thermal insulation

[cliffed]

Hi 2 all,a ring main is installed in a roof space of a bungalow using 2.5 t/e,protected by a 32amp rcbo,the cable laid on top of existing insulation,years later,we r all goin green,the thickness of the insulation exceeds 100mm,how does this effect the final ring circuit calculations etc.If installing a 2.5 radial in the same conditions ref method101 the cable is derated to 17 amps,so is that the case of a ring main circuit,ie;32 amps now rated at 17amps,& protected by a 16 amp rcbo & not a 32 rcbo cheers hav a nice day,ta ra bab

 

[malcolmsanford]

Have a read of 433.1.5 and I think you will find that you will have to de-rate that RCBO if your calculations are correct.

 

[Engineer54]

Are you saying, that you have 100mm of new insulation laid on top of the existing insulation and thus surrounding the ring main cables in insulation??

If so, then yes you will need to apply the appropriate de-rating factor. Unless of course you can bring the ring main cables to sit on top of the insulation, as they were originally. Then as they are not surrounded in insulation, no de-rating will apply.

 

[cliffed]

i wonder how many existin properties r left with that scenario,& how many consultants never ever take termal insulation into consideration,how many ring main circuits hav u seen on a 16 amp rcbo,i hav never put a ring on less than 32 amps.The insulation surounds all the cable,

 

[Octopus]

People installing loft insultion have little or no respect, or knowledge probably, for the impact they are having on electrical installations

 

[Superspark23]

That is why some councils want cabled fixed in trunking when ran in a loft, and from there copex used for the drop to the celling through the insulation, must take ages to do!

 

[mark_]

Sorry Im a bit late jumping in on this topic but does anyone know what the calculation to size the ring main protective device is? Eg the Iz of 2.5 installed to method 101 is 17 amps what is the maximum mcb size?
I'm guessing its Iz x1.6 which give 27 amps so a 20 or 25 mcb would be fine.

Help please ive been looking for this for ages.

 

[Richard Burns]

From the above posts I am no longer sure but I would say that for 2.5mm T&E cable installed to method 101 (T&E cable clipped to a wooden joist and covered by >100mm insulation) the current carrying capacity of the cable is 17A, since you have a RFC there are two cables each taking half the current so CCC of the circuit is 34A maximum size protective device is 32A assuming no other rating factors apply.

 

[mark_]

Hi Rich, thanks for trying to help. I ended up speaking with my NICEIC area engineer so I asked him for some advise. He said 433.1.103 (new green book) tells us for a 32a ring each conductor Iz must be a minimum of 20a. So to work out the minimum IZ for a 20a ring you would calculate

20/32= 0.625

20x 0.625 = 12.5a

So a 20a ring main would therefore need each cable to have a minimum Iz of 12.5a (perfect for install method 103).

To go back to the original scenario with cables installed to method 101, a RFC in 2.5mm t+e could be installed on a 25a mcb.

Sorry to answer my own question but I thought it might be useful to someone else like me who has to design circuits for well insulated buildings.