Can you overload a lighting circuit

i think the answer to your question is yes , you can overload any circuit and cause the breaker to trip , you need to calculate the design current for what you have plus your additions .

If the total possible load on a circuit is insufficient to cause overload then you need not provide any overload protection.

You must still provide both Short Circuit protection and Earth Fault protection.

Both of these (and overload protection) may be provided by a fuse or MCB .

An RCD may be used for Earth Fault protection, usually as a supplementary protection along with such fuse or MCB or on its own on a TT system (a system whose earthing relies on a rod/plate etc in the ground).

There is nothing special about a lighting circuit in that sense - and there is nothing stopping you having a other loads on the lighting circuit - fans, window openers, TV amplifiers and alarms are often added to 'the lights' in small houses and flats, if it is convenient to do so. However, normally the breaker should be sized such that it trips at a level lower than that at which the cables would suffer damage (so not less than 1mm2 and a 6A or 10A breakers or not less than 1.5mm and not more than a16A breaker)

If the cable is undersized for the breaker, while unusual, this may be OK, so long as there is no risk of the load ever being increased, and the breaker is not soo big that it would not trip in a short circuit. Anything with sockets on (common for large buildings to allow whole fittings to be removed or re-located) should be designed assuming the load may change, and size the breaker and cable rating accordingly.

Yes you can have a 13A socket on your lighting circuit, but if you are daft enough to plug in a kettle instead of a small load, you can expect to be plunged into darkness, but not for the wiring to be damaged.

So if the cable is rated correctly for the breaker overloading the circuit will only cause the circuit to trip and no further damage?cheers guys

In general, yes the breaker should trip before any damage results.

There are exceptions however if the original design or installation was of poor quality. For example 1mm cable on a 10 amp MCB is usually fine, but if large numbers of cables are bundled together and/or the temperature is unusually high, then the cable might become dangerously heated by an overload before the breaker trips.


Repeated trips should be investigated.

I remain flummoxed by this OP. Can you expand on the circumstances Mr Jack96? It is inadvisable to add to a lighting - or any other - circuit until it goes pop............ then decide OK - that's enough. It cant be that hard to add up whats on the circuit and decide if the additional load can fit onto the existing circuit?


You last said, "So if the cable is rated correctly for the breaker overloading the circuit will only cause the circuit to trip and no further damage? cheers guys "


This is correct up to a point...........If you loaded a 40A lighting circuit up to 60 amps, on a 6mm circuit - it would last for - about an hour I believe - perhaps more -  before tripping - repeated loading to 60Amps on a cable not rated for this would stress it out and lead to eventual failure of the cabling...... Id guess. The same would apply to any size cabling/mcb relationship I'd immagine. 


What was explained above, however, was that (Under unusual circumstances)  if the cable is too small for the MCB/fuse it's connected to, but the cable is rated correctly for the load with no possibility of there being an overload AND then cable/mcb relationship was OK for short circuit protection. Then this is OK. 


There must be graphs to show how overloading a cable repeatedly and the heat produced will degrade the PVC over time.

Agreed - there is a small "gap" in overload protection - for small overloads of long duration. The regs require us to design in a way to avoid that, but if it is allowed to happen and there's not much margin between cable's rating and the protective device's rating then long term the cable can suffer.

   - Andy.

This seems a rather odd question. What additional loads are envisaged?


Don't forget that if it is 1 mm² cable, the circuit is limited to lighting (and the odd bathroom fan, etc.) Table 52.3 .

A 13 A socket is expected to be able to deliver 13 A.

Humm, what about a 13A socket on a 32A ring where it just so happened two other sockets were already used to supply 13A loads?

   - Andy.