Morris Minor Owners Club

I'm posting here being electrical although not directly minor related. Hoping someone could help please. I'm going to get another garage built and want to run power to it from the house. How do I work out the cable size over a distance of 25 metres, it will be buried so armoured cable.

It depends on what amount of power you want in the new building. You need to know the total wattage of all items that will be in use at any one time. For example: 4 x 35 watt lights + 1 x 2Kw fan heater + 1 x 2KW compressor + power drill + radio etc. Once the load is known, then the cable can be sized accordingly. 25m is not an excessively long run and so should not pose any issues.

Decide now if you think you may want 3 phase power in the garage in the future...

Thank you, I have added up the total, compressor, heater , grinder lights, drill, radio and although I won't be using all at once thought it might give me an element of safety. The total would be no more than 3500w I calculate it would take me over 13amp. using Amp= watts over volts 3500 divided by 250 = 14 amps. I've heard that voltage drop could occur, would this be an issue over 25metres? If so is there a solution.

Lots of factors involved here like distance, type of cable, etc.

TLC has a calculator that gives an approximation.

my garage is about 70 foot away from the fusebox in the house
i've got a huge compressor , welder lights , sockets etc etc

i played it save and used 25mm cable.. same size as the mains cable coming into the house

fuse box in the garage and a kill switch and consumer unit in the house

better to put the biggest down you can afford ... cost me £150 just for the cable

The calculator pointed out by silloyd is really good. Not sure about beko's 25mm cable - it's the cross section area of the conductor that counts, not the diameter of the outer sheath. You could just get away with 2.5 sq mm, but I recommend using 4 sq mm conductor which will give you a much better margin for extra bits and pieces. You need to ensure that you have the correct connection in the house, it's not just a question of sticking a plug on the end. Once the cable has been laid in the ground, I recommend getting a qualified electrician to finish the ends.

I used 6mm SWA cable to absolutely guarantee that there would be no unacceptable voltage drop (>5%) over the ~35m length, even at the highest usage that the remote MCBs can handle (6A lighting + 16A sockets = 22A x 230v = 5.06kW). I will never get near that level of usage but the extra cost of the next cable size up is small relative to the time, cost and inconvenience of laying a replacement or additional cable in the future.

I got my cable from Online-Cables Ltd a few years back as they were reasonably priced and they sell various types by the meter. Postage is a few quid so you may be able to get a better deal locally as SWA is heavy stuff.

I buried mine 600mm plus under a stone path (nice and safe!) and let a 'leccy sort the ends.

I've got a feed from a 32A RCD protected MCB in the house Consumer Unit to an RCD protected CU (sockets and lighting) in the Garage (~5m) and then on from there to a shed at the bottom of my garden (+~30m) with another RCD protected CU (sockets and lighting).

Whilst it is possible for an earth trip in the shed or garage to take out the house (I tested it and discrimination is separated enough and most sensitive RCD flips first), I'll live with that potential inconvenience (only had one non-testing trip in 3 years due a faulty plug on something) for the safety of knowing all cabling is as protected as can be.

Many thank for all this advise, I've used a calculator to help with the cable, not sure it's the one suggested though, and to play safe 6mm is my preferred size, although 4mm would cover me. I would also like to know if I can use this cable to feed the lighting in the garage, by perhaps spurring off a socket. Or would I need multi cores in the cable run. In fact how many wires should I have within the armoured cable.

All I can come up with from that calculator is;[frame][/frame]
I don't think buried cable is the only option either, cantanery (overhead) is another option without the danger of getting cut by a spade or plough.
You don't have to just bury the cable on it's own either some overflow pipe is what I plan to use when I get tired of running extension leads to my shed

Thanks mark, I know there is a lot of do's and don'ts regarding this subject. From what I've found out so far is the need to bury at least 18inches deep fill 6 inches then insert a tape warning of a cable below then backfill to surface. I'm pretty ok with that but other factors outlined in my last post still evade me.

It's just about how much power you need, unless you plan to light a stadium the load from lights is going to be trivial.
Unless you intend to employ an octopus to switch everything on at once to achieve the highest possible load I doubt that voltage drop will be an issue.
Give some thought to the more awkward bits like where the wire exits the house and not letting water get in.

I was going to suggest 6 sq mm cable - but waited to see what others would come up with. It will be fine.........

Thanks again everyone, and yes I have been considering the entry/exit points, thinking I'd use metal conduit for the cable before it went underground with a waterproof box on the external wall to connect back to the consumer unit.

If metal - would need to be galvanised - even then - it will rot, just take a bit longer! Armoured cable is fine without any other protection.

I only intend to use conduit for the short length up the wall before going through the wall. All this has come about through wanting to build a garage on the site of an old garage, the existing garage has had power for years, from an underground cable but what this cable consists of I don't know, I do know there is a 13 amp plug in the house that plugs into a socket, which allows power to this garage. I know there are regs for new power supplies but have started to wonder if I could get away with using this existing power supply and build the new garage around it. It would certainly save me some hassle.

Worth finding out what is in situ, if you are lucky there could be a conduit that you could put fresh wire in.
At least check that the existing wire isn't rubber and cloth sheathed.

Yes Mark will investigate this line further, although if I do start modifying it I'll know doubt need to abide by the latest regs.

I think in England you are 'not allowed' to do your own electrics, certainly around the house? No such rule up here in Ecosseland...yet! But I assume if the garage is connected by a 13 amp plug - then that must be ok?? Certainly limits your power to 13 amps!! Anyway I would certainly run it via an RCD.

I've heard a rumour to that effect, not sure if, or how it's enforced.
I wired my house and fitted the central heating system, so far so good, don't tell the building police though