SuperlecDirect - ElectriciansForums.net Electrical Suppliers
This official sponsor may provide discounts for members

Discuss Ring Main Help in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

I

irishsailor

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

Can anyone shed a little light on this please

How is the cable factors worked out on a ring??

32A Circuit breaker 2.5 T&E clipped direct 27A

How is the calc worked out for the above as I wa on the understanding that the cable capacity has to be greater than the Fuse rating??

Can you please explain

Irish
 
uHeat Banner - Forum Discount Available
This official sponsor may provide discounts for members
W

WayneL

  • Thread Starter Thread Starter
  • #2
Hi mate,

If it was a 2.5 t&e radial the capacity would be 27A and you would use
a 20A breaker.

Being a ring, the current carrying capacity is higher (not sure how much-haven't got
my regs on me) - but its more than 32 A.

Hope that helps,

Cheers,

Wayne
 

ian.settle1

-
Mentor
Arms
Hi mate,

If it was a 2.5 t&e radial the capacity would be 27A and you would use
a 20A breaker.

Being a ring, the current carrying capacity is higher (not sure how much-haven't got
my regs on me) - but its more than 32 A.

Hope that helps,

Cheers,

Wayne

It is because you have 2 x 2.5mm T&E in parallel that is why you can use a 32A breaker on a ring circuit.
 
I

irishsailor

  • Thread Starter Thread Starter
  • #4
So does that effectively mean that u have 54 amp in the 2.5
 
A

amberleaf

  • Thread Starter Thread Starter
  • #6
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
 
Last edited by a moderator:
Instyle LED Lighting Specialists UK
This official sponsor may provide discounts for members

Reply to Ring Main Help in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Instyle LED Lighting Specialists UK
This official sponsor may provide discounts for members
Top Bottom