Understanding the Markings on Electrical Switches

African American woman fixing light switch
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Your home has a variety of switches that are used both alone and with a combination of other switches to control lighting, outlets, and appliances such as furnaces and garbage disposers. Homes with newer technologies such as LED lighting or solar panels may have also specialty switches that control 12- or 14-volt power circuits.

All electrical switches have different labels stamped onto the metal strap or imprinted on the plastic body of the switch. These markings allow you to make sure that you are using the right switch for the circuit and application. Here are some of the markings you might see on a standard wall switch:

UL Label

A circled UL marking on the metal strap of the switch indicates that the device has been tested for safety by the Underwriters Laboratories, an independent testing agency. Virtually all switches made by reputable manufacturers will carry this label; don't buy a switch that doesn't have this marking. While there are other testing agencies, UL is the best-known and most reputable.

Amp and Voltage Rating

A marking that says something like "15A 120V" indicates that the switch is approved for use on a 15-amp, 120-volt circuit. Virtually all wall switches are designed for 120-volt circuits, and either 15 or 20 amps. This marking is normally on the front of the metal strap.

AC Only

This marking indicates the switch is designed only for alternating current wiring. This is not a marking you normally need to worry about, since all standard household wiring is alternating current. This marking is usually on the front side of the metal strap.

Some switches are used for applications such as on/off switches on battery-operated devices such as radios. These are DC switches rather than the AC wall switches used in most house wiring. Some solar-panel systems may also use DC switches somewhere in their wiring.

Use Solid Copper or CU-Clad Wire Only

The label tells you what the types of wire the switch can be attached to. By default, most switches are intended for use on solid copper or copper-clad aluminum wire (CU is the chemical symbol for copper).

CO/ALR

If a switch is not rated for "Solid Copper" or "CU-Clad," it may have the label "CO/ALR." This indicates the switch is approved for use with either copper or aluminum wiring. If your house happens to be wired with aluminum wire (not common), this is the type of switch you will need to use.

No ALR

If a switch has a marking such as the letters "ALR" inside a slashed circle, it indicates that the switch cannot, under any circumstances, be used with aluminum wiring. Because aluminum wire has a different expansion and contraction rate than copper, there can be a danger to using a copper-only switch with aluminum house wiring.

12- or 14-gauge AWG

A marking such as this (or using similar phrasing) indicates the switch is intended for use with 12-gauge (20-amp) or 14-gauge (15-amp) wiring. Not all switches carry this rating, but where it is found, you will find it either on the metal strap or on the plastic body of the switch.

GR

This label, usually found on the plastic body of the switch, will identify the push-fitting opening where the grounding wire is supposed to be inserted. It will normally be near the green grounding screw on the switch (GR stands for green).

Strip Gage (Gauge)

This marking identifies a convenient measuring gauge that shows you how much wire to strip from the individual wire conductors for connection to the screw terminals. There may be two such gauges on the switch body: one for screw terminals, one for push-in fittings.

For Permanent Incandescent Fixture (or For LED Fixture)

Dimmer switches often carry markings to indicate what kind of lighting fixtures they can be used with. Older traditional dimmers are appropriate for only traditional incandescent light fixtures, while newer switches may be rated for CFU (compact fluorescent), LED (light emitting diode) or halogen light fixtures—or all types. This rating is sometimes printed with a paper label on the side of a switch body.

Country of Origin

The switch will have a stamp or label indicating the country where it was manufactured, such as "U.S.A.", "Made in China," or "Made in Mexico." There is normally no reason to be concerned about foreign-made products, provided they carry a UL listing.

THHN or THWN

On switches that have wire leads, such as many dimmer switches, the insulation on the wire leads will be printed with a label such as "THHN" (thermoplastic high heat resistant nylon-coated) or "THWN" (thermoplastic heat- and water-resistant nylon-coated). These markings indicate the quality of the insulating jacket on the wires:

  • "T" stands for thermoplastic, the type of insulation covering the wire itself.
  • "H" stands for a heat-resistance of up to 167 degrees Fahrenheit.
  • "HH" stands for a higher heat-resistance, increased to a rating to 194 degrees Fahrenheit.
  • "W" stands for moisture resistance.
  • "N" stands for an additional nylon coating that makes the wire both oil and gasoline resistant.

The labeling on the leads will also indicate the conductor size and what the wire is made of, either aluminum or copper.

UL-listed dimmer switches all meet industry standards, and you normally have no reason to read and interpret the ratings printed on the wire leads of a UL-listed device.

Commercial vs. Standard Grade

Although a switch may not be labeled as one or the other, manufacturers typically offer switches in both standard (household) grades and commercial grades. This is sometimes indicated only by the price difference, but the distinction is quite real—commercial-grade products are the choice of professional electricians who are interested in long-lasting products that won't fail. DIYers are also well advised to invest in quality switches rather than buying $.99 switches that are likely to fail within months.