10mm swa cable +60m Ask questions and find answers to many subjects relating to electrics and electrical work

[stuweston]

I am running armoured cable to a summer house approx 60/70 meters from the house CU (underground :( ). My question is, will a 10mm swa sized cable be suitable for this distance or will 16mm swa be more suitable? also does the armoured cable also need to run in a conduit of some sort? I will have a 2 circuit CU running 32amp ring (3 double sockets) and 16 amp light circuit in the summerhouse. rgds, Stu

[sparx]

Hi stuweston,
presuming you have notified LABC of your part-p work,

Answer depends on expected load, if we assume 32Amps including lights ect. then for worst case distance, using PVC /SWA table 4D4B gives a volt drop of 4.4 mV /Mtr/Amp or:
4.4 X 70 X 32 =9856mV/1000= 9.8Volts dropped which exceeds allowance of 4% of 240v or 9.2Volts, not forgetting you are only allowed this much to the furthest point of the longest final circuit in garage.
Next consideration would be earth loop impedance to ensure tripping of the supply device, MCB/Fuse, over such a distance it may be difficult to achieve even with 16mm2 only way then is to not connect armouring at summer-house end and use RCD incoming switch with an earth rod,T.T. system.
So either : move summer house closer!, accept lower loading, or go up to
16mm cable or more!. BTW I hope you meant 6A lighting circuit!
SWA can be run directly buried as long as deep enough not to be hit by spade/fork etc. and must be half back-filled then yellow marker tape put over it then final back-filled so someone later finds tape before cable,
regards SPARX

[stuweston]

Hi Sparxs, and many thanks. I'll have an electrician commission everything. I want to do all the runs and labour stuff first. i've dug a trench 24-30" deep and i'm at the point of ordering the cable so i can back fill what is a very long muddy trench! very best rgds, Stu

[ericmark]

Some figures to start with. Until June were are allowed 4% volt drop so that’s 9.2 volt after that we are allowed 5% power and 3% lighting 11.5 volt and 6.9 volt. 10mm² cable loses 4.4mV/m/A and 16mm² is 2.8mV/m/A so looking at 70 meters and 48 amp we get 14.8 and 9.4 both fail. But unlikely to draw full amps so working backwards for 10mm² before June 30 Amp total after June 22.5 Amp total as you have lights and for 16mm² before June 47 Amp total after June 35 Amp total. If no lights after June 37 and 58.5 amps. Not sure why 16 Amp lighting 6 is the norm and with some light fittings until June 6 amp is max I see after June up to 16 amp.
So forgetting what is in summer house if you feed 10mm² cable with a 25 Amp MCB or 16mm² with a 40Amp MCB then you can’t really overload the circuit in which case if using 10mm² using then 20amp to 2.5mm² radial for sockets and 6 amp for lights would fit the bill nicely. Assuming little or no volt drop within the Summer house. Of course the Part P etc will all be taken into consideration. Even 16mm² does not have 25mm² steel for earth so you will need 3 core cable and use copper earth.

[stuweston]

thanks for your time on this ericmark. as a layman i had to read it 3 times to grasp the concept. but i now understand the issues, especially correct earthing. 10mm2 looks fine but i've decided for 16mm2 for a more robust installation and some margin. My initial post mentioned conduit? I wondered if it was good practice to run trunking so that some poor sole wont have to dig up +14cubic m of clay and flint if they want to upgrade the supply in the future...additionally now ive dug the trench i am considering laying some MDPE pipe (possibly dig another 10cm down)... is it permissable to run water under the SWA cable with a suitable barrier, say sand? thanks again for your help. rgds, Stu

[ericmark]

It is not normal to lay any water with power but not my field, Cable normally laid in soft sand below and above and tape or tiles above that. As to conduit on very big sites they have laid pipes for cables with manholes either end but I would think that is OTT. Remember who ever is signing the certificates will need to see the cable which is one reason for ducts of course. But if you have ever tried to pull a 16mm cable through a duct I think you would also see the case against them. I would recommend 3 core not 2 giving you a core for earth even if you in the end use TT. At such a distance it will all need testing and as I have said many times before the meters used are expensive and some interpretation is required inspection and testing is not a DIY job you will need an electrician even if you live in Scotland and don't come under Part P and you will be better getting an electrician involved at start rather than find there is something wrong and having to repeat the work. June sees new regulations and you need to either complete before or after and decide which regulations you are going to work to.

[stuweston]

ok, so i need to leave some of the cable exposed for inspection, i'll have to do dome more research on the water. many thanks again for your time, rgds, Stu