I Can't See How This Works...

[R1CH-G]

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]

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]

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

 

[Richard Burns]

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).

 

[telectrix]

beaten to it by 6 hours by a night owl.LOL.

 

[Pete999]

Kerchof's law seems to ring a bell in my foggy brain

 

[telectrix]

Kerchof's law seems to ring a bell in my foggy brain

the sum of the currents entering as junction is equal to the sum of the currents leaving the junction. dunno what that's got to do with the OP.

 

[wozz]

With regards to the grouping there's a paragraph under the table that you may be able to use.