Am I missing something?

[revjames]

Interesting point BS, just when I thought I had things sussed.

Regarding the grouping issue I just noticed Note 1 under table 4C1 (page 268) which states:

"These factors are applicable to uniform groups of cables equally loaded"

Does that mean I only need to consider the cables that make up the circuit since they will be the only ones that are 'equally loaded'.......

Can of worms.....

 

[revjames]

Sorry to be still going on about this. I am using the formula Iz = In/Cg

If thats the case then 2 circuits ran together method b gives a factor of 0.8 (4C1 p268)

That means In (value of CB) which is usually 32/ Cg which is 0.8

That comes to 40A which means 6mm for a ring circuit and thats before factoring in length of circuit etc....

Someone please tell me this doesnt apply to Ring circuits!

 

[PC Electrics]

Hi there - been trying to get my head around calculating cable size in a ring final circuit. Its 100M in length in singles in conduit. Trying to work out the volt drop etc - vd x A x M/1000

cant use the 100M figure as its not a radial.

R1+R2 works out at 0.23ohms for 4mm singles and 0.37ohms for 2.5mm.

I know its relatively simple and I know I am missing something basic but its friday night and I want to get cracking on this assignment!

Thanks guys

When calculating the volt drop on a ring your equation should read

VD = (vd x A x m) / 4000

Basically this is your equation (as used on radials) divided by 4. The furthest point on the ring is half way - so divide by 2 for 2 cable runs. Each leg carries half the current, so divide by 2 again.

 

[revjames]

Sorry to be still going on about this. I am using the formula Iz = In/CgIf thats the case then 2 circuits ran together method b gives a factor of 0.8 (4C1 p268)

That means In (value of CB) which is usually 32/ Cg which is 0.8

That comes to 40A which means 6mm for a ring circuit and thats before factoring in length of circuit etc....

Someone please tell me this doesnt apply to Ring circuits!

Thanks PC - understood, I got there by a slightly different route. But the above is what is getting on my wick at the mo!

 

[PC Electrics]

Sorry to be still going on about this. I am using the formula Iz = In/CgIf thats the case then 2 circuits ran together method b gives a factor of 0.8 (4C1 p268)

That means In (value of CB) which is usually 32/ Cg which is 0.8

That comes to 40A which means 6mm for a ring circuit and thats before factoring in length of circuit etc....

Someone please tell me this doesnt apply to Ring circuits!

So by using the Cg 0.8 you get a current requirement that is 1.25 times larger, ie 40A. Now, for a standard ring circuit on 32A each leg is required to have a current rating of 20A. So if our 32A is increased by 1.25, then this requirement is also increased by 1.25, ie 20 x 1.25 = 25A. Looking up cables in table 4D2A the minimum cable size that gives a rating of 25A in method B is 4mm.

There isn't much in it actually as 2.5 is rated at 23A method B! So if in the real world you considered it was likely to be a lightly loaded ring or very unlikley to be fully loaded continuously then you could still use 2.5mm (in a domestic environment).

 

[revjames]

So by using the Cg 0.8 you get a current requirement that is 1.25 times larger, ie 40A. Now, for a standard ring circuit on 32A each leg is required to have a current rating of 20A. So if our 32A is increased by 1.25, then this requirement is also increased by 1.25, ie 20 x 1.25 = 25A. Looking up cables in table 4D2A the minimum cable size that gives a rating of 25A in method B is 4mm.There isn't much in it actually as 2.5 is rated at 23A method B! So if in the real world you considered it was likely to be a lightly loaded ring or very unlikley to be fully loaded continuously then you could still use 2.5mm (in a domestic environment).

Thanks PC that is starting to make some sense! I was getting confused by the ring legs. Dont remember them covering this on the course!

Its a commercial location - a village hall so will put it in 4mm (its all academic and no financial constraints so can put what I like!)