2396 design project teaser

[juz571]

Hi all, I am slowly working through my design to gain this qual, and have come up against a little teaser when starting my ring main design calcs. I have (at the moment) split the ring mains into 5 or 6 seperate ones, and they range in length from about 90M to 130M total length (including drops and return leg). The problem I have is when I do my grouping factors. I was going to run all circuits in a suspended trunking around the building and then take them in conduit down to their positions. Now, as soon as I put 20 (will be more at the origin) circuits in the grouping factor table, I instantly get a 0.38 multiplier, which takes the cable size over 4mm2 and stops me having a ring main. I know I can maybe change to tray and run the cables in a single layer, to increase the multiplier, but in reality with the amount of circuits involved it would take about a 3M tray! Almost all installs I do at work contain many more than 20 circuits, in a single trunking, and unless very long are usually designed at 2.5mm2. I am just wondering if I am missing anything here, or if anyone has any thoughts. Any help or ideas would be greatly appreciated. Thanks.

 

[Farmelectrics]

Done my design last year passed and enjoyed it. I ran basket all around building in my project which had enough space on for cables not to be touching so no need to apply any grouping factors just try and make the installation as easy as possible for yourself there's no cost applied to the installation good luck. I'm sure I done radials aswell

 

[ImpededLoop]

If you are having problems with derating due to grouping, you need to think of a method to reduce the problem.

Out of interest, what kind of building is it this year?

 

[Engineer54]

Many people get confused when determining grouping factor, a couple of things to remember, one that the table is relating to ''Circuits'' not the number of cables. eg, you don't apply a grouping factor to a ring circuit because it has two cables, it's classed as ''one'' cable/circuit. Also the table is assuming that each circuit is at or near it's tabulated carrying capacity. The other thing to remember, is if it can be determined that a circuit will not carry over a certain percentage (think it's 30/35%% but check) of it's grouped rating then it can be ignored for grouping derating purposes.

I generally use my own formula to determine grouping factors, but won't help you to follow a BS 7671 based course /exam...

 

[ImpededLoop]

Agree with Eng54. Bear in mind that C&G have configured the design building to deliberately make you overcome this kind of problem.

 

[Andy78]

If you are having problems with derating due to grouping, you need to think of a method to reduce the problem.

Out of interest, what kind of building is it this year?

We have had a sneak peek but not been issued with the project yet, that's next week I think. It's a petrol filling station. Not sure of the details yet but I think it mainly comprises of power and lighting in the shop building with sub feeds to the forecourt, with finals on the forecourt to be completed by others.

 

[ImpededLoop]

We have had a sneak peek but not been issued with the project yet, that's next week I think. It's a petrol filling station. Not sure of the details yet but I think it mainly comprises of power and lighting in the shop building with sub feeds to the forecourt, with finals on the forecourt to be completed by others.

Cheers, ours was a two storey catalogue shop - basically an Argos.

 

[juz571]

Thanks all of you. I guess I have to satisfy a city and guilds examiner, not look at how it would be "realistically" done. If I have to use 6mm2 radials on socket outlets then so be it. I haven't even mentioned the thermally insulating walls yet (it's going in surface conduit)!

Yes it is a petrol filling station, with restaurant. Which looked very daunting at first, until as mentioned previously, the spec revealed the forecourt is to be installed by others.

Thanks again for all your input and suggestions.

justin.

 

[ian.settle1]

Unless it has changed the project when I did 2400 the course lecturer marked it but had to keep the projects for 1 year in case C&G wanted to carry out an audit on them.

 

[ImpededLoop]

Unless it has changed the project when I did 2400 the course lecturer marked it but had to keep the projects for 1 year in case C&G wanted to carry out an audit on them.

That's still the case. Apparently C&G visit each centre once a year and check a sample from each course.

 

[D Skelton]

Thanks all of you. I guess I have to satisfy a city and guilds examiner, not look at how it would be "realistically" done. If I have to use 6mm2 radials on socket outlets then so be it. I haven't even mentioned the thermally insulating walls yet (it's going in surface conduit)!

Yes it is a petrol filling station, with restaurant. Which looked very daunting at first, until as mentioned previously, the spec revealed the forecourt is to be installed by others.

Thanks again for all your input and suggestions.

justin.

No no no!

The project is there to determine how YOU, a potential designer, will overcome certain problems.

You have the freedom to design the circuits in any way you want, so long as you can justify your decisions and back them with regulatory guidance you will be absolutely fine.

I'll give you an example;

You have 20 x 20A radial socket circuits running in a piece of trunking. Does a grouping factor apply to any of these cables? In short, unless any of those circuits are running at more than 30% of load factor for... and this is the crux of the matter; extended periods of time, No. If any are, but they are still no running at 100% of load factor, a different derating factor compared to those shown in appendix 4 will apply.

In most realistic scenarios, no grouping factor will apply to any of those 20 cables.

If anything, you'll get marked down if your project shows 4mm cables running all over the place for lights and 10mm cables for socket outlets (which is what will happen if you ignore all the guidance at your disposal in appendix 4 other than the group derating factor table) as this demonstrates not only that you haven't a clue about how cable grouping affects their temperature but also that you have no idea how to apply this knowledge practically.

 

[Guest125]

Why are you avoiding running a 4mm radial for the sockets? We never install a ring in commercial circuits.

 

[Engineer54]

Why??........

 

[Guest125]

Less work. Easier. Less testing. we only do ring mains when wiring in T&E.

 

[Leesparkykent]

Less work. Easier. Less testing. we only do ring mains when wiring in T&E.

What about computer socket circuits?