help please

[astropete]

hi al,l newbee here and this is my first post.


now the question i am about to ask may sound dumb, but here goes

we all know that 2.5mm t/e cable is use for ring mains, but how do we prove that. a question on a course work i am doing requires a ring main circuit to be designed from stratch. this is how far i have got (now no laughing)

i have given 7200W as a maximum for the ring main

so Ib = P / U
= 31.3 Amps
In = 32 Amp RCD

It = In / CFs
It = 32 / 0.85 x 0.88 (thermal and grouping)
It = 32 / 0.748
It = 42 Amps

now 2.5mm takes 27Amps max when clipped direct, do i devide the calc by two

ie It = 42 / 2 = 21Amps (so the 2.5mm t/e can be used)


please help

the man (pete)

 

[yellowvanman]

Ring circuits covered by 433.1.5. For a ring circuit each branch should have a current carrying capability of 20A, if the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity of the cable.

What you need to do is take the 27A for 2.5mm and apply the thermal and grouping effects to that, and I believe that will be greater than the required 20A.

Thats my 2p worth anyway!