6mm to 2.5mm for sockets

[Dustydazzler]

I would probably opt for a 20a rcbo at the house and just mcbs in the shed , just the way I would do it

 

[Roberts7286]

I that case, it doesn't need RCD protection.

The RCD was to protect the sockets/lights in the summerhouse

 

[brianmoooore]

The thing that will really push up the costs is when additional equipment at the summer house means the cable and fusing you install now isn't adequate, and the whole lot has to be upgraded.

 

[brianmoooore]

The RCD was to protect the sockets/lights in the summerhouse

That is required, of course, and it's better that the RCD is at the summer house, not 60m away.

 

[Roberts7286]

The thing that will really push up the costs is when additional equipment at the summer house means the cable and fusing you install now isn't adequate, and the whole lot has to be upgraded.

I discussed "future proofing" with the customer but she's adamant she only need 2 double sockets and a light

 

[brianmoooore]

I discussed "future proofing" with the customer but she's adamant she only need 2 double sockets and a light

Heard that many times, only to find, a couple of years later...........
In spite of the old saying, the customer is rarely right.

 

[pc1966]

If you are looking at absolute minimum cost then feed the SWA from a RCD protected supply so you can use 2-core cable, then minimum to meet 5% VD and 13A use is 4mm.

If you go for a non-RCD supply your end of cable Zs becomes a factor, though for small SWA the 2C armour R1 is less than the copper R1, but not once above 2.5mm. Here is a crude calculation and assuming TN-S 0.8 ohm Ze and not computing parallel armour + copper R2 in the 3C case:

mm^2 \ length (m)​	60​	V/A drop​	A for 5%​	A for 3%​	R1+R2 2-core​	R1+R2 3-core​	Zs for TN-S 2C​	Zs for TN-S 3C​
1.5​	31​	1.86​	6.2​	3.7​	22.3​	24.2​	2.1​	2.3​
2.5​	19​	1.14​	10.1​	6.1​	16.2​	14.8​	1.8​	1.7​
4​	12​	0.72​	16.0​	9.6​	12.5​	9.2​	1.6​	1.4​
6​	7.9​	0.474​	24.3​	14.6​	10.1​	6.2​	1.4​	1.2​
10​	4.7​	0.282​	40.8​	24.5​	7.8​	3.7​	1.3​	1.0​

So if going for a 13A fuse with max Zs 1.84 ohm and no RCD (very unlikely...) you would be OK for 2.5mm but only 10A max load.

At 4mm you could probably disconnect on OCPD for a 20A B-curve MCB (Zs 1.75 ohm) but if only an out-house I would split tails, put in a small CU with RCD and 20A breaker to feed the cable, and for the out house just have the sockets dropped to 2.5mm as you suggest and a 3A FCU taking off power for some LED lights.

If there is any chance of it being used for office (i.e. PC) or with a TV (wanting Netflix, etc) then I would suggest they put in some armoured CAT-5 or CAT-6 network cable at the same time as there is a trench for the power cable.

 

[happysteve]

To keep costs down.... Terminate the 6mm SWA into a 45A (or maybe even 20A) metal-clad double pole switch at the shed end acting as a "main switch," from there to your double sockets and a 3A SFCU acting as a light switch.