Seeking advice on supplying two outbuildings

[Muzza80]

Hi all,
I'm after some advice and clarification on a few points for a proposed supply to two outbuildings in my garden, a shed/workshop I've just built and a small BBQ cabin further down the garden, neither of these have any water/gas supplies into them and are of timber construction.

Firstly, I'm not an electrician (obviously) however I do hope to do as much of the work myself as I can but I'll be paying for the entire install to be checked by a qualified electrician prior to them connecting everything to mains inside the house. As I live in Scotland the work is not notifiable to my understanding.

My supply is TN-C-S so I gather the outbuildings will need to be earthed locally by the means of an earth rod rather than exporting the earth.

I have no additional capacity in my house consumer unit so will need tails to a new small metal 1 way CU in house for the external supply. The run from the house to the shed/workshop is 15m and I'll use 10mm SWA and the shed/workshop to the BBQ cabin run is 25m of 3-core 10mm SWA (this is already buried during recent garden re-landscape)

The shed/workshop will have the following:

5 way insulated consumer unit with 30ma RCD
1 x 6amp MCB for lighting circuit (12 LED downlights)
1 x 32amp MCB for 4mm2 radial socket circuit (8 double sockets for various power tools, desktop computer, small fridge & occasional use of a small electric heater)
1 x 16amp Type-C MCB for a single 16amp plug (for occasional use of a small MIG welder)
1 x 32amp MCB to supply BBQ cabin

The BBQ cabin will have the following:

2 way insulated consumer unit with or without 30ma RCD??

1 x 6 amp MCB for lighting circuit (for 10 LED downlights)
1 x 16 amp MCB for socket ring (for 4 double sockets for low power devices & occasional use of a small electric heater)


The earthing requirements, can I get some clarification?

As not exporting the earth from the house supply it would be a local earth rod (or rods) behind the shed (nice damp clay ground, never really dries out), copper 5/8inch rods with 6mm2 cable running in conduit into the shed CU earth bar?Can this then be taken via the 3rd core (this cable is already in place) of the SWA from the shed to the BBQ cabin or does the cabin also need to have a local earth rod fitted?

Protecting the armour of the SWA, the armour would be earthed at the source end via gland banjo's to the TN-C-S earth in the house is that correct? The house-shed run of SWA and the shed-cabin run would both enter an adaptable box (using glands obviously) on the exterior of the shed, to protect the armour of the shed-cabin run can the two banjo's be bolted together in the adaptable box to give continuity of the armour, therefore the 25m run from shed-cabin would also be protected have I got that right?

Regards RCD's...

I'll be protecting the shed circuits and cabin with a 30ma RCD in the shed CU, I'm not sure if there is any need for an additional RCD in the cabins small CU and also at the supply end? I've read about time delayed 100ma RCD's at the supply end however, although that seemed to relate to the domestic supply being TT rather than the outbuildings themselves.

I'd really appreciate any advice anyone can offer on these points or indeed any aspect of the proposed installation.

Kind Regards

 

[flameport]

My supply is TN-C-S so I gather the outbuildings will need to be earthed locally by the means of an earth rod

Why?
Unless specifically required in a very small number of situations, there is no need to throw away a perfectly good earth connection and use your own.
Installing earth rods and the rest just adds significant cost and complexity.

Insulated consumer units will not comply.*
Lighting is not wired as a ring.
If there is an RCD in the first CU, you do not need or want one in the second CU.
A 16A socket ring is not required, a 16A radial will serve the same purpose with less cable.
Multiple MCBs will not discriminate on short circuit faults.
The mess about earthing requirements and connections to armoured cable is entirely avoided by using the supplied TN-C-S earth.

*cue wrangling by those who 'interpret' things differently and argue over irrelevant semantics and minutiae for the next 100 pages.

 

[EFLImpudence]

*cue wrangling by those who 'interpret' things differently and argue over irrelevant semantics and minutiae for the next 100 pages.

It is futile discussing the matter as it is up to everyone to make up their own minds according to the wording.
No one KNOWS.

 

[Muzza80]

Why?
Unless specifically required in a very small number of situations, there is no need to throw away a perfectly good earth connection and use your own.
Installing earth rods and the rest just adds significant cost and complexity.

Insulated consumer units will not comply.*
Lighting is not wired as a ring.
If there is an RCD in the first CU, you do not need or want one in the second CU.
A 16A socket ring is not required, a 16A radial will serve the same purpose with less cable.
Multiple MCBs will not discriminate on short circuit faults.
The mess about earthing requirements and connections to armoured cable is entirely avoided by using the supplied TN-C-S earth.

*cue wrangling by those who 'interpret' things differently and argue over irrelevant semantics and minutiae for the next 100 pages.

Thanks for the reply.
Sorry, of course the lighting would be radial my mistake.
It was my understanding that PME earth is not supposed to be exported outwith the "equipotential zone, i.e my house? It's often done but not supposed to be done?
I thought the requirement for metal CU's was for a dwelling only and the plastic ones were still within regs in an outbuilding such as a shed/cabin?
16A radial, makes sense.
When you say the multiple MCBs will not discriminate on short circuit faults, can you elaborate? Do you mean that the local MCB for the circuit in the cabin for example might not trip under a fault on that circuit but the 32amp one in the shed CU might instead? And likewise, a fault in the shed might trip the 40amp MCB in the house supplying the shed rather than the MCB for that circuit?

Thanks, I do appreciate your advice.

 

[JohnW2]

It was my understanding that PME earth is not supposed to be exported outwith the "equipotential zone, i.e my house? It's often done but not supposed to be done?

IF you export the earth, the outbuilding will become part of the equipotential zone. If there are any "extraneous-conductive-parts" (e.g. metal water pipes, structural metal etc.) which enter the outbuilding and are, or may be, in contact with the earth, then they have to be adequately 'bonded' to the PME earth in the house (as do such things in the main house), so as to maintain that equipotential zone - which might in some circumstances require that the conductor 'exporting' the PME earth has to be larger than would otherwise be necessary.

One theoretical risk of an exported PME earth which creates much discussion arises if it is possible to touch something (e,g, a metal light switch) inside the outbuilding which is connected to the 'equipotential zone earth' whilst standing on the ground outside. Under certain circumstances, the potential of a PME 'earth' (hence everything within the equipotential zone) can rise to considerably above true earth potential, so someone in simultaneous contact with something within the outbuilding and 'true earth' outside is at risk of an electric shock. Some do not feel that this is a high enough risk to outweigh the advantages (primarily 'convenience') of exporting a PME earth.

I thought the requirement for metal CU's was for a dwelling only and the plastic ones were still within regs in an outbuilding such as a shed/cabin?

The regulation relates to "domestic (household) premises", and it is not necessarily clear what this does and does not include. We have discussed this many times and at least some definitions of "premises" (the regulations do not contain a definition) do include any associated outbuildings. So, who knows!

When you say the multiple MCBs will not discriminate on short circuit faults, can you elaborate? Do you mean that the local MCB for the circuit in the cabin for example might not trip under a fault on that circuit but the 32amp one in the shed CU might instead? And likewise, a fault in the shed might trip the 40amp MCB in the house supplying the shed rather than the MCB for that circuit?

Exactly that.

Kind Regards, John

 

[oldbutnotdead]

It all looks reasonable- whether you actually need the 2nd CU in the barbeque shed is debatable. I'm not familiar with building regs in Scotland so couldn't comment on the need or not for a warrant, what would be wise is to get your electrician on board first (they have to certify that the system design is complaint as well as test and commission) and agree with them (a) the particular switchgear etc you need (b) cable size for the leg not yet installed (c) how deep to dig the trench (d)whether to duct the SWA or not (e) what they're happy for you to do and how much you can do before calling them back in.

While you're digging your trench from house to shed, think about throwing some CAT5 in as well (4 runs in a separate duct)- very handy for all sorts of things- alarm wiring, CCTV- as well as not relying on Wifi for streaming training videos in your mancave

 

[Muzza80]

Thanks for the replies, most helpful.

Assuming I did want to go down the local TT route for the outbuildings, can anyone clarify the below points for me? I'm not 100% I've got it right in my own head.

Thanks!

As not exporting the earth from the house supply it would be a local earth rod (or rods) behind the shed (nice damp clay ground, never really dries out), copper 5/8inch rods with 6mm2 cable running in conduit into the shed CU earth bar?Can this then be taken via the 3rd core (this cable is already in place) of the SWA from the shed to the BBQ cabin or does the cabin also need to have a local earth rod fitted?

Protecting the armour of the SWA, the armour would be earthed at the source end via gland banjo to the TN-C-S earth in the house is that correct? The house-shed run of SWA and the shed-cabin run would both enter an adaptable box (using glands obviously) on the exterior of the shed, to protect the armour of the shed-cabin run can the two banjo's be bolted together in the adaptable box to give continuity of the armour, therefore the 25m run from shed-cabin would also be protected have I got that right?

Kind Regards

 

[JohnW2]

Assuming I did want to go down the local TT route for the outbuildings, can anyone clarify the below points for me? I'm not 100% I've got it right in my own head.

I'll have a go ....

As not exporting the earth from the house supply it would be a local earth rod (or rods) behind the shed (nice damp clay ground, never really dries out), copper 5/8inch rods with 6mm2 cable running in conduit into the shed CU earth bar?

To be honest, even smaller cable would probably be acceptable, but 6mm² would be fine. There's no real need for the conduit unless you feel it is needed "for mechanical protection.

Can this then be taken via the 3rd core (this cable is already in place) of the SWA from the shed to the BBQ cabin or does the cabin also need to have a local earth rod fitted?

Interesting question. I see no fundamental reason why you couldn't use the same earth rod for both, particularly given that you're talking about 10mm² SWA. However, given the fairly large distance (25m) between the two outbuildings, I would personally probably be more inclined to have separate earth rods (that it, if I wasn't exporting the PME earth).

Protecting the armour of the SWA, the armour would be earthed at the source end via gland banjo's to the TN-C-S earth in the house is that correct?

Yes.

The house-shed run of SWA and the shed-cabin run would both enter an adaptable box (using glands obviously) on the exterior of the shed, to protect the armour of the shed-cabin run can the two banjo's be bolted together in the adaptable box to give continuity of the armour, therefore the 25m run from shed-cabin would also be protected have I got that right?

I suppose you could do that. However, particularly if I were using the same rod for both outbuildings, I would be inclined to have the armour of the SWA between the two outbuildings connected to that rod (and NOT connected to the PME earth, hence not connected to the armour of the house-shed SWA) - an added advantage of which being that the armour cold be used as an additional connection (as well as the 3rd core) of the earth connection between outbuildings. To achieve that would be easiest if you had a plastic adaptable box. The most crucial things, if you are not using the exported PME earth, is to make sure that the armour of the house-shed SWA is NOT connected to anything touchable within the two outhouses.

Kind Regards, John

 

[ban-all-sheds]

It is futile discussing the matter as it is up to everyone to make up their own minds according to the wording.
No one KNOWS.

Please don't take this as the start of 100 pages of wrangling, as that would be, as you say, futile.

But people do know - they just refuse to accept it because they don't believe in thinking.

 

[ban-all-sheds]

It was my understanding that PME earth is not supposed to be exported outwith the "equipotential zone, i.e my house? It's often done but not supposed to be done?
I thought the requirement for metal CU's was for a dwelling only and the plastic ones were still within regs in an outbuilding such as a shed/cabin?
16A radial, makes sense.
When you say the multiple MCBs will not discriminate on short circuit faults, can you elaborate? Do you mean that the local MCB for the circuit in the cabin for example might not trip under a fault on that circuit but the 32amp one in the shed CU might instead? And likewise, a fault in the shed might trip the 40amp MCB in the house supplying the shed rather than the MCB for that circuit?

You need an electrician.

Forget notifiable or not, forget what the Scottish Building Regulations are - you are not going to be able to properly design this on the basis of what you think you know plus questions about what you think you don't know.

Have an electrician take charge, and agree with him how much of the basic donkey work you can do under his supervision and guidance. Or put your outbuilding electrification plans on hold for some months while you skill up.

 

[JohnW2]

Please don't take this as the start of 100 pages of wrangling ...

The best way of ensuring that would be to not start Page 1. It's now up to others to decide whether to take the bait you apparently couldn't resist dangling at them!

Kind Regards, John

 

[ban-all-sheds]

The best way of ensuring that would be for you not start arguing and criticising. It's now up to others to decide whether to continue the wrangling you apparently couldn't resist.

 

[securespark]

Or put your outbuilding electrification plans on hold for some months while you skin up.

 

[Muzza80]

I'll have a go ....
To be honest, even smaller cable would probably be acceptable, but 6mm² would be fine. There's no real need for the conduit unless you feel it is needed "for mechanical protection.
Interesting question. I see no fundamental reason why you couldn't use the same earth rod for both, particularly given that you're talking about 10mm² SWA. However, given the fairly large distance (25m) between the two outbuildings, I would personally probably be more inclined to have separate earth rods (that it, if I wasn't exporting the PME earth).
Yes.
I suppose you could do that. However, particularly if I were using the same rod for both outbuildings, I would be inclined to have the armour of the SWA between the two outbuildings connected to that rod (and NOT connected to the PME earth, hence not connected to the armour of the house-shed SWA) - an added advantage of which being that the armour cold be used as an additional connection (as well as the 3rd core) of the earth connection between outbuildings. To achieve that would be easiest if you had a plastic adaptable box. The most crucial things, if you are not using the exported PME earth, is to make sure that the armour of the house-shed SWA is NOT connected to anything touchable within the two outhouses.

Kind Regards, John

Hugely helpful John, thanks very much for taking the time.

I'll take this all on board and have a chat with an electrician and see what, if any, they're happy for me to do myself.

Thanks again!

 

[JohnW2]

Hugely helpful John, thanks very much for taking the time.

You're welcome.

I'll take this all on board and have a chat with an electrician and see what, if any, they're happy for me to do myself.

As others have said, I'm sure that is the way to go. I hope that it all works out well for you.

Kind Regards, John