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Discuss "Clever" client derated the cables! in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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So I'm currently sorting a CU change and kitchen rewire while also fixings some bodges by a have a go client, came back this week to discover that he has decided to insulate the void between ground and first without mentioning it. Now this has derated all his cables by half as they are tottaly surrounded, am I being a bit OTT or am I justified?

Cooker will now have to be low wattage single oven, fortunately the hob is gas anyway, but with the ringmains would I have to put them on a 16amp or a 20 amp? I know that 32 amp MCB must be below the actual capacity of a 2.5 ringmain but by how much would I get away with 20amp? I don't have the book with me!
 
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telectrix

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quote for replacing cables with 4.0mm. :D:D:D. kerrching.
 
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  • #3
Lol this will be done but I don't think he has the budget at the moment, I have already warned him about the potential of overload I'm not worried about the up and down ring, just the kitchen it's a small house with central heating so ne real requirement for regular high current items except the kitchen.
 

Spoon

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What type of insulation has he used?
Can the insulation be moved away from the cable a bit?
 

telectrix

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insulation between floors????? whatever next, loft insulation???
 
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Earthwool throughout, I have moved it on the cables we installed as they were accessable still in the most part, but now I'm changing the CU I am of the mind that I should derate the circuits accordingly in order to keep things as safe as possible for now. I can't lift the floor throughout and move insulation for him without quoting as it's a lot to move, it and if he refuses then I need an alternative that doesn't leave me open to blame for leaving in an unsafe condition hence why I am concidering downgrading a few MCBs...
 
I think you'll be fine after taking diversity into consideration. 20amp rings are perfectly adequate for a small house.
 

FatAlan

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Trainee
Very good point worth considering for future jobs. Must admit that in this day and age of heat and sound insulation it would be prudent to future proof unless discussed with client. Hindsite is a wonderful thing;)
 
T

Toneyz

Are the cables totally surrounded, or clipped to the joists etc?
Agree the insulation is probably not right up to the clipped direct cables. We did the same when we had the floor up on our landing to install heating pipes you can't get it solidly around the cables as you would in a loft space.
 

Wilko

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Arms
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Hi - with book open here - it might be Method 101 (above plasterboard ceiling with insulation cover more than 100mm) which is 17A for 2.5mm. Not making the minimum 20A ccc would mean derating to 20A mcb for RFC. On the other hand if it's 100mm or less, then it's Method 100, which is 21A which is ok for 32A mcb on RFC.
As @telectrix says, 4mm is 22A worst case :) .
 

telectrix

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can't see any point in insulating between floors. cannabis farms need heat as well as ground floor dwellers.
 

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