In a room lit by a single pendant light, controlling that light from a single switch is no great hardship. But if the room contains wall lights it’s useful to be able to control them from different parts of the room and that’s exactly what two-way switching lets you do.
In most rooms, lights or groups of lights are controlled by just one switch. It’s the standard set-up, and electricians call it oneway switching. However, there are situations where a one light, one switch arrangement isn’t very convenient.
Take the light over a flight of stairs as an example. Having the light switch at the bottom of the flight is fine if you want to go upstairs. You can turn on the light before you go up without difficulty. But what happens when you reach the top? You can’t turn the light off again. And suppose you want to come downstairs and the light is turned off? You can’t switch it on without negotiating the stairs in the dark, which rather defeats the object of having a light there at all.
Obviously, what’s needed is another switch at the top of the stairs and a system of wiring that allows either switch to turn the light on and off independently of the other. This system is called two-way switching.
A switch is able to turn a light on or off by completing or breaking a circuit. The one-way switching system, where the light is controlled by a single switch, is the usual system, but the two-way switching system, when two special switches are linked, allows you to control your light from two switches that are independent of each other.
Where it can be used
The example of the light above the stairs is such a common one that providing two-way switching for stair lights is now more or less standard procedure. There are, however, many other situations where two-way switching may be useful.
Think of the advantages of having a switch at both ends of a long hallway. And what about rooms with more than one entrance? It makes sense to have a switch beside each of the doors. The same applies to a garage with a side door in addition to the main one.
There are also situations where two-way switching is not vital but still worth considering. For example, where you have installed wall-mounted bedside reading lamps: it is a great advantage when you can control these from the door, as well as from a switch by the bed. You might also want to install a two-way switch for the main bedroom light so you can turn it on and off without getting out of bed.
And don’t forget the hall light: in many homes this is one-way switched despite the fact that it often serves as a stair light by illuminating the bottom steps of the staircase . It’s all too easy to go upstairs having forgotten to switch it off.
How it works
The key to two-way switching lies in using a special switch at both switching positions.
An ordinary one-way switch has two ter-minals and when you operate it you either make or break the electrical connection be-tween them. For the current to reach the lightbulb and make it work, it must pass down one of the cores in the switch-drop cable, and back up the other. Making or breaking the link between terminals also makes or breaks the link between the two cores and therefore switches the light on or off.
A two-way switch works in a completely different way. It has three terminals, marked ‘Common’, L1, and L2, and when you operate it, flicking the switch one way provides a link between L1 and Common; flicking it the other way provides a link between Common and L2. If you link the terminals of two two-way switches in a certain way, then one switch can complete the circuit while the other can over-ride it and turn the light off again. The reverse also applies.
So, what is this remarkable wiring arrange-ment? Well, in its traditional form – the one normally illustrated in text-book wiring diagrams – the switches are linked with a pair of single-core cables called straps. Each joins the L1 terminal in one switch to the L2 terminal in the other. The switch drop from the light is then divided in two: the first core goes to the Common terminal in one switch, the second to the Common terminal in the other. It’s all run in single-core cable, and is designed for use where your home’s wiring is run entirely in conduit – as it may well be if you live in a flat. In most homes, though, the wiring takes the form of multi-cored PVC-sheathed cables, and in this case, a different two-way switching circuit is generally more convenient. What happens is that the switch drop, run in two-core and earth cable, is connected to only one of the two-way switches: one core to LI, the other to L2. This switch is then linked to its partner with three-core and earth cable, a cable which has rather oddly colour-coded wires – one red, one yellow, and one blue. However, this doesn’t make the wiring up any more complicated. The yellow and blue cores are connected in the same way as the straps in the traditional system, and the red core is used to link the two Common terminals.
Converting an existing switch
That’s the theory, but how does it work in practice? How do you convert an existing one-way switching circuit to two-way switching? You may be surprised at just how simple it is. Using a club hammer and cold chisel, make a hole in the wall to take a one-gang, plaster-depth mounting box at the spot where you want the new switch to go. This should be secured to the wall with a couple of screws and wallplugs.
Then, before running a length of 1.0 or 1.5mm2 three-core and earth PVC-sheathed cable to the original switch position, connect up the new switch in order to minimise the time the power has to be switched off at the mains. The red core should go to the Common terminal with the yellow and blue cores acting as strap wires. The running of the cable shouldn’t pose any difficulty. The cable is taken up the wall, above the ceiling and back down the wall to the old switch.
Chop a channel in the plaster at the wall sections of the run, insert a length of PVC conduit and pass the cable through that. Above the ceiling, if the cable runs at right angles to the joists, feed it through holes drilled at least 50mm below their top edge.
If the cable runs parallel to the joists, simply rest it on top of the ceiling, unless it is likely to be disturbed – as in a loft, for example. In that case it must be secured to the sides of the joists with cable clips. Turn off the power at the mains before removing the fixing screws. Then, ease the original switch from the wall so you can pass the cable into the mounting box.
With the cable in position, all that remains to be done is connect it up to the new two-way switch at the original switch position. Finally, connect the two cores of the switch drop to terminals L1 and L2, screw the switch securely to its mounting box and restore the power.
The same system can also be employed in new work where you require two-way switching. For example, if you are installing a new wall light and want to be able to control it from a switch at the door of the room, as well as from a switch near the light itself, all you need to do is install the light and a new two-way switch in the normal way and then run a length of three-core and earth cable from this switch to the switch by the door . Here, you replace the old one-gang, one-way switch with a two-gang, two-way switch and connect the three-core and earth cable to one gang for two-way switching. Then connect the switch drop belonging to the room’s main light to the other gang, this time connecting it up for one-way switching with one of the cores going to the L2 terminal and the other to the Common.
This method is quite straightforward, but it is not necessarily the most convenient way of setting about the job. Suppose you were installing not one new wall light, but two or more. If they are all to have two-way switches then you’ll be involved in a great deal of cable running and, therefore, a great deal of work. After all, for each light you have to run a cable down to the new switch, back up above the ceiling, then back down the wall again to the second switch position.
This extra work can be avoided by using a junction box and a variation on the traditional two-way switching circuit. You run the circuit using three-core and earth PVC-sheathed and insulated cable. Doing this you are using the cable’s red core as one half of the switch drop and the yellow and blue cores as straps. Then connect them up, together with the cables to the wall lights and the cable supplying the new circuit with power in a large multi-terminal joint box called an RB4. The step-by-step photographs will explain what is happening but note that only one cable need be run to each wall-light.
There is one other kind of two-way switching that could be useful in your home. It’s called an intermediate switching circuit and it means that a light or group of lights can be controlled by three or more separate switches.It’s easy to install because all you do is introduce one or more additional switches into the circuit between the two two-way switches. This can, of course, be very convenient because you can then control a light from as many positions as you like. For example, you could control a hall light from a switch near the front door, from one near the living room door and from another switch on the landing upstairs.
There are two ways of carrying out the wiring but with both methods a special switch called an intermediate switch is needed. This has four terminals: two marked L1 and two marked L2. To install an intermediate switch in a two-way switching circuit all you do is use the switch to interrupt the three-core and earth cable – or the strap wires in a traditional circuit that has been installed in a flat or where the cables are all run in conduit.
The cores from the L1 terminals and l_2 terminals in one two-way switch go to the L1 terminals on one side of the intermediate switch and the L1 and L2 cores from the second two-way switch go to the L2 terminals on the other side. However, with a three-core and earth circuit this leaves a break in the core linking the two-way switches’ Common terminals. One way of solving this problem is to join the ends of the two cores with a cable connector. Once the two red cores have been joined the connector unit is then placed in the space behind the intermediate switch. However, it’s better to interrupt the three-core and earth cable with a multi-terminal junction box above the ceiling near the intermediate switch position. You need six terminals in the junction box: one for the earth cores, one for both the red cores from the Common terminals and one each for the remaining cores.
At this stage you should introduce two lengths of two-core and earth cable which. By connecting up to the appropriate terminals, are used to extend the yellow and blue cores from each two-way switch. When that’s done, run the two two-core and earth cables down to the intermediate switch and connect them up just as if they were the two pairs of yellow and blue cores in the three-core and earth cable. It is worth remembering that if one of the switches in an intermediate circuit is to be cord-operated then it should be one of the end switches. This is because there are no cord-operated intermediate switches available.
Two-way switching lets you control a light from two separate switches.
CONVERTING A CIRCUIT
To convert an existing one-way switching circuit to two-way switching:
– replace the existing switch with a two-way switch
– run 1.0 or 1.5mm2 PVC-sheathed and insulated three-core and earth cable to the new switch position
– connect it to another two-way switch.
This method can also be used to create new two-way switching circuits.
Two-way switching circuits, enabling you to control a light from two or more switch-positions, are at their most useful when it comes to controlling lighting in stairwells, and most houses with two or more storeys are now built with at least partial two-way switching of landing and hall lights. The commonest arrangement allows you to switch the landing light on or off upstairs and downstairs, but the hall light cannot be switched off from the landing.
Obviously it would be ideal if you could switch it on or off both from upstairs and downstairs. This would mean the end of irritating trips downstairs when the hall light has been left on, and the elimination of walks upstairs in the semi-darkness once it has been turned off. The answer is to convert the existing switch arrangements to full two-way switching of the landing light and the hall light. All you have to do to achieve this is add one new cable run and change the hall and landing light switches.
In the hall there is likely to be a two-gang two-way switch already in position near the front door. Upstairs, possibly located outside the main bedroom, there will be a one-gang two-way switch for the landing lighting circuit. This switch will have to be replaced with a two-gang two-way switch, which is the same size as a one-gang switch, while the downstairs switch will have to be connected up in a slightly different way.
The existing two-core and earth cables will continue to serve as the switch drops from the lights, while an extra three-core and earth cable will link the two light switches. The new cable will follow the existing three-core and earth cable running across the landing floor and down the hall wall.