6mm to 2.5mm for sockets

[Roberts7286]

Hi

I've got a job that i've been asked to quote for and i'm struggling with how to do it the best way.

It's a summerhouse and the customer only wants a couple of double sockets and a light in it. My issue is that it's a fair distance from the house where the supply will be taken from. I've done the volt drop calcs and 6mm will be fine upto 20A max load. The existing consumer unit is full so I've got 2 options, spur from one of the socket mcb's (32A) to a fuse spur, then run to summerhouse giving 13A and enabling me to have both sockets without it being a spur off a spur (if I went straight from the MCB) or split supply at house to small RCD main switch DB at house end and use a 20A mcb to feed summerhouse. She mentioned using a kettle in the summerhouse so would like to give her more than 13A really. Ultimately I'm trying to keep costs down as best I can.

Also if I'm using 6mm to solve the volt drop issue can I take that to an adaptable box at the summerhouse then connect 2.5mm onto it to go to the sockets as the 6mm will be too big for the socket terminals.

Cheers Tom

 

[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.