The following diagram shows the most simple of lighting circuits, a single lamp on a single switch. In this instance I’ve shown it with an incoming and outgoing supply as this can be achieved without breaking the rule of not putting more than four cables into a single junction box.
The next diagram shows a very similar circuit to the above except that this time we connect two lamps to a single switch. Adding the extra lamp and its associated cable means we can no longer accommodate two supplies within the same junction box without breaking the four cables rule.
