Max Cable length on ring main.

[2000tdi]

Someone earlier asked the question , what is the max length of cable that can be used on a ring main . The answer being that its not the cable length that is the factor but the area covered , in domestic wiring it should not exceed 100m2 area .

Can someone explain as I cant quite my head around this , surely the area covered is dependant on the length of cable used . Or am I totally missing something here.

How do I go about measuring the area m2 that my ring mains is covering ?

Cheers

 

[Spark123]

Cable length also needs taken into connsideration as if it is too long:
(1) the protective device will not operate fast enough in the event of a fault.
(2) volt drop will become excessive.

The 100m² rule afaik is a crude estimate of the amount of appliances (originally heaters) which would be required in that amount of space for the current capacity of the circuit.
In order to calculate cable length on disconnection time, Ze and protective device need to be known.
The way adam151 worked it out in the other post (here) was with the full 32A was being drawn at the mid point of the ring, it is also possible to work out the cable length for volt drop by design current at each point although much more complicated and all loading must be known (an almost impossible task given the nature of socket outlets), hence the mid point is used for simplicity.