Extension to outsde shed.

[Haydn R]

I wish to extend an existing household ring main by a spur to a shed two metres away from the house under a flagged yard at regulation depth.

I envisage an adaptable box inside the property to join a 2.5mm pvc cable from an existing ring main socket to a 6mm SWA cable taken through the wall then down under the yard at regulation depth to another adaptable box, inside the shed, to join another 2.5mm cable. Ongoing to a double pole switch then to a double socket outlet with its own RCD. Then on via a 2.5 mm cable to a fused connection to supply a lighting circuit.

The 6mm SWA cable would be protected down the house wall and up the shed side. The existing ring main is RCD protected. Is this proposal acceptable?

 

[Jon c]

It is not clear whether or not you are planning to put a 13a FCU next to the socket you will be taking a spur from, and then feeding the shed from that FCU? It is important to do so so that you don't overload that point on the ring. It will of course mean you are limited to a total of 13a in the shed but if you wish to do it that way there's no choice.

 

[winston1]

Pretty pointless using 6mm SWA when the rest is 2.5mm and limited to 13 amps anyway.
What is the reason for the double pole switch?
You don’t need a RCD socket as you say the existing ring you are spurring off is already RCD protected.

 

[sparkwright]

I suppose having the underground part of the cable in 6mm2 does at least making the wiring easier to upgrade to a 32 or 40amp supply in the future without digging up the ground again.

 

[bernardgreen]

Pretty pointless using 6mm SWA when the rest is 2.5mm

Voltage drop may be a factor.

 

[Taylortwocities]

Voltage drop may be a factor.

Volt drop, really?

to a shed two metres away from the house

 

[bernardgreen]

Volt drop, really?

Not in this case but in other situations that the forum pedant enters the volt drop drop may need to be considered

 

[Taylortwocities]

@Haydn R as above, the ring final circuit is RCD protected. All you need at the house end is a 13A FCU to provide isolation and current limit.

2.5mm² SWA will be adequate for a 2 metre distance.

 

[JohnW2]

Not in this case but in other situations that the forum pedant enters the volt drop drop may need to be considered

The forum pedant is obviously capable of anything, but I have to say that I have personally never knowingly experienced any problems due to voltage drop (even when, although it shouldn't have been, it was very high ).

In some senses I'm a bit spoiled, since my supply voltage very rarely falls below 245V, which means that I can have a VD of nearly 30V (i.e. more than double the 'maximum guidance') before supplied equipment gets less that what, if I were less lucky, might be my installation's supply voltage!

It's also worth remembering that there really aren't any meaningful regulatory requirements in relation to voltage drop, in as much as I struggle to think of any situations in which low voltage could impair the "safe functioning" of anything

Kind Regards, John

 

[winston1]

The forum pedant is obviously capable of anything,

Who is this forum pedant?

It's also worth remembering that there really aren't any meaningful regulatory requirements in relation to voltage drop, in as much as I struggle to think of any situations in which low voltage could impair the "safe functioning" of anything

Motors can stall on a low voltage. Think of a freezer with a stalled motor!

 

[Taylortwocities]

Who is this forum pedant?

But, soft! what light through yonder window breaks? It is the east, and Winston is the sun.

 

[winston1]

?

 

[JohnW2]

Motors can stall on a low voltage. Think of a freezer with a stalled motor!

That is technically possible, but it doesn't alter what I said about the lack of 'meaningful regulations' relating to voltage drop. BS7671 says nothing about avoiding VDs which prevent 'satisfactory functioning' (or even functioning at all!) of equipment, merely that the voltage should not drop to the extent of impairing their "safe functioning".

 

[Taylortwocities]

?

Just watching All Is True on Netflix, and I came over all Shakespeare.

 

[bernardgreen]

Appliances that have a fixed power rating and adjust the current they take to maintain that fixed load could take excessive current (*) if the supply voltage dropped significantly. Increasing the supply current will increase the voltage drop in the cable supplying the appliance.

(*) overloading the cable and/or the MCB