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Discuss confused about cable requirements in the Electrical Forum area at ElectriciansForums.net

T

tawraste

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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:)
 
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A

acat

  • Thread Starter Thread Starter
  • #2
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
 
T

tawraste

  • Thread Starter Thread Starter
  • #3
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?
:)
 
A

acat

  • Thread Starter Thread Starter
  • #4
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
 
T

tawraste

  • Thread Starter Thread Starter
  • #5
thanks a lot dude, you have helped me tremendously.
take it easy !
:)
 
D

daver

  • Thread Starter Thread Starter
  • #6
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.
 
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