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

 

[Taylortwocities]

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.

Yes. But what about lighting? You'll need another switched FCU with 5A fuse in it for lights.

In the house,13A FCU will be fine, providing she doesnt have kettle plus heater!
take a look at the current required to actually BLOW a 13A fuse.

PS, I'm surprised that 6mm is necessary. How far are we talking about?

 

[Roberts7286]

Yes. But what about lighting? You'll need another switched FCU with 5A fuse in it for lights.

In the house,13A FCU will be fine, providing she doesnt have kettle plus heater!
take a look at the current required to actually BLOW a 13A fuse.

PS, I'm surprised that 6mm is necessary. How far are we talking about?

Yes I was going to put a spur in for the lights in the summerhouse. It's about 60 metres. You never know what people will plug in so was trying to give as much power but keep costs down

 

[Dustydazzler]

Can you not get back to the consumer unit , split the tails in a henley and pop a new small board in to feed the outhouse submain

 

[Roberts7286]

Can you not get back to the consumer unit , split the tails in a henley and pop a new small board in to feed the outhouse submain

Yeah that was the 20A option. I think that's what I'll do as it will provide a bigger supply

 

[timhoward]

To be honest, my first choice would be neither!

or split supply at house to small RCD main switch DB at house end and use a 20A mcb to feed summerhouse

My thinking would be to split the supply at house with overcurrent protection but not RCD, and have small CU with RCD in the summerhouse . This assumes the house is not TT and the 6mm cable is mechanically protected or in ducting.

Then a future inevitable fault outside (e.g. light fitting filling with water) won't turn off half the house.
Your method still achieves that but is a longer walk to turn it back on again!

 

[brianmoooore]

. This assumes the house is not TT and the 6mm cable is mechanically protected or in ducting.

!

The 6mm2 cable I' be using would be SWA.

 

[Roberts7286]

The 6mm2 cable I' be using would be SWA.

Yes I'm using swa

 

[brianmoooore]

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

 

[Roberts7286]

To be honest, my first choice would be neither!

My thinking would be to split the supply at house with overcurrent protection but not RCD, and have small CU with RCD in the summerhouse . This assumes the house is not TT and the 6mm cable is mechanically protected or in ducting.

Then a future inevitable fault outside (e.g. light fitting filling with water) won't turn off half the house.
Your method still achieves that but is a longer walk to turn it back on again!

Yeah I know what your saying. Did one like that recently. Just trying to keep costs down. There isn't any outside lights etc so hopefully not much that can fill with water at the summerhouse.

 

[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.