Power to an Outside Office - Spur from Ring Final?

[Marcus Vaughan]

Hi All,

Just looking for some reassurance on a job I've looked at today. My status is that I am trying to stay within the realms of 'Minor Works' until I get a bit of experience under my belt. Conscientious decision - but everywhere I go it seems that notifiable work is needed as well. Anyway......

So - a customer has asked me to supply power to an outside office that's about to be built (when it stops raining), and has asked if I can take the supply from a nearby external supply he has spotted (actually supplies a pond). I've done my homework, read around the subject.

TNCS
Pond Supply = 2.5mm but spurred from B32 Ring Final.
Tested (live) Zs near the pond at 1.28 ohms so already a bit naughty.
Ring Final IS RCD protected.
The outbuilding has no extraneous conductive parts.
Not sure on the demand for the office but its pretty small, it has its own 'garage unit' (apparently) so suspect a B16 for sockets and B6 for lights.
New building is a gnats whisker from the man house and ring final.

It needs a new circuit - doesn't it?

I'm clutching at straws because the consumer unit is a long way from the new building, and running a new circuit will be a fair amount of work for a few sockets and a light.

I'm toying with taking another fused spur from the same ring (if Zs is low enough), through the walls and into the new building.

What then though - I understand you need a double pole isolator in the new office - but do I ignore the 'garage' consumer unit or use it? Maybe run a 4mm from the house ring final spur to the garage unit? Not ideal, but if Zs (or Zdb) in limits at the garage unit.....

I'll be interested to hear your thoughts and opinions.....

 

[Spoon]

supply power to an outside office

Is this "Office" just for the person of the house? e.g. it's only going to have 1 or 2 PC's in it and maybe a heater, maybe TV?

 

[Marcus Vaughan]

Yes. I’d say so. Biggest demand likely to be the heater.

Can’t see the 13 amp fuse blowing in the spur...

 

[Spoon]

If you are going down this route then just make sure you state the max 13A supply in your quote and explain this to the customer.
Got to C.Y.A. if the customer decides to put 3 heater into the place then whinges..
Are the cable calcs ok for Zs and other rubbish?

 

[Marcus Vaughan]

It's the cable calculations I'm looking at now. It's a bit close to the mark with the B32. It's probably best if I do dead tests instead of live Zs Tests through the main board RCD...

I like the idea of stating the max 13 amp. on the quote though. Makes sense.

 

[Octopus]

I like the idea of stating the max 13 amp. on the quote though. Makes sense.

In such cases you should ALWAYS state the design load on your estimate or quote - CYA applies!

 

[Taylortwocities]

suspect a B16 for sockets and B6 for lights.

If you are running the supply from a 13A FCU, then what is the point of this second CU?

You won’t get any discrimination, and the source ring final is already RCD protected, so you don’t need another one!

This needs a separate circuit. Sorry if this is outside your experience/ability.
Park it and walk away.

 

[Marcus Vaughan]

Yeah. I might give the customer a choice.
60 metres of SWA and labour (and an RCBO)
Or 4metres of Twin and Earth.
I wonder what they'll go for?

 

[123]

Yes. I’d say so. Biggest demand likely to be the heater.

Can’t see the 13 amp fuse blowing in the spur...

I'd say there is every chance of the fuse blowing. Heater/PC/Printer/TV/Fridge/Vacuum Cleaner etc etc.

Also I find modern Fused Spurs (of pretty much any brand) are pretty garbage if anything like 2-3kW is the load for long periods of time

 

[Octopus]

Yeah. I might give the customer a choice.
60 metres of SWA and labour (and an RCBO)
Or 4metres of Twin and Earth.
I wonder what they'll go for?

Hold on ................ 60m of SWA or 4m of T&E ................ can you explain

And I would NOT be recommending anybody that they try and run a garden office off a 13A spur......................

 

[Marcus Vaughan]

If you are running the supply from a 13A FCU, then what is the point of this second CU?

You won’t get any discrimination, and the source ring final is already RCD protected, so you don’t need another one!

This needs a separate circuit. Sorry if this is outside your experience/ability.
Park it and walk away.

Thanks. I know I wouldn't get discrimination.....
I was thinking the Zs might creep up too high for ADS if I didn't use the garage unit....but then again it's not really a sub main, it's a spur from a ring final. It all sounds like a bit of a hash up..

Your honest reply is appreciated!

 

[123]

Zs shouldn't be an issue as that part of the circuit will be protected by a BS1362 13A fuse.

 

[Marcus Vaughan]

Hold on ................ 60m of SWA or 4m of T&E ................ can you explain

And I would NOT be recommending anybody that they try and run a garden office off a 13A spur......................

New circuit from the consumer unit on one side of the property to the other- probably around the outside (there is no easy route). Or 4m from the ring final through a wall...

I might have exaggerated with the 60m.....not really looked at the new circuit options properly yet. Cc

 

[Octopus]

If they can afford a new garden office then they should really put in a proper dedicated supply .........................

 

[Marcus Vaughan]

If they can afford a new garden office then they should really put in a proper dedicated supply .........................

Good point!