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Discuss New Build regs on insulation in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Sean Kennedy

EF Member
Hi

Has anyone had any experience regarding insulation in walls on new builds as to what size mcb they are using for ring main circuits. I have checked the regs and seen that 100mm > insulation derates the cable capacity a fair bit. The builders are using 140 mm in the walls. I have pointed this fact out to them and told them that 2.5 cable on a 32amp ring is not ok?? The contracts manager says it's fine. He said its some special insulation. Im not to sure about it, even though it won't be me signing the insulation off. I was thinking of 20amp rings as its already wired and awaiting 2nd fix. Any advice please.

thanks

sean
 

Ian1981

Forum Mentor
Electrician's Arms
Trainee Access
Hi

Has anyone had any experience regarding insulation in walls on new builds as to what size mcb they are using for ring main circuits. I have checked the regs and seen that 100mm > insulation derates the cable capacity a fair bit. The builders are using 140 mm in the walls. I have pointed this fact out to them and told them that 2.5 cable on a 32amp ring is not ok?? The contracts manager says it's fine. He said its some special insulation. Im not to sure about it, even though it won't be me signing the insulation off. I was thinking of 20amp rings as its already wired and awaiting 2nd fix. Any advice please.

thanks

sean
These are timber frame houses are they?
Each leg of the ring must be able to carry 20amps minimum on a 32 amp ocpd so if your installation method derates the cable at any point , then larger csa will be required , or a lower rated ocpd.
Have you thought about installing 2.5mm in 25mm pvc conduit reference method A in a insulated wall, as the ccc is 20 amps?
 
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Strima

Electrician's Arms

Wilko

Electrician's Arms
... The builders are using 140 mm in the walls. I have pointed this fact out to them and told them that 2.5 cable on a 32amp ring is not ok?? The contracts manager says it's fine. He said its some special insulation.
Hi - its just possible with correct installation method, see diagram for method 102 in Table 4A2 (p390 BBB). But I’ll bet they’re not doing that.
Being a sarky git - I can’t say what this special insulation might be, but it can’t be the thermal insulating kind of insulation then, can it?:rolleyes:
 

Sean Kennedy

EF Member
Hi Ian

thanks for getting back. Yes that is an option moving forward. Or 4mm radial circuits. The guys who wired the job have said they were not told wall insulation sizes before they wired it. It was only myself who noticed it recently when i started with them. The plaster boards are all fitted now, so i doubt they will be pulling them off to sort it. I think 2.5 is only rated at 17amps in insulation exceeding 100mm. I could use 16amp Mcbs. Worried this could cause probs on kitchen. Though not heavy loads to be fair. Fan assisted oven & wash machine & Fridge freezer. Hob is separate supply.
 

Ian1981

Forum Mentor
Electrician's Arms
Trainee Access
Hi Ian

thanks for getting back. Yes that is an option moving forward. Or 4mm radial circuits. The guys who wired the job have said they were not told wall insulation sizes before they wired it. It was only myself who noticed it recently when i started with them. The plaster boards are all fitted now, so i doubt they will be pulling them off to sort it. I think 2.5 is only rated at 17amps in insulation exceeding 100mm. I could use 16amp Mcbs. Worried this could cause probs on kitchen. Though not heavy loads to be fair. Fan assisted oven & wash machine & Fridge freezer. Hob is separate supply.
You could use 20 amp mcbs as each leg of the ring would require to carry a minimum of 12.5 amps using a 20 amp ocpd and it being a ring.
Ocpd 20 amp ring 20 x 20/ 32=12.5
Ocpd 32 amp ring 20 x 32/ 32=20
It’s roughy a same to comply status as with regulation 433.1.204 that applies to 30 or 32 amp ring final circuits.
 
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mattg4321

Electrician's Arms
Assume you're looking at reference method 101 - for T+E above ceilings covered by thermal insulation >100mm.

Shouldn't you be looking at reference method 102 - in a stud wall with thermal insulation with cable touching the inner wall surface. 2.5mm2 rated at 21A
 

Sean Kennedy

EF Member
Yep. That’s what I did. Lol. Just read it again and realised I was looking at ceilings and not walls. Thanks for putting me right
 

Ian1981

Forum Mentor
Electrician's Arms
Trainee Access
You can only use 4mm radials on a 32 amp ocpd with reference method C so that would be rather difficult to achieve on timber frame new builds
 

Sean Kennedy

EF Member
Hi - its just possible with correct installation method, see diagram for method 102 in Table 4A2 (p390 BBB). But I’ll bet they’re not doing that.
Being a sarky git - I can’t say what this special insulation might be, but it can’t be the thermal insulating kind of insulation then, can it?:rolleyes:
Thanks Wilko
 

happyhippydad

Member
Electrician's Arms
DIY
I'm not too sure what the difference is here between 100mm and 140mm of insulation with regards to a wall (not ceiling). A 32A MCB is used for a ring when the parts of the cable route are in a stud wall (ref method 102). It mentions nothing about the depth of the insulation. What it does mention is the U value of the wall being a minimum of 0.1 W/m²K, so the CCC of the cables (in the tables) must be based on the wall having this value.

I have only just had a skim through the net on U values but the building regs state that a wall must have a U value of at most 0.3 W/m²K (lower the value the less heat loss).

Below is an example of a wall with 140mm insulation (not quite the same as the OP's example but I would have thought the U value in the OP's example would be higher as this has a brick wall as well) showing the U value as 0.27,
https://sydenhams.s3.amazonaws.com/Timber Engineering/Downloads/standard-details-1pdf.pdf
so.... criteria for ref method 102 have been met and the CCC of 2.5mm cable will be 21A.

God knows if that's ok for a 32A MCB though as the regs just ignore rings in the CCC tables, but it was fun researching it on a Sunday morning!
 

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