Ring main calculation help

[mmcg]

Hi all
I have 2 quick questions regarding domestic ring main socket circuit calculations im hoping i could get help with.

1) regards calculating I[SUB]t[/SUB]from table 4D5 in the regs it states that in order to use 2.5mm conductors that it should have a max value of 27A (ref method C), im unsure how to get this figure as I[SUB]n[/SUB] is going to be 30 or 32A. The only correction factor applicable is Ca which in this case is 0,94 so as i understand it i[SUB]t[/SUB]=i[SUB]n[/SUB]/correction factors =30/0.94 =31.9A. Obviously i understand it is common practice to 2.5mm cable, im just wondering how to prove this on paper.

2) regards volt drop in a ring main, i have heard conflicting theories about how to calculate this so was just wondering how other people would go about this?

Any help is very much appreciated

 

[spark 68]

Draw a circuit showing a ring, and look at the effective cross sectional area (csa). :willy_nilly:

 

[malcolmsanford]

For the first part of your question have a read of reg 433.1.103 that Iz only has to be 20amp

The volt drop part I have always used the calc

Divide the length by 4 so for a 60mt RFC it would be = 15 x 18 x 26/ 1000

Bs 7671 pg 258, 6.1 corrections for operating temp.

This calculation give you a ct of .923

I was taught to assume 20 amp at the furthest point and 12 amp distributed evenly around the ring so,

average = (32 + 20)/2 = 26 which is the Ib