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Topic Title: Office Wiring
Topic Summary: Rearranging office circuits to separate metering
Created On: 23 September 2013 10:02 PM
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 23 September 2013 10:02 PM
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delta1

Posts: 120
Joined: 04 March 2009

Good evening,

I'm quite recently self employed, with mostly domestic work under my belt to date. I've worked on commercial and industrial before, but not on the design aspect (monkey see, monkey do). So I'm pricing a job and would appreciate some input. I'll try to be brief.
Small office unit. 3 phase 100A TNCS incoming supply feeding 4 way TP and N board. Earthing and bonding ok. 4 existing offices, one store room and a light duty kitchen (fridge, kettle etc). Approx size 10m x 15 total, single story.
One office room (approx 7x5m) to be put on separate meter from the rest of the installation and have additional sockets etc.

My original thought was to split the tails from the existing Henley blocks, new three phase board via new three phase meter, then refeed/ add circuits as required.

However, I'm wondering if this is overkill for the scenario? Space is a also a little constrained (I know Proteus do a small horizontal 2 way 3 phase board, but, you know....). The old 'single point of isolation was nagging also.'

I'm now thinking of supplying a single phase board from a 40/50A mcb from the existing board, via single phase meter, then rearrange existing circuit protection as necessary to try to get each phase somewhere near to balanced (as far as practicable).
The offices are mostly just computers, the one being changed will house around 6 or so, plus peripherals and a server. I'll probably run x3 sockets circuits; two sides of the room and one for the power pole, all on RCBOs.
Any input on how that sounds would be appreciated. Thanks.

Kind regards
 23 September 2013 11:43 PM
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AJJewsbury

Posts: 11553
Joined: 13 August 2003


My original thought was to split the tails from the existing Henley blocks, new three phase board via new three phase meter, then refeed/ add circuits as required.

However, I'm wondering if this is overkill for the scenario?

It does seem a bit OTT for 35m2 of office space.

The simplest solution I can think of it to put a new (presumably private) meter in one of the tails to the existing DB (L1 say) and then re-arrange the existing circuits onto L2 and L3, leaving the L1 ways for your new circuits. That might be a bit too simple (there might not be enough spare ways for instance) - but perhaps gives an alternative point of view as a starting point.

Failing that a single-phase DB should be sufficient, but I'd be wary of feeding it from an MCB - discrimination will be lousey. A 3-pole (or 4 pole if you prefer or it's TT) isolator in the tails and henley blocks perhaps?

- Andy.
 24 September 2013 11:25 AM
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delta1

Posts: 120
Joined: 04 March 2009

I could leave the existing three phases to the existing board and use one of the spare ways in the Henley block to take a single phase feed to the new board. I then assume I'd have to contact the Dno to fit an isolator (or would labelling suffice?). As it stands, there is one isolator for the existing board, and one isolator for the E7 board, so no single point of isolation at the moment anyway.

Kind regards
 24 September 2013 01:00 PM
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AJJewsbury

Posts: 11553
Joined: 13 August 2003

Labelling might suffice - it has been argued that separate DBs could constitute separate installations within the same building, wo the letter of the regs is satisfied by each DB's main switch. (I don't like that approach, for me if main bonding is shared, they're not entirely separate installations, but it's hard to argue against now that the definitions in the regs have been watered down)

<official>Presumably you're not proposing to work live on the Henleys - so a DNO temporary disconnection or isolator installation would be required first anyway?</official>

- Andy.
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