Can I ask a quick question here?  If the cable is rated at 20 amps, how is it that you can use a 32a MCB?

My understanding is:

Design current </= Protective device </= Cable rating

I presume there is some 'correction' to be applied because it is a ring?

I need to know, my exams are soon  

but 2/3 of 32 amps is 21.33 amps, still more than the 20 amp rating of the cable.  How is this allowed?

[quote author=JerryD  link=1098546631/15#20 date=1099090875]but 2/3 of 32 amps is 21.33 amps, still more than the 20 amp rating of the cable.  How is this allowed? [/quote]

I want to put this to rest  

but to awnser your question, ring mains are an exception where the IEE regulations set out the maxium floor area surved, the type and size of cable that can be used, and maxium cable length

This allows a ring main of 32A using 2.5mm Twin+Earth Cable + 100M of Cable in total.

You could as an alternative have a Radial of 50m of 2.5mm T+E on a 20A CC

Or Radial of 100m with 4mm Twin Cable, with a CB rated at 32A..

[quote author=JerryD  link=1098546631/15#20 date=1099090875]but 2/3 of 32 amps is 21.33 amps, still more than the 20 amp rating of the cable.  How is this allowed? [/quote]
burried in a normal non insulating wall counts as method 1 (the same as clipped direct) and the rating of 2.5mm in that situation is considerablly more than 20A (though not as high as 32A)

[quote author=sparkyjonny  link=1098546631/15#25 date=1099236723]22 or 24A from the top of my head? [/quote]

Table 4D5A says clipped direct has CCC of 27A

Read that awhile back, the findings was the important bit  8)

or in my case 1.5mm

the answer is those who see the current ratings in the sheds and don't know about the concept of voltage drop (or don't realise its significant)

also i still think you can get away with a lot more voltage drop than the regs allow for

And there was I thinking that volt-drop was affected by the electrical resistance of the cable too........!

TT