Sips and stones may break my bones...

While sizing the SWA (Steel Wire Armoured) cable I checked out a few forums and found when people ask for advice on the sizing it gets very complicated indeed - in fact on more than one forum you can see it starts to get a little hostile toward the questioner oddly. Also electricians seem to often be unable to agree on earthing and RCD requirements around it and so on. I asked the sizing question on one forum and got ripped to bits by a couple of guys who insist it's not possible for ordinary folks to do electrical work and have it overseen and certified by a part P registered electrician, and then just started attacking me, saying I must be lying! Very odd and such a contrast to how we all behave on this forum. At times people don't agree on an answer but its always civil as far as I can see.  Anyway, by using online calculators some electrical wholesalers provide, I was able to find an answer, and when later asking my nephew (who is a professional electrician who does most if not all of the certification for then firm he works for and has now kindly offered to do ours too) got the same answer from him.  Basically, for a 50 metres run with a 20KvA supply, we've gone for 25mm SWA, which . So that's ordered and comes in with carriage around the £350 mark.

 

My father is a retired electrician and growing up I would help him out (when I was younger, on rewires I would wriggle under floorboards pulling cables from room to room  - but I was always well paid.) and so naturally we'll be doing our own electrics. I've actually found in the regs that it specifically covers a  person (not a certified person) carrying out the work under the supervision of a part P registered electrician  who must satisfy him/her self that the work is up to code before issuing a certificate.  My father as a sole trader was himself qualified but of course now he is retired is no longer certified, though keeps up to date with new regs out of interest. In fact, we actually ended up with 3 qualified sparkies willing to oversee and certify us - 2 friends and one a relative, my nephew, so I think that pretty much settles that one!

 

The differing answer/arguments over SWA sizing are interesting though as most people answering immediately asked the questioners what current they expected to draw and so on and some quite esoteric questions at times - on one level this makes sense, yet when whole housing estates are wired up, every single new householder is not asked this of course, and naturally, subsequent owners of a house will install different appliances, or might have a welder in their garage and so on - so clearly there must be an "average" or norm that is installed without all the hoo-ha. Distance is a very relevant question of course for voltage drop, but beyond that, it seems overcomplicating -  I mean which of us in moving into a new house digs the cable up to see what diameter it is?   I found this site has a simple to use calculator http://www.doncastercables.com/technical-help/

 

Anyway, we've ended up with a reasonably large size at 25mm 3 core SWA (actually rated at 124 amp so waay above our requirement) making the voltage drop negilgible, so I'm happy with that.

Not sure if I'm allowed to mention who I buy from, (if not, just delete this bit please mods or message me and I will) but I've found TLC-Direct to be excellent over the years, and I usually cannot beat their prices. But what's important is even their cheapest stuff is branded and usually British made too. I've had a lot of good cheap low voltage electronics from China over the years, but for mains  voltages I prefer to know it's made to code. I've noticed you can buy UK mains fittings on eBay from China, but having seen some tear downs online of some 240v rated chi9nese goods... (Bigclivedotcom on youtube for one) er, no thank you!

Edited August 1, 2017 by curlewhouse

I know certain electrical forums tend to be hostile towards DIYers, not something I personally support.

 

At the length you have, it's the voltage drop (measured in volts not milliamps  ) that is the limiting thing that determines the cable size to use, not the current carrying capacity. And the discussions about maximum demand can get interesting as well.

 

There is nothing wrong with an unskilled person installing cables under the supervision of a skilled person who is going to test and sign off the work, but some in the trade don't even like doing that. Personally if someone wants to do the more tedious manual work then I am happy with that.

 

Make sure you bury the cable deep enough, with some yellow "electrical cable below" warning tape in the trench just above it.

 

P.S, I would have put your meter box in your wall at the back corner (top as viewed in the plan) It would drastically shorten the run to the house, but perhaps make the garage run slightly longer. 

Yes, me too. Northern networks wanted it top right of the plan unfortunately, so we have 2 runs, one right round the back of the house into the utility room and the other to the garage/workshop. It's all in place now (with yellow marker tape), though for the Northern networks bit to our box they insist they'll only accept rigid "tile" covering and rigid ducting - even though their national body accepts coilable ducting and you can't buy the rigid tile except in massive quantitites (I scrounged some as I only need about 2 metres).