Electric to detached garage

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I would like to change the electricity supply to my detached garage. The garage is about 1 meter away from the house.

The house was rewired 12 months ago to 17th edition. The kitchen is on its own ring - 10 sockets, 6 of which are almost never used. Its not possible to run SWA all the way from the CU, so I want to use kitchen ring.

My plan is to put a suitable junction box to extend a leg of the ring main down the inside to about 50 cm from the floor. Drill through the outside wall run T&E into the back of an external SWA junction box. Then run 4mm 3 core SWA cable down the outside wall (clipped) into the ground to a depth of 45 cm, then take it across to the garage (underground w/ warning tape 15 cm below ground level) and come up on the inside of the single block garage wall. Run the SWA up the wall (clipped) and connect to garage CU.

Does this seem OK?

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What load demands would be in the garage?
As you will already have RCD protection within the house on the circuit you are connecting to, there would be no requirement to have additional RCD in the garage.
 
In a word, NO

1. If you're going to spur from the ring final circuit to supply anything other than one socket outlet (single or double) then you'll have to install a 13A fused connection unit, this will limit the power available in the garage to 13A. It renders the second CU in the garage pointless, you might as well go straigt into a socket then have a second FCU for the lights. Oh and no need for 4mm cable if you're limited to 13A, 2.5mm will be plenty over that distance

2. If the house is wired to the 17th ed. the socket circuits will alredy have RCD protection so the second RCD in the garage serves no purpose, as any faults in the garage will trip both RCDs and you'll loose power to the kitchen anyway.

3. Your sketch appears to show 2 cables being spurred from the ring final, both connected to the SWA, why?

4. Have you given any consideration to the type of earthing arrangement you have, and the possible consequences of exporting the house earth to the garage?

5. You said CHANGE the electricity supply to the garage, that implies there is one already, what's wrong with it?
 
Ive rebuilt the garage - the old supply was disconnected and removed inside the house when the house was rewired. The garage is being signed off by BCO including the wiring.....

- I have a TN-S system in the house, what would be the best way to earth the SWA.

- What about running 2 runs of 2.5 mm swa and extending the ring main into the garage? Is that acceptable/possible? If not, used a 13A FSU wouldn't really be a problem, but would prefer no to have that limitation.

- yes the house does have RCD protection on all circuits, so an isolator switch in the garage would suffice?
 
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Ive rebuilt the garage - the old supply was disconnected and removed inside the house when the house was rewired. The garage is being signed off by BCO including the wiring.....
Then it will have to be done by someone who knows what they are doing with the necessary equipment.

I have a TN-S system in the house, what would be the best way to earth the SWA.
You connect it to 'earth' after calculating that it will be within the required limits.
You may also require bonding conductor(s) from extraneous metal parts, if any, in the garage back to the Main Earthing Terminal.

What about running 2 runs of 2.5 mm swa and extending the ring main into the garage? Is that acceptable/possible?
If the circuit is then still within the required limits.

If not, used a 13A FSU wouldn't really be a problem, but would prefer no to have that limitation.
If you will require a larger supply then it will have to be done differently.

yes the house does have RCD protection on all circuits, so an isolator switch in the garage would suffice?
It would but may be inconvenient.
 
Although it may be possible to supply the garage from an existing ring the question has to be what are the readings on the ring as it stands? The earth loop impedance must be under 1.44Ω and the volt drop when feeding lights needs to be under 3% I would guess we would take 26A as being the current when working out volt drop to point of fuse or MCB before it feeds lights so the loop impedance line - neutral would need to be very low likely around 0.6Ω to comply with volt drop so in real terms to comply with regulations it is unlikely you can use the kitchen ring. Regulation 314.1 may also be a problem division of installation again down to your inspector and how much he will allow you to bend the rules.

Although using HF florescent or LED lighting in the garage it would likely still work with a 5% volt drop safely it is all down to the local authority building inspector and to do anything which does not comply with regulations without his go ahead is clearly a silly move.

Four core SWA with both lighting and power may be a way around the problem, but really this is something which needs to be worked out on site and drawings however good are not really going to help.

Step one must be to measure the loop impedance in the kitchen and see how close it is to the limits once you have these figures you can work out if you are likely to get away with slight stretches to the rules clearly you would need to ask the LABC inspector first but without having the figures he is unlikely to give you an answer.

He may insist on third party testing it depends on his skill and if some one from outside is brought in likely they will want to strictly follow the rules so safe method has to be a supply direct from consumer unit.
 
- Why was no consideration given to this when the house was bing re-wired?
- Why can you not run swa, or any other cable, back to the house CU?
- Do you have an unused shower or cooker feed in that part of the house?


Daniel
 
- Why was no consideration given to this when the house was bing re-wired?
Because that's what happens when people try to do design work on the basis of asking questions as they go along about the things they happen to realise they don't know about.

Over and over again people like that get bitten by the unknown unknowns.
 

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