Date   : Fri, 04 Sep 2009 17:19:50 +0100
From   : rs423@... (Mick Champion)
Subject: Leccy @ Acorn World '09

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. (Say 38 amps to be on the safe
>>>       
>>  
>> 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.


> Maybe you're thinking I^2 R for the power lost due to heating the cables?
> If each cable is rated individually (due to the plastic it's sheathed with
> and the trunking it's installed in) to, say, 100W then Imax = 10A.
> To keep the total power dissipated in the trunking constant at 100W the new
> Imax is 14.14A,
> Sprow.
>
>
>   
One more e-mail to reply to, then time for a definitive read on the subject.

Mick.