Garden Office supply

[MikeBar]

Terraced house, edwardian, TNS, row of similar houses.

Building a shed/garden studio in the rear garden.
Intend to fit an RCD protected CU in the ched, supplying-
power sockets inside on a 16A series circuit (not a ring)


If possible, would also like to supply
underfloor heating 16A
power socets outside shed 16A
shed lighting 6A

So a five way CU, max demand I’m setting at a generous 32A

Supplied via three core SWA XLPE buried in the garden and then run down the entry and around the side of the house on cable cleats, into the front vestibule of the house through the timber surround of the window above the front door and into an isolator, then into the house CU which is in the vestibule. Planning to feed it from a 32A mcb (rcd protected) at the house CU.

Planning to use the house earth, will earth the armouring, bury the cable at 500mm etc.

Cable needed is approx 42m, maybe a bit less, will measure accurately shortly.


Comments are very welcome- I know it is notifiable work and I’ll need an electrician to supervise and test/inspect/ issue installation cert. If there is anyone who would be interested to do this (for a fee of course) I’d be very grateful.

Query- what surface to mount the CU on at the shed? does plasterboard qualify as non combustible? what else might? MDF?

Query- Any reason I can't use 4mm SWA?

Query- I bury the cable with tape, in a bed of sand- is any further protection where it emerges from the ground or otherwise needed, cable tiles aren't a requirement anymore are they? I’m worried about the short run up from the ground into the shed, imagining rats knawing on it.

Query- I could possible fit an earth rod too, I am thinking that will help if I run into Zs problems, with my lighting circuit plans particularly, would like to put it all in without an earth rod then fit one if I need too. Any comments are very welcome.

Thanks very much to everyone.

 

[MalcofArabia]

No reason at all as 6mm^ is within the 32amp range providing you take into account the run of cable and the disconnect times, that is all part of your installation design. As 42m is a fair run. I would use 6mm at least or even 10mm

Be very supprised if you will find a sparks to sign off your work as he can only self certify his own work. your best recourse is to contact the LABC and ask them what you plan to do and they will tell you what you CAN actually do in this regard.

Though you have a TN-s system you might find the DNO can be a little wary of exporting their earth to an outside building. It is mainly for PME but all DNO's are different you will need to ask them

If your having services in this shed/office then you will need to run a 10mm^ bonding cable from the service right back to your Main Earth Terminal, not the one in your local CU.

Personally I would TT the system much less hassle. Most CU are mounted on a chipboard at the mains so do the same.

Is there a reason you are using XLPE cable as I'm sure your installation is not going to go above 90c conductor temperature.

Be a pretty enhtusiastic rat to get through a SWA. The depth varies from 400 to 500 mm^ depending on where it is to be buried ie garden, drive etc.

 

[ban-all-sheds]

Intend to fit an RCD protected CU in the ched ... Planning to feed it from a 32A mcb (rcd protected) at the house CU

Why two RCDs in series?

run down the entry and around the side of the house on cable cleats, into the front vestibule of the house through the timber surround of the window above the front door

Query- what surface to mount the CU on at the shed? does plasterboard qualify as non combustible?

Yes.

Query- Any reason I can't use 4mm SWA?

Yes - voltage drop will be far too high.

Query- I bury the cable with tape, in a bed of sand- is any further protection where it emerges from the ground or otherwise needed,

No.

cable tiles aren't a requirement anymore are they?

If the designer deems that the risks of damage warrant them, yes.

I’m worried about the short run up from the ground into the shed, imagining rats knawing on it.

Put it in steel conduit or behind heavy duty capping (like the stuff they use on telegraph poles).

Query- I could possible fit an earth rod too, I am thinking that will help if I run into Zs problems, with my lighting circuit plans particularly, would like to put it all in without an earth rod then fit one if I need too. Any comments are very welcome.

If you run into excessive Zs problems then you've got more pressing things to attend to than the outbuilding supply.

Comments are very welcome- I know it is notifiable work and I’ll need an electrician to supervise and test/inspect/ issue installation cert.

And design it - all these questions should be asked of him, and his decisions are the only ones that matter.

 

[ban-all-sheds]

No reason at all as 6mm^ is within the 32amp range providing you take into account the run of cable and the disconnect times, that is all part of your installation design. As 42m is a fair run. I would use 6mm at least or even 10mm

I make VD 9.8A for 6mm².

[EDIT] 9.8v even... [/EDIT]

Personally I would TT the system much less hassle.

Personally I think that discarding a perfectly good TN-S earth is perverse.

Is there a reason you are using XLPE cable as I'm sure your installation is not going to go above 90c conductor temperature.

It probably mustn't go above 70°

 

[davelx]

Even 10mm^2 only just does it at 42m run; it gives you 2.6%/5.91V VD and you are allowed 3% as is it supplying a lighting circuit - and that 3% has to include wiring in the shed too.

 

[MalcofArabia]

Ahhhhhhh pity the heady days of 5% are no longer with us, point taken BaS I did say 6mm^ at the least but I thought 10mm^ was the one, but I wanted to throw that back at the OP when I mention his design.

As you know BaS there is nothing prohibiting exporting the Earth as far as design and Regs go it is the DNO that object The majority mostly object to PME but in London a while back a DNO objected for a TN-S system I was on, that's why I advised him to check

Tongue in cheek the XLPE as I'm sure the PO just picked that up from somewhere and perhaps did not know the difference between that and PVC which is easier to get and cheaper

 

[MikeBar]

(quote) If you run into excessive Zs problems then you've got more pressing things to attend to than the outbuilding supply. (quote)

I meant a measured Zs value low enough to give me safe disconnection times, safe enough to limit touch voltages to a safe value. To lower the Zs value, if necessary, when the system is tested, I am imagining that at that point, I could fit an earth rod in addition to the house earth. Thus lowering measured Zs. Is that good reasoning?

The cable run is 45M so will use 6mm SWA.

2 RCD's- well yes I take your point, but I'm not about to remove an RCD from the shed CU without a better reason.

If I could find anyone I felt confident with to design this system I would, believe me.

 

[MalcofArabia]

Even 10mm^2 only just does it at 42m run; it gives you 2.6%/5.91V VD and you are allowed 3% as is it supplying a lighting circuit - and that 3% has to include wiring in the shed too.

Would you not class it as a distribution circuit feeding a CU and then apply the 3% for lighting from the new CU

 

[davelx]

Even 10mm^2 only just does it at 42m run; it gives you 2.6%/5.91V VD and you are allowed 3% as is it supplying a lighting circuit - and that 3% has to include wiring in the shed too.

Would you not class it as a distribution circuit feeding a CU and then apply the 3% for lighting from the new CU

It's measured from the "origin of supply", generally the DNO supply terminal. Otherwise you could have, say 180V at the end of a string of distribution circuits and still comply - which is not the intention at all.

 

[davelx]

duplicated - please delete

 

[MikeBar]

Even 10mm^2 only just does it at 42m run; it gives you 2.6%/5.91V VD and you are allowed 3% as is it supplying a lighting circuit - and that 3% has to include wiring in the shed too.

Would you not class it as a distribution circuit feeding a CU and then apply the 3% for lighting from the new CU

It's measured from the point of supply, I believe.

This is what I believe is correct and is why in the original post i contemplated not having a lighting circuit, just power and using desk/fllor lamps I suppose.

Thanks everyone once more

 

[ban-all-sheds]

I meant a measured Zs value low enough to give me safe disconnection times, safe enough to limit touch voltages to a safe value.

So did I.

If your Ze is so high that an R1+R2 of a couple of '00 mΩ makes Zs too high then investigating the Ze should be a higher priority than building a workshop.

To lower the Zs value, if necessary, when the system is tested, I am imagining that at that point, I could fit an earth rod in addition to the house earth. Thus lowering measured Zs. Is that good reasoning?

No.

1) It won't be an earth rod, it will be an extraneous-conductive-part, and will require bonding back to the house MET.

2) Work out how much an earth rod in parallel will lower the Zs, given that it'll be 2-3 orders of magnitude greater in resistance than a TN- Ze.

2 RCD's- well yes I take your point, but I'm not about to remove an RCD from the shed CU without a better reason.

That implies you have a good reason for having it.

What is it?

If I could find anyone I felt confident with to design this system I would, believe me.

Keep looking, because you need to find someone.

 

[DetlefSchmitz]

I make VD 9.8A for 6mm²

Actually voltage drop is usually measured in volts.

 

[ban-all-sheds]

 

[MikeBar]

er Ban all Sheds- Zs will vary with the length of cable run, that's what I'm concerned with here. One way to lower a Zs value to meet a disconnection time, as I'm sure you know, is to make a better earth provision. I'm thinking of doing that by putting and earth rod in, if need be.
The imaginary Ze problems you refer to, if they ever arise, demand a different solution as I'm sure you also know.

Your point about extraneous conductive parts isn't valid, for obvious reasons. I sort of wish I hadn't posted now, but hey, thanks everyone again.

I would like to find someone who could design and test/certificate this system for me, I have looked. Anyone in the Midlands is welcome to get in touch.