Lighting circuit cable selection

[RQQ]

For selecting the correct cable size how do I determine the cable length in a loop in lighting circuit? Will it be the distance from the consumer unit to the final ceiling rose? Do I have to take into account the cables running from each ceiling rose to the light switch? I have attached a simplified circuit diagram. I think the csa of the conductors and cpc will be 1 mm2 but i have to prove this in my calculations. This is part of a uni coursework so i'm only looking for some advice not for answers. Any input would be greatly appreciated. Not sure if i've posted in the correct section.

 

[Wilko]

Hi - think about the reason for the calc (i.e. what will set the limit and push you from 1 to 1.5 say. Is it current rating, is it voltage drop?). Then work out the worst case (longest path) path that current flows and this will give you the length you need for the calc. Hope that helps a bit.

 

[Richard Burns]

Your longest length of cable is the limiting factor that you need to take into account, the line conductor from the rose to switch and back adds to the length of the line conductor.
so consider a fault from switched line to earth in ceiling rose 3.
Also as an aid to calculation consider the switch to rose as a line earth or line neutral length then you can just measure total length of cable without having to break it down to line, neutral, earth. This would be slightly more onerous than reality due to the reduced csa earth in twin and earth but errs on the side of safety.