Is this one going to get to 4 pages??????? 
Applying de-rating factors to cable and protective devices?
- Thread starter Newboy1996
- Start date
That probably depends on how long holmslaw continues to wilfully ignore what it says in Section 6 of Appendix 4 on pp 206-207 of his copy of the wiring regulations.
You know, the publication which he doesn't actually use, but from which he quotes facts and regulations which people realise are so indisputable that they cannot argue with him.
You know, the publication which he doesn't actually use, but from which he quotes facts and regulations which people realise are so indisputable that they cannot argue with him.
Hopefully notIs this one going to get to 4 pages???????
You can apply them either way around as davelx says, as long as you do the right thing with them i.e. if you have your overcurrent protective device rating, the correction factors should be divided i.e. a 32A device with correction factors totalling 0.5 then it is 32A / 0.5 = 64A. This is the minimum value which you need to select your cable from the tabulated ones in the appendix of BS7671.
Likewise, if you have say a flat 10mm twin and earth (4D5 clipped direct = 64A) but runs through 500mm> of thermal insulation, the correction factor of 0.5 may be applied to the tabulated cable CCC to give the CCC for the cable, in this case it will be 32A.
It's just quicker to apply correction factors to the protective device and choose your cable as it only needs to be done once.
If you apply them to the cable then you may well find that the current carrying capacity of your chosen cable after factors have been applied is less than the rating of your chosen protective device therefore you'll need to select the next cable and carry out the calculation again. It's trial and error until you select the right cable.
If checking installed cables again semi enclosed fuses (PIR) then the only way to do this is to apply correction factors against the protective device, NOT the cable.
Quote BS7671:2008
It is therefore more convenient to apply all the rating factors to In as divisors.
If you apply them to the cable then you may well find that the current carrying capacity of your chosen cable after factors have been applied is less than the rating of your chosen protective device therefore you'll need to select the next cable and carry out the calculation again. It's trial and error until you select the right cable.
If checking installed cables again semi enclosed fuses (PIR) then the only way to do this is to apply correction factors against the protective device, NOT the cable.
Quote BS7671:2008
It is therefore more convenient to apply all the rating factors to In as divisors.
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