Power to an Outside Office - Spur from Ring Final?

[Spoon]

I'd still give the customer both options. See which one they want. 13A may be enough.
Talk to them and tell them what they could possibly get for their 13A.. 1 PC, Printer, 2kW Heater, TV........etc

 

[Pete999]

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.....

No point in spurring the outbuilding with a 4mm2 cable from a ring final wired in 2,5mm2, spurring the office, the office will be limited by the fused spur you have spurred from, in this case 13Amps, "what does that tell you?
Your best be would be to do the job properly and run a new supply from the house CU.

 

[elsparko]

had one of these myself to quote for, main CU 45metres from "office" or glorified shed, 10mm to supply a new CU, chances are theres going to be kettle, fridge, lights, pc, printer, t.v, heater etc.

better to have spare than not enough.

 

[Marcus Vaughan]

No point in spurring the outbuilding with a 4mm2 cable from a ring final wired in 2,5mm2, spurring the office, the office will be limited by the fused spur you have spurred from, in this case 13Amps, "what does that tell you?
Your best be would be to do the job properly and run a new supply from the house CU.

Fair enough. It probably is only about 500 MM of cable from FCU in the house to the first socket in the office!

 

[Octopus]

Fair enough. It probably is only about 500 MM of cable from FCU in the house to the first socket in the office!

Send him off to a shed and tell him to buy 2 x 10 meter extension leads then ............ probably less than £20 and he can have 26A ....

 

[Taylortwocities]

Marcus, if it is that close, why not just extend the house ring into the office and provide a 3A FCU in there to provide power for the lights?

No new circuits, no new consumer unit, just a double length of 2.5mm T&E ???

 

[WillWX]

In my experience, if you’re ever at that point where you’re clutching at straws or you’re leaning towards to wrong side of the fence, it’s best to write the idea off and just put it to the customer and tell them how it is, as much as they don’t like it.

Falls under C.Y.A somewhere..

 

[Marcus Vaughan]

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

Well - I just looked through a lot of old posts with regards to this. I understand the Physics which says this is safe if disconnection times met, but not convinced it's compliant.

It's relevant to me right now, as a separate job on this site was to add additional external lights to an existing outside light - maybe 40m of cable. When I investigated the existing light it also turned out to be on spur from the same ring final as the mentioned before. It would take the Zs way over the max allowed for the OCPD.

Hmmmn. Need to get off the fence with this one. Tricky. Certainly would be useful to extend Spurs beyond their OCPD Max Zs limits - but is it compliant?

 

[Marcus Vaughan]

Marcus, if it is that close, why not just extend the house ring into the office and provide a 3A FCU in there to provide power for the lights?

No new circuits, no new consumer unit, just a double length of 2.5mm T&E ???

Good option that. Not too difficult either. There are sockets either side of a fireplace in the house and the office will be on the other side of that wall. Heavy duty conduit straight out and through the whole lot. Lovely. Best do my proper dead test and make sure I can get a few more meters of cable into the ring. Lovely!

 

[robd]

Depends how big an office it is. If it's 1 computer a desk lamp and a small heater (which is not unthinkable) I wouldn't have a problem with running it through a fused spur. If it's bigger and they're going to be whacking multuple heaters and stuff on then it's a no no.

 

[Des 56]

Looking at a job first hand the straight forward solution suggested in post 26 should have been in your thoughts from the outset,particularly as its just through the wall