Date   : Sat, 05 Sep 2009 08:37:17 +0100
From   : info@... (Sprow)
Subject: Leccy @ Acorn World '09

In article <4AA13E26.6010104@...>,
   Mick Champion <rs423@...> wrote:
> Sprow wrote:
> > In article <090903235504@...>,
> >    Jonathan Graham Harston <jgh@...> wrote:
> >   
> >>> because each socket  has two cables feeding it from either side of the
> >>> ring effectively doubling up the rating.
> >>  
> >> Nooooo. times SQR(2), not times 2. Check your parallel resistance
> >> calculations.    
> >
> > Say one cable is 1 ohm, then the resistance of such 2 cables is
> >
> >  [ (1/1) + (1/1) ] ^-1
> >
> > which is 0.5 ohms. That's a straight factor of 2.
> >   
> If the socket was exactly in the middle of the ring, this would be true 
> I guess? Most sockets would be nearer on end of the ring than another.

Resistance is a function of cross sectional area and length

  bigger cross section -> lower resistance
  longer length -> higher resistance

It's linear with length, so half the length is half the resistance etc...
Therefore a socket which was closer to the fuse box might be served by a
short cable of 0.5 ohms, and a much longer one of 1.5 ohms for example (such
that they add up to 2 ohms again).

Then the resistance would be

 [ (1/1.5) + (1/0.5) ] ^-1

which is 0.375 ohms: better!

I chose the centre point of the ring because that's the worst case,
Sprow.