External Supply

[dans77]

Good morning everyone.

I am looking to install a consumer unit into my shed and then run another supply up to my summer house.
My shed is approximately 5m from the house and the summer housde a further 15m.

From my CU in my house I have a 6mm running from a 32A MCB to an outside box from where I have a 2.5mm SWA running to the shed. Ideally I would like to terminate the SWA into a CU and have 3 sockets (for lawn mower, tools etc) and a lighting circuit in the shed, as well as a couple of spare ways for future (pump, filter etc) and also have another 2.5mm SWA running from here to the summer house to another CU which in turn will have 4 sockets and a lighting circuit from it. The CU in the summer house will I think only need a 6A and 16A MCB's

Question is the RCBO inside the CU in the shed would it have to be 32A or less as am I correct in saying if its higher than 32A it wouldnt do anything as the house CU is 32A?

 

[PrenticeBoyofDerry]

Ideally you need to work backwards in your design for the circuits.
The load required by summerhouse
The load required by shed
add them up this will give you your in house CU size breaker, then voltage drop should be considered and then you will come to the required cable size.
EG
Summerhouse: sockets 16A, lights 6A =22A
Shed: sockets 20A, lights 6A and summerhouse 22A = 48A
So 32A MCB at CU within house is not correctly sized.
I also doubt that 2.5mm SWA is sufficient either.
cable calcs
Are you aware of the legal requirements of what you propose to do?
If not take a look here...
//www.diynot.com/wiki/electrics:part_p

 

[dans77]

Thank you for that.

I am aware of the regs, I will be getting an electrician in to install, I am just 'knowledge' building and understanding what my options are and so I have an idea what Im talking about when hiring the electrician.

If the MCB in the main CU was changed/upgraded then would the CU inside the shed have to be equal to or less than?

Also, the 48A would be the maximum load. If I were to say not use the sockets in the summer house whilst using them in the shed would this make a difference or is it purely down to the potential of them both?

Thank you for your help.

 

[ricicle]

Ask your electrician as he is the one who has to take responsibility for the design, installation and testing of your proposed works !

 

[riveralt]

Thank you for that.
I am aware of the regs, I will be getting an electrician in to install, I am just 'knowledge' building and understanding what my options are and so I have an idea what Im talking about when hiring the electrician.
If the MCB in the main CU was changed/upgraded then would the CU inside the shed have to be equal to or less than?
Also, the 48A would be the maximum load. If I were to say not use the sockets in the summer house whilst using them in the shed would this make a difference or is it purely down to the potential of them both?
Thank you for your help.

What about diversity?
What about voltage drop?
What about cable size calculations?
What about the the type of earthing system you currently have in your house and what type you need to have in your shed/summerhouse.
General knowledge is useful up to a point - but what you're asking goes beyond that and into circuit design. Which from your initial questions and subsequent answers you should leave to the electrician you intend to hire.
Given the limited information you have given I would probably suggest you have a separate consumer unit next to your current household CU and forget about one in the shed.
Get three quotations and discuss your options with the electrician.

 

[dans77]

Thank you all for your help, in the realms of a little over me so off to get hold of an electrician and let him/her come up with the plan.
Thank you.

 

[Chri5]

Read up on diversity, total load and real load are calculated?

The swa is underrated for the existing 32amp mcb, since 2.5 core tends to max out at 26 amp as a radial.

Take the light circuits, if low es type- over both sheds how many 20-25w es fittings would you use and have on at the same time?

Hint 6amp fuse x 230v give you a load max of about 37 lights

There's nothing wrong with having a socket circuit on a 10amp breaker if say you are ok with the limitations of the circuit being 10 x 230v or 2300w.

Then again if you have a 3kw / 3000w mower, you would be poleaxed.


Have a think about true loads, add up the sort of equipment you would use and present that to the spark.

Obvious observation would be to direct wire the 6mm on the incoming supply which avoids rcd on an rcd. Update the swa to 6mm and run to a builders / garage cu at shed 1.

I'd then have mcbs for shed 1 lights 6 amp, sockets 20 amp on a 2.5 radial.

Then a 20 amp mcb feeding to the distant shed on 2.5 swa.

 

[PrenticeBoyofDerry]

If the MCB in the main CU was changed/upgraded then would the CU inside the shed have to be equal to or less than?

You need to use discrimination when installing protective devices in series.
So as you move down the supply, you need to reduce the the rating of the device, so they will break first under fault conditions.

Also, the 48A would be the maximum load.

In my example it would, you could offer diversity to circuits but as you have independent distribution at each locations, diversity of 66% of lights is all your going to get.

If I were to say not use the sockets in the summer house whilst using them in the shed would this make a difference or is it purely down to the potential of them both?

In your case it maybe fine and an overload may never take place, but can we assume that for the rest of the in service life of the installation?

 

[dans77]

Thank you Chri5, PBoD,
Attached - I hope is a quick drawing - as I find it easier to understand - of what I think you are saying and what I am looking to achieve.

Does that make sense?

Untitled

• dans77
• 20 Mar 2012
• 1

 

[PrenticeBoyofDerry]

You have a potential load of 56A at the shed and are only offering 32A, you will have overload problems at maximum demand.
Even if you remove the 10A breakers, you still have overload problems and the lights don't need to be on a RFC.

 

[ricicle]

You have a potential load of 56A at the shed......

Don't be so silly !

 

[PrenticeBoyofDerry]

Don't be so silly !

6A Lights
10A Pump filter
20A socket
20A distribuiton
why silly?

 

[Chri5]

Shed 1
Lights would be radial
Sockets would be radial

Shed 2
Feed via 20amp mcb from shed 1 and swa, terminate on metal box with a simple 20amp switch, then loop 2.5 mm through a 3amp fused spur for lights and on to sockets.

Why have a full sub board in shed 2 unless you plan for more than simple light and a few sockets?

 

[ricicle]

Don't be so silly !

6A Lights
10A Pump filter
20A socket
20A distribuiton
why silly?

All my MCBs in my CU at home add up to 172 amps but I only have a 100A main fuse.......

 

[PrenticeBoyofDerry]

What type of main fuse do you have?
Are you saying that this is a good design?