As an industrial safety auditor(though enjoying this profession but eager to switch), I have encountered several times by the client about this nonconformity. This electrical code is directly quoted in NFPA-70 and in BNBC. Thus I feel privileged to clear this code – why socket and light should not be in the same circuit.
The first concept is the problem with higher sized protection (mcb/ mccb). The considering fact is the quality and rating of cable. Every electrical cable has limitations- the commons
Now if anybody places a 63amp breaker along with 4rm cable, what might happen. The 63am breaker will not trip within 0-63amp range but the poor cable which is 4rm will certainly burnt away. This is called higher sized incomer or protection.
Now the problem with socket and light in the same circuit covers the above concept- higher sized incomer. Generally the light circuit is of maximum 5amp capacity because that much current is drawn by the light – lower current consuming appliances.So naturally light circuit has 5-10amp breaker and 2.5-4rm cables.But If some socket is attached in the same circuit, then there is always possibility of attaching a higher current consuming load to that socket. So the socket should be covered with at least 6rm-15rm cable and 25-40amp breaker.
Thus if any of above option is done either the higher sized incomer or undersized incomer discrepancies will be seen.
Secondly- any person might think safe to work on a light circuit after switching off the light, but with socket in the same circuit, the circuit is still energized.
Thirdly, an open socket outlet to anybody has the chance to attach any type of load- inductive(hand dryer, motor), ac, TV. This load is not steady, might cause overloading the circuit and fire in the lighting circuit.