installation of a ring cicuit

[davidbrideaux]

Hi EVERYONE

Im installing a new ring circuit in a renovated Bungalow.
Right down the nitty gritty ok i am running the cables through the joists above the downstairs ceiling and there is going to be 240mm insulation between the ceiling and floor upstairs do i have to use 2.5mm cable or do i have to step it up to a larger size for temperature reasons any feed back would be appreciated THANKS IN ADVANCE!!!!!!

 

[ricicle]

Is there no way you can keep the cables away from the insulation ?

 

[echoes]

It's a good question to ask rather than ploughing on regardless.

I think ref method 101 requires 4mm² cable in this circumstance, but I don't have books & tables to hand. Since you are designing & installing circuits, you should get the On-Site Guide and a decent guide to the regulations, which after some studying will tell you the answer and probably answer many more questions you didn't even realise.

 

[riveralt]

Generally speaking that decision should be made by the electrician signing off the work on the Electricial Installation Certificate - they will also inform your Local Authority Building Control.

You are better off discussing it with them because running cable through insulation can derate the current carrying capacity by up to 50%.

It does seem odd to have insulation between two living spaces.

Why are you running a ring final circuit?

 

[ricicle]

There are two ways to look at this :-

- Follow the regulations to the letter and say yes the cable will need to be de-rated because of the insulation (especially if it is fully enclosed for more than 0.5m)

-Decide you have a properly designed ring final circuit in which the cable will share load evenly with a theoretical maximum of 16A on any one leg which is a fraction more than 50% of the current carrying capacity of 2.5mm² so is not going to be a problem bearing in mind that ring finals rarely spend very long, if ever, at their maximum allowable design current.

 

[davidbrideaux]

Yeh its a bungalow being converted with an upstairs and the engineer has specified that the insulation has to be put in for some reason dont know why but i will be the doing the installation and signing off so i just wanted to check i have looked it up in the onsite guide but the guy whos building the thing is trying to cut corners with the cost.

 

[DetlefSchmitz]

the engineer has specified that the insulation has to be put in for some reason dont know why...

Probably sound proofing...

 

[ban-all-sheds]

-Decide you have a properly designed ring final circuit in which the cable will share load evenly with a theoretical maximum of 16A on any one leg which is a fraction more than 50% of the current carrying capacity of 2.5mm² so is not going to be a problem bearing in mind that ring finals rarely spend very long, if ever, at their maximum allowable design current.

What would you then decide to do about the fact that you no longer comply with the Wiring Regulations, which clearly and unequivocally call for the cable to be capable of carrying 20A?

 

[Chri5]

Run the 2.5mm cables via 2 x 25mm plastic conduit drops for the vertical. once your over the insulation lay the cables over the insulation rather than in it.

That should be over 20A capacity (OSG isn't to hand).

If you really want to be safe, 4mm feeds from CU to each start point of the ring, 2.5mm for the sockets between 1st and last ring positions.

 

[holmslaw]

..

 

[JohnW2]

What would you then decide to do about the fact that you no longer comply with the Wiring Regulations, which clearly and unequivocally call for the cable to be capable of carrying 20A?

I think you'll find that what 433.1.5 states clearly and unequivocally is that a ring final circuit is deemed to satisfy 433.1.1 if the current carrying capacity of the cable is not less than 20A (and that current in the circuit is not expected to exceed that Iz for long periods).

I do not believe that in any way prevents a designer producing a design which satisfies 433.1.1 without satisfying the requirements of 433.1.5's deemed-to-satisfy provision.

Kind Regards, John.

 

[ban-all-sheds]

So if you fail to satisfy 433.1.1 via 433.1.5, how are you going to satisfy it when you have an In of 32A and an Iz of 16A?

 

[ricicle]

Granted I shouldn't advocate DIYers straying from the regulations but they are not the be-all and end-all to anyone who can exercise engineering judgement through experience and knowledge.

 

[JohnW2]

So if you fail to satisfy 433.1.1 via 433.1.5, how are you going to satisfy it when you have an In of 32A and an Iz of 16A?

That's for the designer to decide, and (s)he may even conclude that it's not possible. I was merely pointing out that 433.1.5 does not "clearly and unequivocally call for the cable to be capable of carrying 20A".

The interesting thing, of course, is that, although deemed to satisfy 433.1.1, even an In of 32A and an Iz of 20A does not, in itself, satisfy 433.1.1(iii) unless one makes assumptions about the distribution of loads around the ring - so maybe that gives some clue as to how a designer might try to claim compliance with 433.1.1 with an Iz less than 20A. Of course, the problem could be solved by having an In less than 32A - but, unfortunately, MCBs with In between 20A and 32A are not freely (if at all) available. 25/26A would probably be ideal in the situation which was being discussed.

Kind Regards, John

 

[ricicle]

http://www.fastlec.co.uk/hager-25a-mcb-type-single-pole-p-3201.html