An intermediate lighting circuit is an extension of a two-way lighting circuit that uses three or more switches to operate a single or multiple light fittings.
In a typical UK home wiring situation, lighting circuits usually incorporate the lighting needs of all of the rooms and areas of a single floor, incorporating one-way, two-way and intermediate switching circuits as required. Generally these circuits are protected by a 6 amp miniature circuit breaker (MCB) and are wired using 1.5mm2 'twin and earth' cable. Previously this might have been a 5 amp MCB or fuse, and 1.0mm2cable. The overriding limitation of the circuit is the maximum wattage that can be supplied. Without taking into account any de-rating factors, the circuit can supply a theoretical maximum of 230 volts x 6 amps = 1380 watts. In general practice, lighting circuit wiring should be planned in such a way as to have a maximum load of 1000 watts. In this way there is a small margin left for third party addition or lamp changes which might affect overall load levels.
The circuit
We can see from the picture above that when the switch 1 is switched, electricity can flow from the live supply connection along one of the two 'strapper' wires via the intermediate switch in the middle to switch 2. If switch 2 is also switched to the strapper wire that is currently 'live', electricity can flow to the lamp and pass through the lamp filament to the neutral supply connection, causing the lamp to illuminate. Now when any switch is switched, the switch connects to the other strapper wire and the circuit is broken, causing the lamp to extinguish.
The idea is that any of the switches can cause the lamp to illuminate or extinguish, and this circuit is used in long hallways, stairs, and rooms with more than one entrance/exit. This of course is how the circuit would look if it was done as a bench exercise. Check out our two way lighting circuit article for wiring tips.
NOTE: Always isolate mains power at the consumer unit before starting work.