We would like to run some SWA from the house to a separate garage, total run is about 35 meters, mostly buried underground.
Using the TLC calculator, here
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here for a short URL
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I need to say it is a lighting circuit, as there will be some lighting on it (standard 6A, for a light in the garage). The main power will be the 20A radial.
If I say it is lighting, then it wants me to use 10mm cable for a 32A circuit, which seems overkill to me?
#1 Voltage drop within limits for the final circuit type. Hence cable size must be suitable for the 35m length so as to minimise voltage drop.
#2 All cables must have RCD protection unless Surface Run, >50mm depth, SWA or BS8436 or Steel Conduit. If you run FTE in the house to a jn-box then SWA up the garden, that FTE must be surface run or >50mm depth or in steel conduit or have RCD protection at the house-end rather than shed end. Having the RCD at the house-end means an earth fault on the garage power would take out the lights so you may want an emergency light in the garage if using circular saw. Some people run SWA all the way to the house CU, but in some cases this can be very difficult (solid floors, space).
There is a uk.d-i-y FAQ available - worth reading.
35m is quite a long way - I assume that includes the run inside the house to the CU. If you dropped to 16A radial in the shed and 6A lighting you could design for a 16A supply (16A RCBO in the house) which would probably make 6mm SWA doable. 16A is quite reasonable unless your garage is actually a workshop with lots of power tools in use. For 35m the garage is most likely going to be a TT supply, ie, 2 core SWA with garage local earth rod.
I was planning on MCB protection only in the house, as it is going to be SWA from the CU in the house to the CU in the garage (Well, the SWA would connect to an adaptable metal box under the CU, then I would connect a 10mm earth cable from this using, a piranha nut, to the main earth terminal Then run the L&N to the CU in round plastic conduit (Probably about 15cm).
I would then have a small CU in the garage containing a main switch, a 6A MCB for the lights (run surface, in round conduit) and a 20A RCBO for the sockets (Again run surface, in round conduit) - I don't see the need for an RCBO in the house, as that means the whole supply would go off, including the lights, if there was a problem with sockets, and a trip to the house would need to be made to reset it.
I would also add a 16A MCB for a possibly future freezer.
The distance between the house and the garage is about 15m, so I was planning on exporting the earth, using the amour of 2 core SWA, is that unreasonable for an exported earth?
> 35m is quite a long way - I assume that includes the run inside the >> house to the CU.
Probably ok. What earthing system have you got?
Indeed, not having a RCD at the head end would simplify your design at the far end.
It depends a bit on what earthing system you have at the head end... If its TN-C-S then you will need to export the equipotential zone to the garage as well, which will mean you need enough area of copper (or equivalent) to act as a main equipotential bond as well as an earth.
See the section on exporting an earth in the article above.
It comes down to... #1 SWA armour cross-sectional area - which should be ok. #2 No incoming services, garage floor dry, class-2 tools
If you have incoming services you need 10mm 3-core to bond them back to the MET, you can not use the steel wire armour because its cross- sectional area converted from steel is not equivalent to 10mm copper.
Personally I would use 3 core SWA because it means you are not reliant on the armour integrity. The cost difference is probably not too different. Conversely if you are not exporting the house earth, and garages can have damp concrete floors or class-1 tools, then 2-core is fine with a TT supply & local earth rod.
If price is high on the SWA always worth checking Ebay for a cheap offcut - does happen, you may have to wait a while.
This is actually for my brother's house, and after going round there, I realised he was being a total plank, and the cable run is just 18 meters, I have no idea where he got 35m from!
The garage does not have other services.
If it did have a water supply, in plastic, I assume that would not affect things would it? (It doesn't, I was just contemplating running a plastic water pipe to my own garage)
The earth in the house is TN-S (Provided through the amour of the incoming cable).
With the revised figures, the TLC calculator shows that 4mm cable is fine for a 32A lighting circuit (just)!
So, with a 32A MCB in the house, would it be best to used 3 core cable and earth the core just at the house end, or just connect to earth both ends?
No it would not change anything. On two counts - firstly being plastic it would not be capable of introducing a potential into an equipotential zone, and secondly, not being TN-C-S there is no need to export the EQ zone in any case! ;-)
That makes it simpler then. You could go for local TT setup in the garage (thought it was a wooden workshop?), or at that distance with a TN-S head end you could just export the house earth.
If you are sure that is enough capacity for any future needs then go with that (cable is cheap - laying it etc is not!)
If you go with an exported earth, then use two core, and connect the armour to earth at both ends - it will be your earth[1].
If you go with TT at the remote end, then connect the armour to earth at the head end, but isolate it at the far end, and instead use your earth stake for the local earth.
[1] either with metal clad box at the head end, and a metal CU at the remote end, or the TLC piranha nuts to make a connection if using plastic boxes/CUs. (note you should not use a metal CU if making the workshop TT)
He is digging the trench now, but isn't sure about the depth - it will run under some slabs of a patio, then under the back of a flower bed - am I correct in thinking there isn't a finite depth required, it just needs to be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance to the ground?
If so, would laying it just under the slabs (so it is then visible if they are removed) would be OK (The slabs are bedded in cement, so we will probably angle grin a channel in it), and then two spade depths at the back of the flower bed be sufficient, with warning tape at 1 spades depth?
We would also take photos and make sure this is kept in an envelope near the CU.
As for the termination, I would drill a hole through the wall of the house, then mount a plastic box over this, then thread the cable in the bottom of the box, then out the back, so no glands are used here, the box just protects the cable from bending too sharply. Then it would run under the kitchen cabinets to the under stairs cupboard, and terminate in a metal adaptable box - then run the earth to the main earth terminal near the service head, and the L&N to the CU via a short piece of round conduit.
In the garage, the SWA will terminate directly in a 4 or 5 way metal clad CU, with a main switch, RCBO for the main radial, and MCB's for the freezer (potentially) and an MCB for the lights - all cables run surface, in round conduit
Does that sound OK?
The roof of the garage has some large metal triangle supports - do we need to bond these all together, and then tie that into the earth in the CU too?
Bury SWA at a depth suitable for the likely use of the ground.
450mm depth generally, you may want 600mm depth thro a flower bed or cultivated area.
Cable laying method is important. Clean the trench of stones, lay riverwashed sand bed, lay cable on top, cover with more riverwashed sand, tamp down, then yellow cable marking tape with some sand on top, then fill in. TLC direct do the yellow cable marking tape by the cut metre. Riverwashed sand is used so a) no stones b) provides a colour change to help identify the cable.
You can bury SWA in a flexible underground duct. Perforated land drain pipe (80mm) is about =A325 delivered for 25m; proper twinwall (smooth interior) electrical ducting is much better, much stronger - but about =A375; flexible nylon or polypropylene conduit (not PVC) of 25mm would give a little protection against exploring spade although not much.
Yup, that's the basic idea. Usually 450mm is suggested as a standard depth, with more in places where it it likely the ground above will be cultivated (you can usually assume below two spades depth is safe - so
600mm is often quoted). Use the marking tape etc as described in the other reply from js.
The warning tape usually goes within 6" (150mm) of the cable. Can't see any problem with using the slabs as a capping for what in effect would be a surface run cable.
The general rule here is use common sense. Try not to fix it to things that are "temporary" (like wooden fences etc)
Can't hurt.
Nothing wrong with taking the feed from the CU to the adaptable box in normal T&E (given the total length of the sub main, I can't see there being a problem with the reduction in CPC size for a few inches).
Yup that sounds like a good plan. You could use a split load CU in place of the RCBOs. but there is unlikely to be much if any cost saving.
Yup.
I would not have thought so. It does not sound like they are capable of bringing a potential (even if that potential is "earth") into the zone. Also as mentioned before, since you are TN-S there is no requirement to extend the main equipotential zone of the house out to the outbuilding anyway.
On Wed, 12 May 2010 04:15:12 -0700 (PDT) someone who may be "js.b1" wrote this:-
That is how it should be done and how it is usually done by those with time. If it is being done by "professionals" a good way to check is to ask to see the hand tampers and the hands of the people who have been using the tampers on the sand.
More usually they pick a time when nobody is looking, heave the earth/stones in the trench and run the wheel of a JCB over the earth to firm it up. The end result is often penetration of the outer sheath by stones, followed a decade or more later by water pissing out of the joints and other related problems.
Reminds me of the gas crowd running new driveway runs under other people's driveways to avoid "pattern concrete" and then going bust. Quite why they didn't shove it up the existing cast iron pipe as Transco had done is beyond me, but hey-ho, such is sub-contracting.
The minimum depth, as you say, is a moving target depending on a risk-based likelihood of the cable being disturbed.
The minimum specified by DNO's for service cables is 450mm *to the top of the buried cable or the duct containing the cable*, so you will need to dig deeper than this to lay sand under the cable/duct and allow for the diameter of the cable/duct.
Under agricultural land (where there is a likelihood of ploughing activity) the depth requirement can be as much as 1000mm to top of cable/duct.
Just found another DNO document which specifies a trench depth of 530mm for domestic new connections. They say that for a direct burial they lay their cable in the bottom of this trench, but I suspect they are allowing for an 80mm dia. duct to bring the top of the duct to 450mm below finished level.
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