confused about cable requirements

tawraste · Apr 15, 2009

hi all,
can anyone help me out please? i am studying for a BTEC in electrical installations and i have run up against a very basic problem. i'd be very grateful if anyone can explain...
basically, i'm trying to assess the cable rating for a socket ring main. i am confused because i thought cables had to be rated higher than the protective device, but everywhere i look i see 2.5mm t&e being recommended with a 32A protective device.
when i look at the tables in the electrician's guide, 2.5mm t&e appears to be rated at 21A when sunk directly into a wall.
surely the cable will get damaged before the circuit gets shut off?
i'm obviously missing something here,but i cannot see the wood for the trees.
for reference, i'm calculating the figures for 8 13A sockets on 24metres of cable. no grouping or ambient temperature correction, but the cable will go straight through a 100mm insulated wall in two places. system is TNS, voltage 230v.
thanks in advance,
tawraste

acat · Apr 15, 2009

Hi

Hope this helps with what you are saying 2.5mm is rated at 20A for a radial circuit if its a ring main the cable rating is 40A. The radial should be protected by a 16A fuse and the ring with a 32A fuse

Good luck

Chris

tawraste · Apr 15, 2009

hi, thanks for your reply.
that does help a lot - i'm assuming the reason is that there are 2 paths to and from the consumer unit?

acat · Apr 15, 2009

Hiya

Yes indeed you are correct the fact that its a ring doubles the capacity. As a point of interest that may help you later no diversity allowed on the radial the ring is 100% then 40% thereafter

Regards

Chris

tawraste · Apr 15, 2009

thanks a lot dude, you have helped me tremendously.
take it easy !

daver · Apr 15, 2009

acat said:
Hi

Hope this helps with what you are saying 2.5mm is rated at 20A for a radial circuit if its a ring main the cable rating is 40A. The radial should be protected by a 16A fuse and the ring with a 32A fuse

Good luck

Chris

Acat a 20A is acceptable for a radial on 2.5mm as long as area is not greater than 50sqM.