Shed Electrics Installation

[InterCity125]

Hello Everyone,
Sorry to post a request without answering some questions first but I need some advice for the following installation I am planning.
We are intending to convert a shed into a 'home-office'. I am going to do the work myself and this will involve installing a power supply.
I anticipate the need for:
A few lights (up to say 4)
Sockets (4 doubles)
Heating - possibly storage or other electric type from flex out-let plates
The shed is about 125 meters from the house.
Myquestions are these:
I will have a small consumer unit in the shed for the above circuits BUT, is it okay to take the feed for this from the main fuse-board?
If so is an RCBO or MCB needed - what size?
What thickness of wire to the shed? (??10mm)
Is it esential to use armoured cable, or will T+E in conduit do?
Is an addtional RCD/RCBO for the sockets needed inside the shed?
Is IP56 required - I think not, but..
Many, many thnaks for advice
Aaron

 

[securespark]

To run a circuit of this magnitude 125m requires an extremely large cross section!

Heating. How much? How big is shed? E7 normally used for storage heaters.

To run a 63A submain 125m in XPLE SWA would make it 50mm2.
Even a 40A circuit needs 25mm2, but that puts it right at the top end of the VD accepted.

Never mind 10mm2!!

Is it safe to use portable calor gas heaters in the shed?

 

[plugwash]

id say a 50A cuircuit with 50mm^2 is the way to go here leaving some volt drop spare for the final cuircuits

i strongly reccomend 3 core armoured to give a good earth path especially at theese larger sizes

it would be a good idea to ring round a number of wholesalers to see who has it cheapest but i think about a fiver a meter is the goign rate

terminating that cable is going to be a bit of a pain i think the most practical way is to use an adaptable box with din rail terminals and then use short tails of 10mm from there

 

[InterCity125]

Thanks for the replies.
I thinck I will not use storage heating but use normall wall heaters - the type that can be wired in using flex outlet plates.
Would the 50mm2 still be needed for this, as the current drawn will be lower?
Many thanks.

 

[InterCity125]

Thanks for the replies.
I thinck I will not use storage heating but use normall wall heaters - the type that can be wired in using flex outlet plates.
Would the 50mm2 still be needed for this, as the current drawn will be lower?
Many thanks.

 

[plugwash]

you could possiblly go for a lower rating

how many heaters do you plan to have and at what power each?
and what else do you plan to fit?

 

[InterCity125]

you could possiblly go for a lower rating

how many heaters do you plan to have and at what power each?
and what else do you plan to fit?

I would say 2 heaters at 3kW.
In terms of the sockets I would inamge A PC, Kettle, TV or the such like to be connected. Nothing too heavy - perhaps a micowave or fridge
A Few lights - up to a maximun of 4Amps current

 

[plugwash]

ok first for the long duration loads

right 26A from heating
you say 4A of lighting
a PC for which i would allow at least 3A


so we are talking 33A pretty much solidly when you are wokring at night in winter

I would allow 13A for various short duration stuff (fridge kettle microwave etc)

TVs represent a negligable load unless they are huge so i think we can ignore them

that gives a total of 46A

so i think realistically we are talking a 50A cuircuit here

unfortunately 125m is a bloody long distance and that means volt drop becomes a big issue and to get around it you need

i don't have the exact figures for theese to hand. I think 35mm will actually be ok provided the final cuircuits are fairly short but i would still advise 50mm (you really DO NOT want to ever have to dig that trench again)

i always reccomend 3 core armoured and using a core for earth espeically when using larger sizes as the relative area of the armour goes down in larger sizes of cable

 

[plugwash]

the basic design would be

50A mcb in non rcd side of house CU
10mm tails and single earth OR 10mm twin and earth
adaptable box with appropriate connection method (i think soldering and heatshrink sleeving is probablly the best way for such a huge reduction
main run of 50mm SWA
adaptable box with appropriate connection method
10mm tails and single earth OR 10mm twin and earth
small rcd CU
6A breaker for lights
16A breaker for each heater
32A breaker for sockets

 

[securespark]

35mm2 gives volt drop figure of 8.44V over 125m when carrying 50A.

 

[plugwash]

ok so i was right its just about within spec but leaves very little left for the final cuircuits

so i still think 50mm was correct

 

[InterCity125]

Thnaks guys. Very helpful. I would say that I was way out on the run length, though. I have just measured it at closer to 70m - 75 to be sure. I was well out on that one. Would that mean that 25mm^2 cable is safe or would I still need a larger size?
Digging that trench is still going to be a bu**er, though!!
Just to comfirm - are th trench specs still 300mm deep by 450mm wide? And will the trench need any sort of special covering?
Thank you once again.

PS) the supply fuse will need upping to 100A to cover this new installation. Will the REC do this? If so how much?
I would also like the installation checked - would the REC oblige or will I need a spark?

 

[ban-all-sheds]

Thnaks guys. Very helpful. I would say that I was way out on the run length, though. I have just measured it at closer to 70m - 75 to be sure. I was well out on that one. Would that mean that 25mm^2 cable is safe or would I still need a larger size?

My cable calculator is down right now, but I expect someone will oblige...

Digging that trench is still going to be a bu**er, though!!
Just to comfirm - are th trench specs still 300mm deep by 450mm wide? And will the trench need any sort of special covering?
Thank you once again.

Don't think the width matters. 300mm not really deep enough - sorry. It must be such that it won't be disturbed by normal digging. This sounds like it's down your garden, and if someone ever decides to grow vegetables, and goes in for double digging.... 750mm is regarded as a good depth. Sorry.

You should bed the cable in sand, partly for protection and partly because a change of material acts as a warning, and you should bury warning tape above the cable - far enough above that it does act as an advance warning.

PS) the supply fuse will need upping to 100A to cover this new installation. Will the REC do this? If so how much?
I would also like the installation checked - would the REC oblige or will I need a spark?

You'll have to ask them if they will do it, and what they will charge. They will not inspect the installation.

BTW - have you considered things like insulation, condensation/ventilation for this shed? Ordinary sheds are pretty inhospitable in the winter.

 

[plugwash]

25mm should be fine over that length

when it comes to burying cables

when using a core as earth (which i stronly reccomend) you must still eath the armour preferablly at both ends

450mm is the reccomended depth for private land

but if its running over an area where there is frequent deep digging deeper may be advisable

WARNING ELECTRICAL CABLE BURRIED BELOW marker tape should be placed above the cable deep enough to be out of the way of normal digging but far enough above the cable to give adequate warning between digging up the marker tape

the material that the cable is burried in should be stone free and the general advice is to use sand as it makes it more obvious that something is burried in the event they miss the marker tape

generally the width of the trench will be as narrow as is practical with the digging equipment in use (ie the with of your spade or the width of the bucket on your minidigger)

i think if i had to dig a half meter deep trench for 75M i would wan't a minidigger

 

[ban-all-sheds]

i think if i had to dig a half meter deep trench for 75M i would wan't a minidigger

Or a labourer paid in cash....