Discuss Rings, reference methods and CCC... in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

HappyHippyDad

-
Esteemed
Arms
Reaction score
5,578
Following on from another thread I have just replied to it got me thinking about ring circuits and reference methods.

Appendix 15 shows a ring on a 32A MCB. It does begin with a get out clause regarding thermal effects but it just points you Chapter 42 which deals with fire.

If for example we had a ring which was completely routed in reference method 103 what MCB would we choose? We know that the CCC of 1 x 2.5mm cable is 13.5A but how does this tell us exactly what size MCB to use for the ring?

There must be a simple equation to use based on the CCC of 2 x 2.5mm cables used as a ring, to find out the MCB required?
 
Hi - I haven’t seen any regulation or guide values for RFC outside of “20A minimum ccc for 32A mcb”, but I’m always happy to learn. Design wise the factor would appear to be 1.25 ... ie I assume someone has tested (by long experience perhaps) that 2 x 20A won’t be damaged if protected by a 32A mcb (40/32 = 1.25). So 2.5 floating in stud wall with insulation (method 103) might be protected by 2 x 13.5 /1.25 = 21.6A mcb, ie 20A . Just my thoughts :) .
 
Hi - I haven’t seen any regulation or guide values for RFC outside of “20A minimum ccc for 32A mcb”, but I’m always happy to learn. Design wise the factor would appear to be 1.25 ... ie I assume someone has tested (by long experience perhaps) that 2 x 20A won’t be damaged if protected by a 32A mcb (40/32 = 1.25). So 2.5 floating in stud wall with insulation (method 103) might be protected by 2 x 13.5 /1.25 = 21.6A mcb, ie 20A . Just my thoughts :) .

3/4 of the combined CCC of the two legs.

Interesting...

So with your example Wilko 2 x 20A would be fine on a 32A MCB but not with SL's (i.e 40 x 0.75 = 30A)

Two electricians who know their stuff but 2 different ideas. That's why I'm asking as there doesn't seem to be a definitive answer. I'll be interested to see more replies!
 
The loading is deemed to be 20 amp at the furthest or mid point with a further 12 amps evenly distributed around the other leg of the ring hence 20 amp minimum for the Iz of the cable is required for compliance with 433.1.1 Overload protection for a 30 or 32 amp ring final circuit
 
The method Wilko is probably the more modern method, using 32A instead of 30A.
40 divided by 30 = 1.333.
2 x 13.5 / 1.333 = 20.25A.
 
Last edited by a moderator:

Reply to Rings, reference methods and CCC... in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock