under wire terminals, such as those on switches and outlets.

Insulation Color

Don't forget, the color of the wire tells a story of its own. It indicates whether the wire is used for a hot, neutral, or a ground wire. In this article about wire color, you can learn the common uses of specific wire colors; some are used for 277-volt installations, while others are used for 240-volt installations, for example.

 For an electrician, wire color may indicate which voltage and transformer configuration are being used. And universally, color is a convenient shorthand that tells us which wire is a "hot" wire, which is a "neutral wire" and which is a "ground" wire.

Whether you need wire for normal conditions, for high heat, or wire that is sunlight-resistant or water-resistant, the labeling and color-coding will help you get it right.