general domestic premises there should be a separate ring
Final circuit for every 100mm2 of floor area. must remember
That the maximum load that can be connected to a ring protected
By a 30A / 32A fuse is just over 7 kW, and the number of circuits Selected accordingly
rule of thumb
( 433.1.5 p : 74
Ring final circuit volt drop
Ring final circuit : ( cable doubled )
<<< Cable length to farthest point >>>
is ½ ( cable length )
2.5mm2 Line conductor :
1.5mm2 Protective conductor :
to the farthest point in effect two cables are run , so ( mV / A / m ) is halved and the length
of doubled cable to the farthest point is half the length of cable in the ring , given a multiplier of ( 4 )
the design current ( Ib ) in each leg of the cable is not to exceed 20A ,
for ring circuits the equation becomes :
Multiplier
4 x 11.5 x 1000
Lvd = -------------------------------------- for ring circuits
Ib x ( mV / A / m ) x Ct
Ring final circuit wired with 2.5mm2 regs : 4D5 , p: 282 ( mV / A / m ) = 18
mV / A / mand assumingIbis 32A and Ct is 1 then
*
( The maximum circuit length is then the smallest of 2 ) to 4 )
Multiplier5% amp mV meters
Lvd = 4 x 11.5 x 1000 / 32 / 18 = 79.9 m
*
Multiplier
Answer 4 x 11.5 x 1000 div 32 div 18 = 79.9 m
Answer 2 x 11.5 x 1000 div 32 div 18 = 39.9 m
However , in a ring circuit the current in a ring will not be the same all round the ring .
If it is assumed to be 20A at the far end and additional 12A is evenly distributed ,
The average current is ( 32A + 20 ) / 2 = 26A , and
Answer ( 32 + 20 = 52 div 2 = 26amp
Multiplier5% amp mV meters
Lvd = 4 x 11.5 x 1000 / 26 / 18 = 98.3 m
*
If Ct is to be calculated assuming Cg is 1 , cable Ib is 26 / 2 , conductors It is 20 and tp is 70oC ,
Then Ct = 0.923 and Lvd = 106.5m