DIY Electrical: An Introduction to Light Switches

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One of the most common tasks that a DIY Electrician will be called upon to perform is replacing a defective light switch. A light switch has a simple task to perform, turning one or more lights on and off, but there is nothing simple about a light switch.

Single—pole light switches are used when the lights are controlled from only one location. Three—way light switches are used when the lights are controlled from two locations. Four—way light switches are used in conjunction with two 3—way light switches when the lights are to be controlled from three or more locations. Wiring these switch circuit is pretty straight forward and will be covered in a future article. In this article I will be concentrated on all the differences between these switches and why those differences are important to know. All light switches of the same type are not created equal.

The Anatomy of a Light Switch.

Every light switch has a treasure trove of information written on it that the DIY Electrician must know in order to perform his work so it complies with the requirements of the National Electrical Code and so that his work is safe. Unlike most electrical project that the DIY Electrician will undertake, replacing an existing light switch with a new one does not require a permit nor an inspection, so it is up to the DIY Electrician to work correctly and safely.

What does the writing on this switch mean?

The Wire Compatibility Rating: At the top of the mounting strap on the front side of the switch you will find the Wire Compatibility Rating. On this switch the CU AND CU-CLAD WIRE ONLY (the markings may also appear as CU/CU-Clad) means that this switch is designed and approve only for use with Copper and Copper-Clad Aluminum wire. This switch cannot be used with AL (Aluminum) wire. Switches that are designed and approved for use with both Copper and Aluminum wire are marked CU and AL or CU/AL. To use a switch designed and approved for use with CU and CU-CLAD wire with AL wire presents a fire hazard.

The Amperage and Voltage Rating stamped on the bottom end of the mounting strap indicates that this switch is designed and approved for use on 120 VAC, 15 Amp circuits only. Switched that are designed an approved for use on both AC and DC circuits would be indicated by “Volts AC/DC” or “VAC/VDC” on the strap.

The Underwriter's Laboratory Approval indicates that device was tested and approved for the use indicated by the UL.

On the back side of the switch you will find more essential information.

The wire gauge rating shows that this switch should only be used with AWG 12 and AWG 14 solid-copper wire. It should not be used with wire larger than AWG 12 or smaller than AWG 14 and it should be used with stranded copper wire. With the American Wire Gauge (AWG), the larger the number the smaller the wire.

The Push-In Fittings are the holes for back-wiring the device. Back-wiring is the quickest method to use when connecting a switch or receptacle, but it is the least preferred method by the professional electrician. Although it is approved by the NEC and by local building codes, the connection depends on spring tension and those spring contacts can weaken with time allowing the wire to work loose.

The Push to Release slots are where you would insert a small screwdriver or some other tool to release the spring tension so you can remove a wire from a back-wired device.

The Strip Gauge indicates how much insulation should be removed from the wire when back-wiring the device. The strip gauge works equally well for judging how much insulation to remove when side-wiring the device.

How to identify the type of switch you are working with?

The type of switch (single—pole, 3—way, or 4—way) may be identified by the number of terminal screws on the switch. Each of these switches may have one additional screw, a system grounding screw. A system grounding screw can be identified by its green color and Hex Head.

The single pole switch

The single—pole toggle switch, often referred to as a S.P.S.T (Single—Pole Single—Throw) Switch on electrical diagrams, can be identified by its two brass colored terminal screws. Newer S.P.S.T. Switches will have a third, green colored grounding screw. Single—Pole do have a top and bottom an the “ON” stamped on the Toggle should be on the underside of the toggle when the toggle is in the up position. With most switches this will also place the terminal screws on the right hand side of the switch's body.

The 3—Way Switch

The 3—way switch has three terminal screw—two brass colored screws and one black colored screw. The two brass colored screw connect to the circuits “Traveler Wires”. The “Traveler Wires” connect two 3—way switches together or connect a 3—way switch to a 4—way switch. The black screw connect to the wire bringing power into the first switch and carrying power from the second 3—way switch to the lighting outlet. three—way switches may have a fourth screw, the green system grounding screw. three—way switches have no top or bottom and there are no “ON” and “OFF” stamped on their toggles. Their “ON” and “OFF” positions depends on the positions of the other switches in the circuit. Technically speaking, a 3—way switch is a Single—Pole, Double—Throw (S.P.D.T.) switch with no “OFF” position.

The 4—way switch

A 4—way switch has four brass colored terminal screws. The first set of terminal screws connect to the traveler wires from the first 3—way switch. The opposite set of terminal screw connects to the traveler wires going to another 4—way switch or to the second 3—way switch. These switches may have a fifth screw, the green grounding screw. Technically, a 4—way switch is a Double—Pole Double—Throw (D.P.D.T.) switch with no “OFF” position. Like with the 3—way switches, their “ON” and “OFF” positions depends on the positions of the other switches in the circuit.

Now that you understand what all those letters and numbers really mean you know about light switches than the majority of DIY Electrician and are well on your way to becoming expert home electricians.

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