Sub main board question

[maluka]

Hi All.

I'm finally getting to start the installation that i'd asked advice on in October. However there have been some changes i've had to make i may or may not be on the right track.

The plan was to put in an 8 way cu (sub main) with 6 RCBOs' to divide up a computer suite, 25 PCs', data projector, printer, e.t.c, all radials (high integ earthing).
The 8 way being fed by the original radial circuit from a Federal DB in a riser about 50m away (cable run). The plan being to replace the 30A Federal RCBO with a 40A Federal MCB, those of you with much more experience than myself have just seen the flaw in my plan, Federal boards are now obsolete and i'm a little uncomfortable with using secondhand MCBs' (ebay).
So i decided to let sleeping dogs lie and not touch the Fedral except for the folowing.

So plan B from the riser a 2 way cu sub main with a 40A MCB (63A pole switch) to the computer suite in 10mm T&E to the 8 way cu.

My question concerns the tails from the 2 way cu:-
Where do i connect them, on the incoming or bus bar side of the Federal board (each phase protected by a 100A fuse)?
Or directly to the incoming supply bus bar box?
Can i use the 10mm T&E?
Or does it need to be larger that 10mm?

The total length not being longer than 2 meters.

Your thoughts would be appreciated.

 

[SPARTYKUS]

Got a picture, not really following it ?

 

[telectrix]

try the fuse co. for obsolete MCBs.

 

[Guest123]

I'm a bit confused by the sub-main supply end connection also. If there is a busbar chamber, you could supply directly from this with a suitably rated switchfuse to protect the sub-main. If there is a change in CSA, the switchfuse will need to be within 3M of the chamber.

WRT the actual sub-main being 10mm T&E, an issue that immediately springs to mind is the size of the CPC and the possibility of high earth leakage currents from all the IT equipment.

 

[maluka]

Lenny
Effectively the 2 way with 40A MCB is what i was trying to achieve being similar to a switch fuse you suggested and it is well within the 3m distance so i'll wire it directly into the bus bar chamber.

However i didn't give the CPC any thought. I'm not sure but i think that the CPC in 10mm T&E is 4mm so does this mean that i'll have to run an additional 6mm CPC.
As the installation stands when all pc's are on, i've measured up to 60ma for short periods, 2-3 seconds, however from time to time it stays there for long enough to trip the RCBO. As a result i've disconnected 6 PCs' as an interim fix. It was the story "this looks like a good room to turn into a PC suite" not looking at all if the supply will support it.

 

[needasparks]

I did a internet cafe some years ago now and it was designed with no more than six pc's per circuit for that reason. Its assumed 5mA per pc on design (can't find the spec ATM). What reasons does it require RCD protection in your design ?

 

[maluka]

Needasparks.
one word.... children. The whole site is RCBOed and RCDed.

 

[needasparks]

Sorry, could not see in the OP. If that is the case, throw the problem back to the designer.
Why would anyone put that amount of PC's on a circuit.

Edit
whoops just seen, your the designer.

 

[maluka]

 

[needasparks]

just seen the T&E bit. Is this a school ? can you use T&E in the place ? would not have thought so. Personally i would go for the swa with the additional core with armouring for possible high currents.
You can still get some old federal (not all) types, I had to on a couple of jobs last year. Don't see you have an option other than changing to a standard mcb. sorry for all the questions, i missed your original post about it all.
I agrre about the ebay route, I will look back during the week where i got mine from when i'm back in the office.
Stick a photo up as i may even have one in the stores your welcome to it if it helps.

 

[Knobhead]

Not having a go at you maluca but it would have been better had you continued your original thread http://www.electriciansforums.net/electrical-forum-general-electrical-forum/43670-little-advice.html It would have saved some confusion.

View attachment 10647

 

[needasparks]

Think i'm putting mud in the water here so will sit on the bench, there seems too much i don't know on this one from the other thread.

 

[maluka]

Tony Will do next time

 

[maluka]

needsaparks
You right about the T&E issue, however it will be run in metal trunking above a suspended ceiling and the 8way cu will be in a small office adjacent to the suite.

 

[needasparks]

Yes I thought it would be, possibly low smoke even, its just that you will have a sub-board supply with possibly high currents running on the earth so how do you intend on addressing this if your using T&E.?
My guess is the pc suite is surface trunking or dado compartment, thats how most do them. sounds like just adding additional circuits to split it up would be the sensible answer on more rcbo's.

 

[maluka]

Thanks needasparks.
Just been on to the doncaster website as i was unsure of the csa of the cpc of the 10mm T&E and as it comes out at 4mm. My plan will be to running an additional 6mm cpc.



Edit
Sorry for the late response but i could not reply posts till today

 

[netblindpaul]

Why are you using Flat Twin & CPC in trunking there is no need?
In fact it is detrimental as the sheath takes up unnecessary space.

 

[supasparxs]

if your running trunking why don't you wire everything in singles and use MCB's?
if there is a suspended ceiling why don't you use cable tray above the ceiling?

 

[maluka]

We had a drum of about 70m of T&E so couldn't justify the £200 spend on singles. As for the trunking, there is plenty space as it is 100mm x 100mm and about 20% full.

 

[Engineer54]

We had a drum of about 70m of T&E so couldn't justify the £200 spend on singles. As for the trunking, there is plenty space as it is 100mm x 100mm and about 20% full.

Save your T&E for another job and use singles on this installation. Your already having to supply a separate single CPC conductor, so bite the bullet and do things properly by using the right installation cable....