Ring main in insulation

[jynxy]

First of all i would like to say Hi to you all.

Im doing some research for a course and need to understand about ring main in insulation.

I understand that because of the insulation it degrades the rating of the cable, so how do you overcome this ? i keep seeing about regulations and things but no actual detail on what to do.

is it a case of using thicker t&e cable or running in conduit, perhaps both ?

The research is for a kitchen so will need to apply this to cooker cable aswell but i guess it will be the same answer.

i dont have access to any regulation books etc, so if you could please explain and provide reference points from books i could look into this further

Thanks for your help.

 

[telectrix]

basically, because the insulation prevents air circulation and cooling of the cable, you have to derate it.simply put, if more than 400mm of the cable is in insulation, the current capacity is halved, so for example, if you reqiure a current of 20A, a 2.5mm cable ( ccc 27A clipped direct), would only have a ccc of 13.5A in insulation, so would not do for 20A, you would have to utilise a cable with a clipped direct rating of 40A, so 6mm would be required.

 

[sedgy34]

the above sounds about right

 

[malcolmsanford]

As our learned friend Tel says degradation of cable insulation can be found when in his example your stressing the cable by overloading it.

another form of degradation can be environmental. If you run a bare PVC cable in a very low temperature it would eventually affect the insulation, in this example the cold would make it brittle and prone to cracking.

That is why there are many forms of cable and cable containment that would be utilized in all environments

In a domestic installation sheathed PVC cable is perfect, it's durable, lightweight easy to work with and easy to terminate, but would really not be suited for very high or low temperatures, and so it's insulation would degrade.