I Can't See How This Works...

R1CH-G · Mar 18, 2014

Here's the scenario:

Ring Final Circuit

Design Current = 20 Amps
Circuit Breaker = 32 Amps (may well be using an RCBO - but that's another story)
Ring length total (end to end) < 45 meters
Correction Factors - grouping factor of 0.78 (reference method E with 9 other circuits)

Gives an Iz of just over 41 Amps

Where am I going wrong here, as I think wiring the ring in 10 mil T&E is going to be a little difficult!

NickD · Mar 18, 2014

Assuming I've understood the question, you've applied all your correction factors and need a cable with a 'raw' clipped-direct capacity of > 41A? OSG p.144 says 6mm clipped direct has 47A clipped-direct capacity. But also that would be for a radial not a ring, wouldn't it? Whole point of the ring is you can get away with smaller cable. Can't remember how you work it out for a ring. Would be interested to see the working from someone who can.

R1CH-G · Mar 18, 2014

Essentially you've got it right, but was using table 4C1 - item 4 for the reference method (basket).

Richard Burns · Mar 19, 2014

In essence a ring final circuit must have a corrected current carrying capacity of 20A for the conductors.
BS7671 states that this is the minimum CCC for ring final circuit conductors and the OSG states that at a ring is designed with a 20A load at the mid point and then 12A (to make up 32A) distributed evenly around the ring => equivalent to 26A at the mid point of the ring. and Appendix H OSG gives the guidance that the minimum CCC is 20A for a 32A ring final circuit.

In your scenario the ring length is <45m and the guidance in BS7671 says a 32A ring can be 100m2 and the OSG states that the maximum length can be up to 106m, so you would have a compliant circuit for length.

You need to apply the 0.78 grouping factor (and assuming there are no other factors) then you have 20A/0.78 = 25.64A
Therefore you are looking at a single phase multicore PVC covered cable installed to reference method E that can carry at least 25.64 A, from table 4D4A this would be a 2.5mm² cable (31A CCC).