domestic, help with switched fuse for sub boards in mansion, cable selection...

[Smudge]

Hi all

I've started working in my boss's mansion (10 bedroom, 3 floor plus loft), and i need a little help/guidance with something. The incomming supply is at the front door on the ground level (3phase). As the majority of the loading is on the ground floor, i will have a db next to the incommer to provide for it. I will then have another db on the second floor to supply the 2nd and 3rd floor circuits. My question(s)...am i right in saying that both boards will need a switch fuse at the incommer? will this give suitable protection to the supply cable for the 2nd+3rd floor db? Will this have to be an armour cable? (the run is approx 20m). And can someone be so kind to show me a link to a wholesaler that sells such a suitable switch??

Its by far the largest domestic instal i've been involved in, so i'm on foreign territory, snipers and insult throwers, please be gentle. (and my boss is not a spark).

Thanks for reading...

 

[Guest112]

is your boss willing to pay to fit a smaller but correctly rated 3 phase db at the incomer and then you could supply your submains from there to the other dbs in the house? it would be a good idea to install a 3 phase iso at the origin for ease of isolation in the future

 

[Smudge]

the incommer is above the door in a small porch, enough room for a sgl db but not much else, uless it were very small and elongated!?! When i suggested at a cost to get scottish power to move the incommer to a better position, i got the roll of the eyes and a 'its there now, so it is aswell staying there'.

 

[Guest112]

ah right so pretty limited to what you can do then im sure youll get a reply from an experienced member soon mate

 

[Rockingit]

You need to have an isolator at the primary point of entry, or thereabouts, but where you choose to site your primary db is up to you. You may want to consider fitting a 4p isolator switch in a small din rail box where the whole thing starts from, then running an SWA of suitable size (5 core) to your 'main' ground floor db (as a 3ph db). You could then extend ph2 up to the 1st floor and ph3 up to the 2nd floor using 3 core SWA fed from 32 or 63A MCB's as you deem.

Things to watch for are cable lengths for your voltage drop allowances, getting your bonding to make sense and making sure the 2 way lighting circuits between the floors don't mix the phases up.

Good luck, tell us how it goes!

ps - any wholesaler will help you get a small 4p iso to fit, and you don't need to protect the cable between it and the first board unless you want to as it's straight onto the DNO fuses.

 

[Smudge]

would it be unreasonable to expect scottishpower to install a 3ph version (4pole) switch of their normal double pole grey/red isolator for single phase supplies? at a cost obviously. Then i could run the 5 core armour from this to where ever i want on the ground floor, supplying a 3phase db. Then from this, have a board per floor off each of the phases on 63A breakers? Does this have to be armour cable (these are effectively acting as your tails from the isolator)? Sorry for the need of total clarification as i'm a domestic/3 phase virgin!!

 

[Rockingit]

Hi Smudge, what you've just said pretty much echoes what I put earlier, so I think you've got your answer!

It doesn't HAVE to be armoured cable running out to your sub-boards (aka a 'sub main'), you could do it in singles in trunking instead, but SWA is far easier to run around than having to do all the surface dressing of trunking and pulling in of singles, and you'll almost certainly find it's cheaper, too. If you've limited experience of working with armoured, it's not that scary, and I'm sure we can give you a forum lesson in termination of glands!

As for asking the DNO for an isolator - always a good idea if you can persuade them but they may not give it to a 3ph location. You should be able to get one yourself for under £50, DNO will be more.

 

[Smudge]

believe it or not, i work with 3phase and armours everyday!! but in an industrial capacity within a factory. I was a 'house basher' as an apprentice, but now struggling to visualize meaty armours in a nice domestic setting!!

Thanks for your advice, hopefully got it sussed now, and looking forward to getting back into domestic, even if it is only for a couple of months.

 

[KFH]

I am with EoN and they put in a three phase isolator for me for free. It did take a lot of phoning around though before anyone would admit responsibility for doing anything like this.