Getting power to a detached garrage

[Craig_r]

Hi,

I've recently moved into a new house (new for me, but not a new build). The house has a garage at the end of the rear garden that I'd like to converting into an office.

The garage was built in the late 70s, it was originally supplied with 2.5mm T&E which was run underground in a metal pipe. the garage had an old wire type fuse board which I'm guessing at the time so would the house of had. This supply has been disconnected at both ends some point.

What would be the best way of supplying power to the garage/office?

The CU in the house doesn't have any spare space for connecting the supply and the CU is in a cupboard where with only a couple of centimetres of room on either side so probably couldn't fit a larger CU. Would splitting the tails with a Henley block and fitting a second smaller CU below the main CU, then running SWA to a new CU in the garage be the way to go?

The garage is about 30m-40m from the location on the CU in the house.

I'm going to be doing most of the building work myself ie bricking up the old garage door and insulation. I will be getting a electrician in to do that side of things, but was just looking for some advice on what the electrician should be doing. as with all fields their are good and bad workers, and I'm just looking for the correct information.

Thanks

 

[ericmark]

If it was me I would select some thing in existing CU to be transferred to a new consumer unit and replace the vacated slot for a MCB as large as possible for that make.

If there is an option to have a non RCD protected supply to new consumer unit that is what I would do.

As for cable twin and earth should not be used outside so much would depend on what will go up the existing pipe. But most likely it would be replaced with SWA cable.

For you the big thing is under or over 13A supply For lights and running a computer 13A is ample but for a heater it's not. So using a portable gas or oil heater or even a wood burner would mean power supply can be much smaller.

At 13A total you can use a simple fused connection unit supplied from any of the ring finals no need for consumer unit at house or garage end. Over that figure then two consumer units and work involved rockets.

Also what other services are required. Be it LAN, Telephone, water although there needs to be some spacing they can all go in same trench.

I have talked to a few people about volt drop. In theroy for lights 3% limit but if you use lights which will work at 150 volt although in theroy 3% is limit in practice working on 5% as used for power will not be a problem.

So step one is work out what power you need remembering over 13A the price will rocket.

 

[ban-all-sheds]

So step one is work out what power you need remembering over 13A the price will rocket.

Why?

 

[JohnW2]

So step one is work out what power you need remembering over 13A the price will rocket.

Why?

eric's point appeared to be that a 13A supply could theoretically be achieved simply by adding an FCU to an existing sockets circuit. Since the OP has no spare capacity in his present CU, any other approach is likely to involve appreciably more work, hence cost.

Kind Regards, John

 

[winston1]

Really the first question should be, "Is there any RCD protection at the house end?"

 

[ericmark]

To wire an outbuilding with a maximum of 13A requires cable, glands and a switched FCU (for lights) and a FCU if not already RCD protected a RCD FCU some lights and sockets.

Over 13A in this case two consumer units are also required OK I suppose in the outbuilding you still don't technically require a CU until you exceed 32A if one is OK with lollipop design of a ring but I would think most electricians would want to fit a consumer unit as without it would be hard to terminate 4mm³ SWA into two 2.5mm² cables I would not want to terminate those into a double 13A socket terminals are just not big enough. So in practice over 13A means a consumer unit in the shed.

Of course the heavier the cable the more expensive as well.
13A = 4mm² if volt drop for lighting is used. It would do 53 meters before the 6.9 volt drop is exceeded.
16A = 48 meters max still needs 4mm².
20A = 32 meters max with 4mm² so needs 6mm² which would allow 51 meters.
25A drops to 40 meters using 6mm² so right on the limit. So 10mm² will allow up to 67 meters.
32A = 51 meters using 10mm².
40A again right on the limit using 10mm² with 40 meters max.

I am sure anyone can price up cable as can be seen 30 - 40 meters when on the edge can make a big difference. Also design current and size of MCB are not quite the same thing.

In the grand scheme of things 4mm² around £47 and 10mm² around £128 and 2 x FCU = £11 and two CU = £50 and that's with a RCD so £22.50 for one FCU and one RCD FCU. I am not looking up price of glands but looking at over £100 between 13A and 40A supply.

A calor gas heater is also around £100 so there is very little in the price of installing electric or gas heating. An Eberspächer will cost far more but likely save in long run using cheap red diesel.

I have used in the past many home made heaters the rocket mass heater is one of the most efficient even if home made. But step one is to decide how to heat the office. Wood burners start at around £170 however I would prefer to just switch on the heat.

My aim is to get the poster to consider the options. He makes it plain he is not going to DIY the electrics but he still needs to decide on heating method before anyone can advise on what he needs electric wise.

 

[ericmark]

Really the first question should be, "Is there any RCD protection at the house end?"

£14.10 for a RCD FCU £6.88 for one without RCD protection don't think it really matters in the grand scheme of things.

 

[JohnW2]

Would splitting the tails with a Henley block and fitting a second smaller CU below the main CU, then running SWA to a new CU in the garage be the way to go?

That would be the conventional method of getting a significant supply to an outbuilding - although, as eric has suggested, a 'less nice' way of getting a small supply (up to 13A) would be to spur off an existing sockets circuit. Unless you had thoughts of also using it for other purposes (additional circuits) in the future, you wouldn't necessarily need a 'second CU' - having had the tails split, you could have just a switch-fuse installed for the outbuilding supply. Provided that none of the run of cable from the switch-fuse involved T+E cable buried in a wall, you would not need RCD protection at the house end, but could have it at the garage end.

Kind Regards, John

 

[Taylortwocities]

Really the first question should be, "Is there any RCD protection at the house end?"

Ahh no.

Ericmark already asked the first question

So step one is work out what power you need

Nothing can be planned without having that key bit of information.