Supply to a shed/studio....

Discussion in 'Electricians' Talk' started by Andy Sparks, Jan 12, 2020.

  1. Andy Sparks

    Andy Sparks New Member

    Hi Screwfix community,
    I'm a recently qualified electrician and doing a job at the moment to supply a studio in the clients garden with power.
    He needs 13.2kw of power and already has a small CU installed in the studio. Will need to look at this and reconfigure, test all existing circuits installed, and introduce an RCD.

    His house CU is 100% populated, he needs more than a MCB could give him anyway.

    Plan was to split his existing CU tails into 2 sets using 2 pole Henley block for 25mm2 CU tails. 1 set would be for his house CU (inserting an isolator if I can squeeze it in), the other going to another 100A isolator supplying 16mm2 SWA. Debating whether to use 2 or 3 core SWA. Use the 3rd core for a belt and braces additional layer of CPC protection.
    The run of SWA is approx. 15m, 5 of which is from the back of the house to the end wall of the studio.
    There is a concrete foundation to a fence which I could easily clip the SWA to for this run but it is at foot level, albeit tucked away. Would you recommend digging a trench for this part of the run and mark it etc, for regs, or be OK to go for the easier route? (paving slabs and patio would have to come up!)
    For the rest of the run it could be clipped to the back of the studio which is not accessible before entering studio through a junction box.
    Need to check what the cable connection from junction box to the studio CU. If not sufficient, will increase in cable diameter.

    Anybody have any thoughts, or suggestions?
     
  2. nigel willson

    nigel willson Screwfix Select

    If your recently qualified, do you not know about earthing arrangements on outbuildings . What is the earthing arrangement in the main house,TNC-S, TNS, TT, ? Loading 13KW, what they running ect
     
  3. Andy Sparks

    Andy Sparks New Member

    The main house earthing arrangement is TNS. The loading of the 13kW is between a water heater, 4kW, a couple of plug in driers, lighting circuit, ring main, but I have not seen the CU yet, but aware a few dedicated circuits will have to be used in the studio.
    No extraneous -conductive parts in the studio.
     
  4. Bob Rathbone

    Bob Rathbone Screwfix Select

    Andy, do not use the house earth in the shed, use it for the SWA but drop it at the CCU in the shed. Install an electrode and 30mA rcd to protect it all. Correct disconnection time can now be assured. Assuming you have a 100A service fuse, you must use a switch fuse or 60A CB rather than an isolator at the circuit origin, you are making a cable size reduction from 25mm to 16mm. As for my comment on the earth, others will disagree, but you have to install a 30mA rcd whichever way you choose, an electrode is cheap and will give you 100% compliance, no grey areas. When does the client intend to install the hot tub? it's a large supply for a shed, beware of future expansion plans, use the earth electrode route and be future proof.
     
    Sparkielev and nigel willson like this.
  5. Tony Goddard

    Tony Goddard Screwfix Select

    Plan was to split his existing CU tails into 2 sets using 2 pole Henley block for 25mm2 CU tails. 1 set would be for his house CU (inserting an isolator if I can squeeze it in),

    I would run the consumer side meter tails into a special isolating switch (widely available, the type the DNO use) then into a DP henley block and out to the existing CU and the supply for the shed. The DP block will take up less space. The isolator being before the henley means it can be turned off if needed without pulling the cut out.

    the other going to another 100A isolator supplying 16mm2 SWA.

    You need some up front protection here, not just an isolator, a 60A switchfuse unit or a small "garage CU " with isolator and 60A MCB

    Debating whether to use 2 or 3 core SWA. Use the 3rd core for a belt and braces additional layer of CPC protection.

    As Bob says, don't export the earth, use an electrode at the shed making it a local TT installation. Incidently, if you did export the earth you would have to use the third core as earth as you cannot use the armour on a 16mm SWA as a CPC (you can only do that on the smaller sizes)

    The run of SWA is approx. 15m, 5 of which is from the back of the house to the end wall of the studio.
    There is a concrete foundation to a fence which I could easily clip the SWA to for this run but it is at foot level, albeit tucked away. Would you recommend digging a trench for this part of the run and mark it etc, for regs, or be OK to go for the easier route? (paving slabs and patio would have to come up!)

    Clip it, avoids a lot of work, no issue with this

    For the rest of the run it could be clipped to the back of the studio which is not accessible before entering studio through a junction box.
    Need to check what the cable connection from junction box to the studio CU. If not sufficient, will increase in cable diameter.

    Anybody have any thoughts, or suggestions?

    Incidently, he needs a lot of power, it's a studio, not putting a kiln in there is he??, what is the studio made from??. I wouldn't put a kiln in a wooden building without some serious forethought!!
     
  6. Bob Rathbone

    Bob Rathbone Screwfix Select

    That is why I asked about the hot tub.
     
  7. Andy Sparks

    Andy Sparks New Member

    Thanks guys for all this info and your time. It is really appreciated. I've not much room for Isolator, DP Henley and Fused switch but will play around tomorrow with the space. Also looking at the exit from the back of the house for my SWA. Quite a few bends to get to the perimeter of the garden......16mm2 SWA bend radii is quite big! Going to have to break the connection here somewhere as no way I can get around the brickwork.
     

    Attached Files:

  8. Tony Goddard

    Tony Goddard Screwfix Select

    You can't put anything on that board at all, it's for DNO equipment only. You might get away with an isolator and maybe the henley on there but not the switchfuse.
     
  9. Andy Sparks

    Andy Sparks New Member

    No hot tub, or kiln! He's using the studio for a dog grooming business, and needs a water heater, and potentially a couple of driers at 2.8kw a piece as well as the sockets and lights. He may want to convert into a studio flat at some point as well, hence the future proofing. Now looking at fire risks and provision.
     
  10. Tony Goddard

    Tony Goddard Screwfix Select

    Doesn't sound like there is much of a fire risk. You could site another meter cabinet right next to the existing one to house the switchfuse and any other gear for the studio, thus keeping it out of the house.
     
  11. Andy Sparks

    Andy Sparks New Member

    Hey Tony, Thanks for that. Unfortunately there is no space around the existing cabinet. House consumer unit above existing DNO cabinet.
    May have to remove coat hooks, or put below DNO cabinet.
     

    Attached Files:

  12. Tony Goddard

    Tony Goddard Screwfix Select

    Ah I see, DNO cabinet indoors, thats novel!! I reckon you might get something in above the coat hooks, some units can go sideways on, looks a bit odd but functional.
     
  13. Bob Rathbone

    Bob Rathbone Screwfix Select

    Hi Andy, you have just highlighted the issues here with the future plans. I know we have no control over what the customer does with our installation once it is finished, but we should try to look into the future. I see a hot tub at the bottom of the garden within 3 years, might only be a 'Lazy Spa' but a hot tub non the less.
     
  14. Andy Sparks

    Andy Sparks New Member

    Hey Guys!
    Just wondered what your view was on placing Garage type CU/isolator at approx 50cm from the floor in a cupboard under the stairs in a 80s build? Aware CU in a new build should be between 1350mm-1450mm from the floor, but for what I am trying to do with meter tails, SWA and DNO cupboards this feels the best option.
     

    Attached Files:

  15. Sparkielev

    Sparkielev Screwfix Select

    Are you using the garage unit for your switch fuse ? Lewden do a switch fuse unit that comes with 3 bs 88 fuses 40a, 60a 80a, can you drill the brick wall instead of bending the swa, and feed it through
     
  16. Andy Sparks

    Andy Sparks New Member

    Yes ... using garage unit for switch fuse. I have that exact one which is in the picture. I can drill into the wall but was not going to chase the swa into the wall if I did not have to. To get my feed into the garage unit the easiest route is from the bottom of the DNO cabinet, but does mean my switch unit is only 50cm from the floor. To raise the unit up the wall it either gets in the way of the customers coat rack and means my feed from the DNO unit is really complicated and there is not an obvious clear route.
     

Share This Page