grouping of cables factor

[crow11]

hi all, the job i am doing for my first assesment is garage supply in conduit(i have previoulsy posted about that) and replacing the c/u, on assessing the job today for the c/u all of the cables come down from the ceiling in a wooden box covered with a wooden front, i removed the front cover off to inspect and all the cables are just bunched together none of them clipped, there isn't much room in there when the front is on there is roughly about 12 cables, my main concern is there is a 6mm cable roughly about 15m there running a 8.5kw shower which works out 37A load for the cable which i will have to put on a 32A mcb according to the regs i have to apply a 0.45 factor for 12 cables together,could someone please advise on how to do this calculation, so i can work out if it takes my cable rating below 32A, or my other option is to extend the box work so i can clip each cable seperatley which would then be classed as method c as there in no insulation covering any of the cables, would it still be classed as method c with it having wooden front, sides and back cheers

 

[somersetsparks]

IMO that would be ref method B. But its open to interpretation!

 

[crow11]

yes it is open to interpretation, was just going for method c as it says in BS 7671 page 315 clipped direct so that the gap betwen the cable and the surface is less than 0.3 times the cable diameter, just wish sometimes the regs made things more clear

 

[somersetsparks]

I think that is to define it from 'free air'.
Under floorboards counts as B, So I would class that boxing as a method B. What I don't understand is why if you are wiring it in conduit this boxing comes into play?

 

[crow11]

only the garage is being done in conduit, which doesn't go that route, its just because i am doing a c/u change and all the cables in the box work is from the existing installation, obviously how it is at the moment in the box work needs the grouping of cables factor applying just need to work out if the shower cable after the factor taken into account would take my cable below my 32A,if so then would have to do as i mentioned extend box work and clip all the wires direct or change cable to 10mm, with it being my first assesment i dont want the assesor to turn round and fail me saying after the grouping factor the cable isn't big enough, just trying to cover all problems he could pick up on

 

[somersetsparks]

Ahh I see, I wouldn't personally worrty about that cable, whether you have it on a 32A, 40A or 50A mcb it will still be pulling the same amount and still be 'technically' overloaded.
Could the lid of the boxing not be fixed in place when your chappy turns up? :smartass2:

 

[crow11]

i could make it so it doens't come off, just my luck i get a funny assesor who will look at the box work and say there's alot of cables in that size of box lol

 

[Farmelectrics]

on page 340 bs7671 table 4d5 there is no method b so what method woul you use for cables under floor boards

 

[bigbaddave]

Don't worry too much about your asseament. I did a C/u change for mine many years ago and I worried and worried about what was going on so spent about 3 days on the job checking everything. Removed every socket, light switch followed every cable possible. Applied grouping factor ect ect. Removed spotlights in a bathroom. When they guy turned up we had quick chat and then Looked at the consumer unit and did a few tests. As long as you only put what work you are responsible for on your cert then you don't become responsible for the whole install. You wouldn't be expected to If the boxing was unacceptable to remove then to do so. Just note it on your cert. Obviously if you have 1.5mm cable on a 63 mcb running a shower then I'm sure he will have something so say about that but as long as your confident with testing and the work your responsible for is spot on then you will be fine.