Date   : Thu, 03 Sep 2009 17:00:53 +0100
From   : philb@... (Phil Blundell)
Subject: Leccy @ Acorn World '09

On Thu, 2009-09-03 at 16:18 +0100, Mick Champion wrote:
>Jason Flynn G7OCD wrote:
>> Mick Champion wrote:
>>> A 32 amp breaker  is only permissible because each socket
>>> has two cables feeding it from either side of the 
>>> ring effectively doubling up the rating.
>>
>> It's more complicated than that: the cable that should be used
>> is dependent on the material it is enclosed in and the length.
>   
> I don't doubt any of that. However, what do you mean by "material it is 
> enclosed in"? Do you mean aperture the cable rests in, or the cable 
> covering (ie PVC)? If you refer to the aperture,  please tell  more.

Both are significant; the rating of the cable depends on its own
construction materials and on the situation where it's mounted.

The former determines the maximum allowable operating temperature for
the cable: mineral insulation can run safely at higher temperatures than
XLPE, which in turn can run hotter than normal PVC.  The latter
determines the temperature rise that is likely to occur for any given
amount of dissipated power in the cable: one that is buried in
insulating material, or bunched together with many other cables, will
get hotter at the same current than another cable sitting in free air.

Incidentally, going back to what you wrote above, the allowable current
for a ring tends to only be about 150% of the current that would be
permitted in an equivalent radial circuit, not double.  This is one of
the reasons that ring circuits are often not an efficient use of
resources: rather than installing a single 32A ring wired with 2.5mm
cable, you might well be better off using that same 2.5mm cable to
install two 20A radial circuits and hence gain 40A capacity in total.

p.