reference method for current carrying capacity

[sherman0251]

Installing new shower. Trying to work out wether the exisisting cable is big enough the cable is ran through a stud wall with no insulation into the loft then through approxx 100mm through rockwool to pull cord then cable from switch to shower in stud wall with no insulation.

Which referance method would I use?
I'm using referance method B as over 90% of the cable is installed in a stud wall without insulation

 

[sparkybird]

IIRC you have to use the 'worst case', which for you will be the rockwool but the method will relate to how the cable is run eg in the middle of the insulation, or resting against the plasterboard. See p 265 of Regs for info.

all that said, I also remember that the old OSG (16th) had something about applying the method only if it related to 500mm or more of the cable. I've not got my 17th to hand, but might be worth checking as it can make a big difference to cable size.

 

[BS3036]

For 100mm insulation you should multiply the ref method C rating by 0.78 . If that is then the worst case section (as sparkybird said) that applies to the whole cable. (p.125 of On-Site Guide).

 

[GaryMo]

Is the 100mm of rockwool covering the cable in the loft? Is the cable clipped to a joist or in contact with the plasterboard ceiling?
If so then method 100# and no de-rating.