Am I missing something?

[revjames]

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

 

[NozSpark]

If you count your furthest point as being 50m from the origin and you have 2 sides of the ring sharing the current equally..

Then work them out as 2 radial circuits and combine the results

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[revjames]

OK then using that thought:

18mvx50x16/1000 = 14.4V (2.5mm)which is more than 5% we are looking for 11.5V

11mvx50x16/1000 = 8.8V (4mm) which is acceptable....

Still got all the other factors to apply such as grouping. I did ask this question before as you know!

 

[revjames]

hang on do I have to double those figures? confused

 

[NozSpark]

That was the advice I got for doubling up the conductors in 6mm 4core SWA.. (apart form halving current)

 

[revjames]

hmmmmmm still not sure - this is for an assignment on the 2391-20 so it has to be right. while what you say sounds feasable Noz, I need to know its right. cant find anything helpful in OSG GN3 or even GN1......

 

[green-hornet]

No need on a ring to calculate 100m of cable as its dual flow, twin return etc.

The calculation should only be done on 50m of cable and not 100m, no need to calculate 50m and then double it.

One of the reasons ring circuits came about was to reduce volt drop, and the amount of cable used.

 

[Apache]

No need on a ring to calculate 100m of cable as its dual flow, twin return etc.The calculation should only be done on 50m of cable and not 100m, no need to calculate 50m and then double it.

One of the reasons ring circuits came about was to reduce volt drop, and the amount of cable used.

To be pedantic you are working out the volt drop of 100m and halving it

How far can you walk into a forest? (half way, then coming out again)

 

[revjames]

Thanks guys

Thanks guys so on the basis of your replies can you confirm I am right in my calculations above and assertion that 4mm is necessary for this circuit?

Thanks

 

[NozSpark]

Thanks guys so on the basis of your replies can you confirm I am right in my calculations above and assertion that 4mm is necessary for this circuit?Thanks

SO long as it passed your other calcs then I'd say so..

 

[revjames]

SO long as it passed your other calcs then I'd say so..

I wont know until I get an answer about the grouping factor. Cg for 20 circuits in trunking is 0.38........

at least I have something to be going on with, theres 6 rings in my assignment!

 

[septiclecky]

hmmmmmm still not sure - this is for an assignment on the 2391-20 so it has to be right. while what you say sounds feasable Noz, I need to know its right. cant find anything helpful in OSG GN3 or even GN1......

I have done this course, you do not have to stick to the Distribution Boards which they give you in the project.

You can put in more DBs if you want supplied off the orginal given DBs which will cut done on circuit lengths, volt drops etc

 

[revjames]

I have done this course, you do not have to stick to the Distribution Boards which they give you in the project. You can put in more DBs if you want supplied off the orginal given DBs which will cut done on circuit lengths, volt drops etc

I realise that. However I have looked at it and decided for simplicity to keep it down to the 3 DBs as per drawing.

The thing i am stuck on now is the grouping factor,

It

In/Ca Ci Cc Cg

Ca ambient doesnt apply as not expected to be more than 30C

Ci thermal insulation doenst apply as there isnt any

Cc not a BS3036 so doesnt apply

therefore the only factor that does apply is grouping Cg

My question that no one has yet been able to answer (not being rude just issuing a challenge)

If the circuit runs in trunking part of the way (with lots of other circuits) and in conduit for the rest of the way (on its own) on what do we judge the grouping? ie 36 circuits coming out of main DB in same trunking then getting less and less as it goes around the building. Even if it was only 20 circuits table 4C1 says Cg is 0.38 on that basis all the cable sizes in this installation would be massive!

 

[green-hornet]

You must always calculate for the worse case scenario.

 

[revjames]

So in a house where for example there is 2M of MT5 jam packed with cables from the DB and then all bunched tightly together through 25mm holes in joists you would apply the relevant grouping factor?

 

[Lurch]

So in a house where for example there is 2M of MT5 jam packed with cables from the DB and then all bunched tightly together through 25mm holes in joists you would apply the relevant grouping factor?

I was looking for where it says this since you first asked the question (or the other similar one, or wherever it started!) but somewhere it says that below a certain distance of run grouping factors need not be applied otherwise you'd be applying them to every installation ever done due to as you say above the final run down to the board or where they pass through joists.

 

[green-hornet]

Yes as this would effect the capacity for that cable to perform, although once clear the cooling effect can take less than 1m, the effect of the bunched cables will effect capacity.

This is best seen with thermal imaging, where you can actually see the heat build up.

Another aspect of grouping is spacing, many cable sizes can be dropped if conduit or trunking sizes are calculated properly.

 

[revjames]

I was looking for where it says this since you first asked the question (or the other similar one, or wherever it started!) but somewhere it says that below a certain distance of run grouping factors need not be applied otherwise you'd be applying them to every installation ever done due to as you say above the final run down to the board or where they pass through joists.

Thanks - if you could find where it says that the even better. I guess I will look at the plans and try and think of it on the basis of how many circuits are together in the last say 60% of the run.

 

[BS3036]

I'm not really in a proper state to comment at the moment, but...

Assuming the derating factors are not a problem, a 100m ring is a standard 2.5mm circuit in the OSG. The current draw should be distibuted around the ring, rather than loading it all at the furthest point. If you really did this, the two paths mean half the current goes each way. And then the load being distributed down the length of each half means around half the voltage drop at the end compared to what it would be in all the load was at the furthest point.

So compared with a single 100 radial, I would say divide by 8.

However, in practice, I think that it is assumed that the load may be not quite so evenly distributed, so the factor is between 4 and 8.

 

[NozSpark]

I remember looking at grouping factors not so long ago...

There's also something abot being able to ignore circuits which are loaded to less than 60%