Where in the regs....

[daveede]

Hi all

New here been reading awhile but this is the first post.

This maybe a silly question but where in the regs do you get the ring final cable carrying capacities?

For example its common knowledge that a 2.5mm ring takes a 32a breaker but a radial is 20a ... ignoring other factors for a minute.

The regs give current capacity for radials but not rings afaik

where does this info come from??

is it just a formula like radial + 60% gives the ring amps, e.g 20a * 1.6 = 32a

Thanks guys

 

[Sintra]

Have you read 433.1.5

 

[daveede]

Thanks, hadn't seen that reg before, it seems to explain the common knowledge of 32a for a 2.5 ring but only for that 2.5 ring.

Still wondering how rings in general are calculated, e.g if you wanted to know the carrying capacity of say 4mm or 6mm rings or any size (not that they would be used).

I know its not double the radial even though the csa becomes double, some wiggle room must be allowed for certain effects.

 

[Guest123]

The general rule of thumb is to multiply whatever OCPD you by 0.67. this will give you the CCC of the cable that can be installed.

Take a caravan park for instance, assume you wanted to install a 100A ring main. Multiply the 100A by 0.67 and you arrive at 67A therefore a 16mm SWA would be sufficient even though the max CCC in appendix 4 is less than 100A.

What is more important though is to ensure that all loads are evenly distributed across the ring main.

 

[daveede]

on closer reading

"the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable."

Seems like they might not really rate a 2.5mm ring for 32a, if its can only be applied for a short time.

 

[daveede]

@ lenny

Thanks for the reply but i always thought..

Ib < In < Iz

So what your saying is that...

In > Iz

Is this taking into account the max allowable fault current duration calculation? so it would still operate the ocpd before any damage to the cable insulation?

Edit: not sure my response made much sense tbh

What i mean to say is that In > Iz, when diversity is taken into account? and not run at the full 100A rating of the ocpd? otherwise would the cable be damaged running at 100A permanently.

ps. sorry im a noob so probably asking stupid stuff

 

[Bradzz]

Im also lost, im ready to learn, im going to have a look through that part of the regs.

I think i have it that is the ccc for each leg of the ring, like a domestic ring, ccc is less than the 32A protective device so as a ring the circuit is prorected.

As you have said as long as all is balanced ie 16 A down each side of the ring, or 50A down each side, thus the cable then exceeds the ocpd.
if im wrong accept my apologies.