In this guide you will find the explanation of one of the most difficult electrical circuits to understand. This is one of the simplest ways to connect a three-way switch. First check out the "Tips" section to see other popular ways to make this type of circuit.
Steps
Step 1. Choose the right cable size
In each circuit, each wire must have the same diameter. If they come from an electrical panel or fuse box, they must be made of copper and with a diameter of 12 which is the minimum size for making connections to a thermal magnetic circuit breaker or a 20 amp fuse; a diameter of 14 is the minimum for connecting a magnetothermic switch or a 15 amp fuse (in circuits of this kind it was not possible to use aluminum cables for many years). If you draw electrical power from a nearby outlet, the cables in the new connection must be the same diameter as those feeding the outlet or the circuits of the other devices.
Step 2. Choose the suitable cable type
The power supply should be "2 way" (or conductors) plus a ground wire. The most popular cables that have these characteristics are:
- NM (often called "Romex") and UF type cables (both have 2 or more insulated wires wrapped in a plastic jacket - one white, one black and possibly other colors - and another non-insulated wire). NMs are used indoors and UFs are used outdoors, exposed to the sun or underground.
- Cables of type BX, MC and AC. These are armored cables. They are very similar to each other and have only small differences (constructively they are made with a woven metallic coating, which encloses two or more insulated wires - one white, one black and possibly other colors including green - helically wound by steel strips or aluminum). Cables that do not have an insulated green wire use the outer metal jacket as a ground conductor. None of these types of cables can be installed outdoors or underground. If the power supply comes from an armored cable without the green wire (12 or 14), a metal box must be used for grounding, which discharges from the cable armor to the box itself, and to the grounding circuit. to the ground by means of a specific green hexagonal head screw which is threaded into the appropriate pre-marked housing on the metal box, or by means of a specific green tweezer.
- All cables have "trade names" which essentially derive from the number of insulated conductors apart from the grounding ones and the type of construction; for example: "twelve two Romex" or "fourteen three BX". A 12/2 NM, BX, AC, or Romex cable with two conductors of diameter 12, plus a grounding always with a diameter of 12. A 14/3 NM, BX, AC or Romex cable has three conductors of diameter 14 and one always earthed from 14. There are also special conductor wires for armored cables that provide specific precautions and methods of use. It is not possible to use Romex conductor wires, even if similar in appearance, in an armored cable. NM or Romex cables are easier to use, require no special tools or setups and cost less. For these reasons they are widely used.
Step 3. Disconnect the power
This is a very important step. Don't omit it.
Step 4. Install a two-way cable between the power supply (power outlet, electrical box, etc.)
) and the first switch box. Before cutting the cable, leave approximately 20 - 25 cm inside each box (the junction box and the switch box) to facilitate easy connections to the switch and power supply. With a clamp, connect the ground wire to the ground circuit. If the power is coming directly from an electrical panel or fuse box, the cable should be cut so that it is long enough to connect to the furthest branch point (circuit breaker or fuse, earth or ground pin. neutral) without the need for joints. The ground wire should be connected to either the neutral pin or the ground pin (but only if there is a separate ground pin). If all ground wires are connected to the same pin and all white wires to another pin, the ground and neutral connections must be kept separate. Never put a ground wire on a plug with a cable to which only the white or gray wires are connected, and vice versa. Connect the black wire to the phase or thermal breaker / fuse, and the white wire to the neutral or neutral pin in the electrical panel.
Step 5. Install a three-way cable from the first switch box to the light fixture box
Before cutting the cable, leave approximately 20 - 25 cm inside the switch and system box to facilitate easy connections and additions. A three-way cable has an "extra" wire than a two-way cable, and is almost always coated with red insulation. This wire is for installing the three-way switches.
Step 6. Install a three-way cable from the second switch box to the light fixture
Before cutting the cable, leave approximately 20 - 25 cm inside each box to facilitate easy connections to the system.
Step 7. Connect the ground
In all electrical boxes and connections, all ground wires must be connected via clamps, nuts or other approved systems. For each terminal leave a piece of green wire (20 cm) uncovered to connect it to the green screw of the grounding system in each box (switches, sockets, light points, etc.). If the breaker boxes are made of metal, these must also be grounded with the green ground screw or green tweezers. This type of ground connection must be made in each box where a cable enters and for each appliance that has a terminal for grounding. It is highly recommended to make these ground connections first, to be able to arrange them easily at the bottom of the box - so that they do not interfere with the rest - leaving only a small guide wire to connect easily to the appliances. Grounding connections may not be made on plastic, fiber or other non-conductive material boxes.
Step 8. Connect the power cables to the first switch
As mentioned above, connect all ground wires. The two-way cable of the main power enters the switch box from below and the phase wire (black) connects to the diverter terminal of the three-way switch. In the three-way switches there is only one of these diverters. Usually this terminal is identified with a screw of a different color (usually dark) than the two screws of the other terminals (not counting the green ground screw). In the above circuit figures, at the points described above, the diversion terminal is the one at the bottom right of both switches.
Connect the white (neutral) wire of the three-way cable directly to the white (neutral) wire of the two-way power cable with a clamp (you do not need to make any connection of the white wires to the switch)
Step 9. Plug the three-way cable into the switch
The three-way cable enters the box from the top. The red wire connects to one of the two free terminal screws (in the figures above these are the top and bottom terminals on the left of the three-way switch). It is indifferent to which of the two it is connected.
Connect the black wire to the screw of the last free terminal of the switch
Step 10. Connect the wires in the light box
If you haven't done so already, first connect the ground wires as explained above. There will be two three-way cables in the light box. One comes from the box of the first switch and has the white neutral wire; the other comes from the box of the second switch and its white wire will become what is called the "leg" of the switch.
- Mark the switch leg. Mark both ends of the white wire in the three-way cable that was mounted between the second switch and the light box with black electrical tape. By doing so, whoever intervenes on the circuit will later know that that white wire is no longer neutral. It is a recent but widespread practice when a white or gray thread becomes a load. Precisely because it happens often in switches, it has been given the name of the "leg" of the switch.
- Connect the two red wires with a clamp.
- Clamp the black wire from the first switch to the "leg" (the white wire with insulating tape) from the second switch.
Step 11. Connect the three-way cable in the second switch box to the switch
As mentioned above, connect all ground wires. Connect the black wire to the switch deviation terminal screw (again, the deviation terminal screw is a different color from the others).
- Connect the red wire to one of the two free terminals (it doesn't matter which).
- Connect the "leg" of the switch (white wire with black electrical tape) to the last free terminal on the switch.
Step 12. Connect the implant
There should be only one black, one white and one ground wire left in the light box.
Step 13. Complete the entire circuit
Tighten all the clamps and check that there are no exposed neutral or load wires. Put all the cables in an orderly way inside the boxes and fix everything with screws. Fit the plates and covers. Reconnect the power and check that everything is working properly.
Part 1 of 1: Installation with the Australian Method
Step 1. Disconnect the power (and verify that there is no current in the circuit)
Step 2. Connect the ground (green) and neutral (black) to the system (to green and blue respectively)
Step 3. Connect the load (red) to the central terminal of the first switch; connect the switch wire (white) to terminal 1; connect the wire of the second switch (white or red) to terminal 2
Step 4. Connect both switch wires (terminal 1 and 2 respectively on switch 2) and the common terminal to the red wire (which is then connected to the light point)
Step 5. In the lamp holder, connect switch wire 1 of the first switch to switch wire 1 of the second switch; and connect the switch wire 2 of the first switch to wire 2 of the second
Step 6. Connect the red wire from the second switch (already connected to its terminal) to the active terminal in the lamp holder (red or brown)
Advice
- This third system is used when the switches are close enough, but the light point is far away. For example: the switches are close to the two entrance doors on the same wall of a room and control a light point in the center.
- One terminal = one wire. It is not possible to connect more than one wire to a single terminal screw. Beyond that, the wires should wrap clockwise around the screw. Only the whole threads need to be wrapped around the screw. The twisted wires must be installed with the aid of the special ring or U-shaped terminals (pressed or welded) on which the screw is tightened.
- The second system is used when the switches are far away and the light point is between them - as when the switches are one at the top and one at the bottom of a staircase and the light point is closer to each other than to the other. It is also the only possible way to modify an existing chain light point so that it can be controlled with two three-way switches.
- A 120V / 15A system is designed to withstand up to a maximum of 1,440 watts of continuous load (heating, lighting, etc.), so just a few lights will be enough to get to the limit of a 15A / # 14 (14 gauge) circuit. Just for comparison, a 120V / 20A system is designed to withstand up to a maximum of 1,920 watts of continuous load (heating, lighting, etc.). The maximum load of the system - in this case in watts - is calculated by multiplying Volt x Ampere x 0, 80, where Volt and Ampere are given and 0, 80 is the coefficient required by the legislation to downgrade the capacity of the system to 80 % of its maximum. Applying this formula, we could say that the maximum amperage of a 15 Ampere system is 12 amperes: Intensity x 0, 80 = maximum load. So for a 20 amp system: 20 x 0, 80 = 16 amps. The legislation requires that the capacity of each plant be reduced by 80%. If a larger load is connected, fuses, circuit breakers and electrical wiring with a sufficiently larger size / diameter must be installed.
- If the power is coming in from several places, it could affect the checks you do for any problems with the switches. This first of three methods is often used to extend a current source - such as an outlet - that is near a switch or the other. It is the method described in detail step by step in the article above.
- If your system is protected by a 15 amp fuse or circuit breaker, use Romex # 14 (14 gauge) copper wire, which is smaller, easier to use, and cheaper. There are very few circuits that mount three-way switches on a 20 amp system. It is not necessary to use # 12 wires with wires from a 14-wire circuit. 12 gauge wires are required by law for kitchens and dining rooms, and for household appliances (washing machines, refrigerators, etc.) that need a 20 amp power supply (# 12 wires are installed in some bathrooms to be able to use hairdryer and the like, but it is not a regulatory requirement).
- When making changes to the system, always check the fuses and circuit breakers to which you connect the new light points or the new power sockets. If you install a # 14 wire to a system protected by a fuse or circuit breaker larger than 15 amps, you are breaking the law and, more importantly, you are running serious safety and fire risks. This concept is valid for all types of electrical cable. Never install wire with a diameter smaller than the capacity of the fuse or circuit breaker in a system: gauge 6 - 50 amps, gauge 8 - 40 amps, gauge 10 - 30 amps, gauge 12 - 20 amps, gauge 14 - 15 amps. It is not allowed to connect smaller wires to the electrical panel - unless they are intended for a transformer for the doorbell or similar.
- The use of screw terminals is preferable to the "rear insertion" ones provided in some switches or in some sockets, which for convenience allow the insertion of the stripped wires into suitable holes for connection without the need to tighten any screws. In the long run, these press connections wear out and can fail.
Warnings
- Never use materials of different nature (copper and aluminum).
- Check your local wiring practice. Different color combinations may be in use in your area.
- Before starting any type of work on the electrical system, remember to disconnect the power.
