Supply To Shed - New Build, Best Way?

[elephant]

Hi, I`m hoping someone can advise me the best way to provide a power supply to a (large) shed at my self build project.

I should state that it is in Scotland (i.e. no part P) and Building Control have granted warrant for the build, and will inspect the electrics eventually, however BCO doesnt seem able/competent to advise.

The layout is that the incoming supply and consumer unit is at the front of the house, shed is at the back of the back garden, and the best route for the cable is through the house (while it is under construction). Earthing is TN-C-S and load at shed was based on 6kW (although this is deliberately oversized to allow for future expansion). I'm proposing this solution:

Meter
-to-Incoming Isolator Switch (live), Henley Blocks for (N) and (E)
-to-Consumer Unit
-to-32A MCB on RCD protected way
-to-6mm Twin & earth,length 15m
-to-junction box at back perimeter of house (indoors)
-to-6mm 3 core SWA cable (outdoors, buried), length 15m
-to-garage consumer unit in shed
-to-MCBs for sockets & lights in shed

My questions are:
1. Should the shed consumer unit have an RCD or is such "doubling up" bad practice?
2. Would it be better to connect the shed cabling to henley blocks rather than the house Consumer unit? (I have seen this suggested elsewhere, but wonder if the cable runnning through the house would need metal conduit to satisfy 17th edition as it would not then be RCD protected.)
3. Should SWA be used for the whole run? (simpler, but this would seem to be more cost and more difficult to terminate at the (plastic) house Consumer Unit).
4. Do I need an earth rod at the shed?
5. Have i missed anything?

I`d appreciate it if any expert on here can cast their eyes over this and offer their opinion. Thanks.

 

[Paul.M]

Whilst under construction run none rcd protected swa from the house c/u to a metal isolation switch in the shed, this way you getlocal isolation and a good swa connection. Feed from switch to an rcd protected c/u. Even if Scotland is not under part p I would still protect a shed with rcd's due to the ability to plug in outdoor appliances.

 

[somersetsparks]

Personally I would run armoured all the way back to the DB and then not have RCD protection for the submain, alternativley just make sure the 6mm T+E is 50mm deep in any walls it runs through in the house, thus avoiding an rcd.

I would then put small metalclad CU in shed with RCD incomer.

 

[IQ Electrical]

Personally I would run armoured all the way back to the DB and then not have RCD protection for the submain, alternativley just make sure the 6mm T+E is 50mm deep in any walls it runs through in the house, thus avoiding an rcd.

I would then put small metalclad CU in shed with RCD incomer.

Would the 2.5mm cpc in the T+E satisfy the regulations as a combined cpc/main protective bonding conductor on a PME system? (assuming the presence of extraneous-conductive parts)

 

[somersetsparks]

No I don't think it would, I was assuming no exteranneous conductive parts!

If that is the case I would take swa all the way back to the DB.

 

[johnboy6083]

if i were you, i would install a CU that allows for RCD unprotected ways. From there i would run my SWA directly to a new CU in the garage/outbuilding which will have a 30mA RCD. as its a new build outbuilding, if you do have a water supply, i would guess that it will be in plastic, and as such there will be no need for main bonding of services inthe outbuilding.
For the SWa being bvuried, i would bury it about 800mm deep, and use builders sand to backfill over the cable, so that there are no sharp stones that will damage the cable. Lay warning tape about 300 below the surface (lower if you are planning to dig there).

The ideais that when you dig down, you hit the tape and realise there is a cable below, you can then dig down gently and uncover cable if needed

 

[Engineer54]

You have not indicated the length of cable run from the house CU?? Can you confirm or otherwise the provision of water tap/water pipes at the shed?? If there is, or other extraneous conductive parts, then i believe you will require a bonding cable of 10mm to satisfy PME (TNC-S) requirements. This could be included in a 10mm 3 core SWA cable using one of the cores as your bonding cable.

Run the SWA from CU in a duct within the floor screed if not already poured, as this will be a minimum of 4'' in depth and often more. How are you intending to run the cable from the house to the shed, direct burial, Buried duct, clipped to wall, ...how ??


EDIT ... just noticed .... total 20 meter cable run ...lol!!!

 

[elephant]

Thanks for all your replies so far. Consensus seems to be to run SWA for the whole run, and to just have RCD protection at the shed CU, so I`m just not sure how best to connect the SWA to the house CU (already in place providing builders supply) as it is plastic and won`t support an SWA gland.

Can the SWA be terminated back into the henley blocks/main supply isolator switch?

I should have explained that the section that runs through the house will go in a ceiling void (concrete beam and block floor above the cable), and I can find no mention in the On Site Guide if that requires RCD protection or not. What's worrying me a bit is that 411.3.3 states an RCD is required "for circuits supplying mobile equipment not exceeding 32A for use outdoors" - and I`m not sure if that applies to the sub-main to the shed or only to the final socket circuit in the shed that the mower gets plugged into?

Engineer54, There are no extraneous conductive metal parts in the shed requiring bonding, total cable run is 30m, (15m indoors, 15m outdoors), and direct burial of the SWA is the plan.

Thanks again for all your help.

 

[baldelectrician]

Is this shed for a hairdressing salon perchance?

just starting out in the back garden type thing?

 

[IQ Electrical]

411.3.3 is referring to socket outlets and mobile equipment.

 

[baldelectrician]

I would over spec the supply now.

Run a 6mm or 10mm 3 core armour.
Think about data / phone (if you are using it for business)

 

[Deleted member 26818]

411.3.3 does not mention circuits.
The only reference to circuits requiring RCD protection in BS7671 is for those of special locations.

 

[elephant]

Is this shed for a hairdressing salon perchance?

just starting out in the back garden type thing?

My haircut might look like I had a fight with a lawnmower, but no, no hairdresssing planned - would love to know where that thought came from though?

 

[flasho1959]

Can anyone help with this question.... Have an ouside shed which is supplied by a SWA cable from the house supply. The supply is not protected with an RCD and the CU is an old style rewirable fuse type. The SWA to the shed is terminated into a new CU protected by a 30mA RCD and has a 6A & 16A MCB. If i run two new circuits for lighting and a 13A Socket, will the whole installation back to the house CU need to be tested, Inspected and certified under Part p or just the new circuits in the shed back to there respective CU. Cheers if some one can help.

​

 

[baldelectrician]

My haircut might look like I had a fight with a lawnmower, but no, no hairdresssing planned - would love to know where that thought came from though?

Someone was on MSE forum asking similar questions about setting up a salon at the bottom of their garden.

 

[Guitarist]

Are you an electrician, or the homeowner trying to establish how much you will have to pay for?

 

[Des 56]

WARNING we are under attack,the shed question strikes again