Feeding a Shed Cu from the main CU

[robbo777]

Hi I am putting a Small 2 way CU in a shed to feed 3 x double sockets and 2 x florescent lights. The run from the Main CU is 30m

I am wiring it in conduit and I will be using 4mm singles to supply it with a 20A Breaker at the main, is this ok?

Many Thanks

 

[robbo777]

I take it that you don't have a load of steelwork in contact with the ground (it being a former 20ft lorry back) ?

the whole thing is made from aluminium and is sat directly on the ground

 

[Guitarist]

So you are taking a supply from an existing outbuilding to a massive aluminium structure in direct contact with earth? If so, is your tac room fed from a tncs supply from the house?

 

[GLENNSPARK]

So you are taking a supply from an existing outbuilding to a massive aluminium structure in direct contact with earth? If so, is your tac room fed from a tncs supply from the house?

at last.....someones asked what the earthing arrangements are here.....this will determine wether an s type RCD is required for discrimination purposes.....

 

[Guitarist]

And more importantly, is he going to be standing in a massive extraneous conductive part?

 

[GLENNSPARK]

And more importantly, is he going to be standing in a massive extraneous conductive part?

i can see this developing into an export..lol......

 

[GLENNSPARK]

no...i`d better not.....will go major will this..lol....

 

[Guitarist]

I think people may be keeping out of this one Glenn.....

 

[Deleted member 26818]

It appears that this installation may well be classed as Agricultural/Horticultural.
As such, I would suggest a perusal of Section 705.
The circuit supplying the shed will require a minimum of 300mA RCD protection.
The socket-outlet circuit will require 30mA RCD protection.
The shed will most likely require bonding, as it's an extraneos-conductive-part.
If the earthing system is TT, then the disconnection times will be 1s for distribution circuits and 0.2s for final.

 

[Guitarist]

Do you still keep horses (as you have a tac room)? Where is the tac room fed from, and as what type of supply?

 

[robbo777]

I think people may be keeping out of this one Glenn.....

Why's that then?

 

[Guitarist]

Why's that then?

You started by saying that you were sending a supply out from a main CU to a shed, which most people can easily relate to. Now it appears that the main CU is already in an outbuilding itself and your "shed" is a massive aluminium conductor in direct contact with earth. You haven't told us anything about your main earthing arrangements or whether there are still horses in the vicinity.
I merely meant that the situation seems more complex than the OP suggested and that most people would keep out of this thread without seeing your exact setup. No disrespect meant at all.

 

[robbo777]

This seems to be getting a little confusing so I'll start again with the exact set up.


Firstly there is no house.
The tac room has its own supply from the main supplier which is a TNCS system
there is 1 socket in the tac room and 1 light in each of the stables so 9 lights in total, the light switch for each stable is on the outside of each stable out of reach of any horse and in an IP66 enclosure.
the lights are fed by 1.5mm singles in 20mm PVC conduit over 2m high and inside
There is a 30mA RCD in the CU protecting all circuits

The shed
I want to put 3 x double sockets in here with 2 x flourescent lights.

I will run 4mm singles in conduit to an external junction box where I will come from there in 4mm SWA to the 2 way CU in the shed
I will feed this by a 32A breaker in the main CU and use the main RCD to protect this circuit
I will of course bond the shed.

how does this look?
I appriciate the help with this

 

[jase]

will it be a 3 core SWA? make sure the SWA is bonded, and i think you may have to run a 10mm earth back to the MET to bond the shed/lorry

 

[Deleted member 26818]

To start with, use of a TN-C-S supply appears to contavene both BS7671 and ESQCR.
Is the main supplier aware that the installation is using their PME connection?
If you intend bonding the shed, as the supply is TN-C-S, a 10mm² conductor will be required, or the shed will have to made TT.

 

[Guitarist]

Thank you for clearing that up robbo. How far is the lorry/shed from the main CU? Just wondering if 4mm will be adequate as you are feeding it from a 32A supply. Have you done the VD calcs?
Also, regarding the bonding, what spinlondon said.