CCC of a ring?

[HappyHippyDad]

Just wondering how we calculate the CCC of a ring. The tables give the CCC of the cable depending on reference method but how do we convert this to a ring?

It clearly isn't the same as using the CSA for a 2.5mm cable and then doubling it as the cables are joined rather than running separately. We talk diligently about reference methods but then can't apply them to a ring.

 

[littlespark]

If the cables are installed in the exact same reference method throughout the length of the circuit, then work it out to one cable, and double it.

If differing methods, then go with worst case scenario

 

[Lucien Nunes]

The RFC is a special case. I cannot recall reg numbers but the required CCC after derating is 20A. Historically, it was 2/3 of 30A which was then the normal OCPD. Any cable that achieves this, such as 2.5 PVC T+E or 2L1.5 MICC clipped direct, is considered suitable. The 20A figure was arrived at from the design and typical usage of an RFC; you cannot derive it by comparison with other applications because the RFC concept involves some specific assumptions that are not applicable to radials.

 

[HappyHippyDad]

The RFC is a special case. I cannot recall reg numbers but the required CCC after derating is 20A. Historically, it was 2/3 of 30A which was then the normal OCPD. Any cable that achieves this, such as 2.5 PVC T+E or 2L1.5 MICC clipped direct, is considered suitable. The 20A figure was arrived at from the design and typical usage of an RFC; you cannot derive it by comparison with other applications because the RFC concept involves some specific assumptions that are not applicable to radials.

Thankyou Lucien,

When you say the CCC is 20A are you referring to each of the 2.5 conductors? If not, then I am confused as the OCPD is 32A yet the CCC is 20A.

 

[Lucien Nunes]

Yes, 20A per leg, hence the necessity for assumptions about how the load is distributed around the ring to arrive at a compromise. Some people are aghast at the idea of such a great disparity of Iz<In, but those people have never tried running a whole house on 2.5 T+E.

 

[happysteve]

433.1.204 is the relevant reg. 20A per leg (as above) is the relevant bit, so if the reference method and/or rating factors mean Iz is below this, then you need a bigger cable. There is also a requirement that "the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying-capacity (Iz) of the cable."