Cable size advice needed

[devonspark]

Hi

I'm a diyer planning to run SWA to an outbuilding and just wanted to get some advice on what size I need.

This is what I am planning.

The cable length is 65m. the load will be no more than 32A at any one time, a few sockets, maybe a 3KW heater and a couple of lights.

At the house end it will be connected directly into the CU, via a dipole switch, on a 32A RCD on the socket side of a split CU. it will terminate in the outbuilding in another diplole switch which then connects to a small CU. There will ber RCDs in the outbuilding also on all circuits.

First of all, what cable size will I need. Is 6mm sufficient or should I go to 10mm.

Also, what would be the best way to earth the outer cable. The cable will be burried at a depth of 0.5m with warning tape at a depth of 15cm.

I am aware that I need to submit to the council for part P so I really want to check my plans before I submit them.

TIA

 

[pmac10]

for what you described 6mm would be sufficient, current carrying capacity of 37A ish i think. if you wish to err on the side of caution, 40A RCBO and 10mm, wiring regs will tell you exactly.

 

[patricklondon]

sorry devon, a bit controversial but many on this site myself included are very wary of giving advice to diyers mostly because its too easy to get potentially fataly wrong. the cost involved with paying labc compared with paying a Part P registered electrician or the cost of it going wrong make it a job well worth getting someone in.

 

[sparks1234]

Hi

I'm a diyer planning to run SWA to an outbuilding and just wanted to get some advice on what size I need.

This is what I am planning.

The cable length is 65m. the load will be no more than 32A at any one time, a few sockets, maybe a 3KW heater and a couple of lights.

At the house end it will be connected directly into the CU, via a dipole switch, on a 32A RCD on the socket side of a split CU. it will terminate in the outbuilding in another diplole switch which then connects to a small CU. There will ber RCDs in the outbuilding also on all circuits.

First of all, what cable size will I need. Is 6mm sufficient or should I go to 10mm.

Also, what would be the best way to earth the outer cable. The cable will be burried at a depth of 0.5m with warning tape at a depth of 15cm.

I am aware that I need to submit to the council for part P so I really want to check my plans before I submit them.

TIA

You should really be installing this as a TT system.

Protect the cable with a 100ma RCD. Isolate the cable from the shed fuseboard and install an earth rod and main RCD board.
There is no need to install another isolator in the shed

 

[pmac10]

i missed the DIY er part but stand by my reccommendation, however DIY ers should not be messing with @32A AC, i think i recall 50mA AC as lethal current? could be wrong, its been a while.

 

[electricAl]

There`s gonna be an outhouse full of RCDs protecting this install.
Not entirely sure, but it sounds as if the cct is protected back at its origin.

Sparky1234. Humour me would ya bud - why the 100mA?

 

[Spudnik]

No need for any RCD on an SWA submain.

 

[ian.settle1]

for what you described 6mm would be sufficient, current carrying capacity of 37A ish i think. if you wish to err on the side of caution, 40A RCBO and 10mm, wiring regs will tell you exactly.

Don't forget to allow for volt drop in calculating the cable size as the run is 65 metres

 

[Guest123]

Don't forget to allow for volt drop in calculating the cable size as the run is 65 metres

Took the words right out of my mouth.

6mm will be pretty close to the 11.5V with the loading described. A calc will confirm.

 

[Jimmyb]

voltdrop @ 5% (Max) with 6mm swa will only go 49meters so 10mm it is which at 32amp load (which seems a big load) would be ok for upto 81 meters.

 

[devonspark]

Thanks for the replies, I will go with 10mm. I said 32A as that will be the rating on the RCD in the house but under normal circumstances the load will be well under 20A, but going for 10 will enable for some expansion if required.

I should say that even though I am a DIYer I feel that I am fairly competent and earlier this year I rewired an entire house which was passed with no problems by the inspector.

I have considered doing the part P course but I don't do enough work for the continuous assessment.

 

[JonAW]

I haven't done a cable sizing but just wanted to give my input on the method of instalment.

If you supply the submains with rcd protection and also have rcd protection in the c.u. at the install then you are likely to experience nuisance tripping as putting 2 rcd's in series reduces their sensitivity.
My advice would be to supply the submains from a 40A mcb isolated only by a double pole mains switch. Then at the load side create a TT system by installing an earth spike as the main earth and using a 30mA double pole rcd as the main switch in the c.u. Don't forget to bond the steelwork if there is any! The 30mA should not cause nuisance tripping as there are so few circuits being installed.(The reason i suggest a 30mA rcd is because if you install using a 100mA rcd then you will have to add a 30mA rcd to supply the sockets, again you would be putting 2 rcds in series.) Make sure you have the earth and the installation in general tested by a proficient electrican as you won't have a leg to stand on should the worst happen.