Earthing Systems for Outbuildings

hello Andre w and this time and have a look at so how you install power to a shed or garage or some remote building and bang on this is totally my home not talking about industrial or that type of installation also you've got your normal house and a short distance away or maybe a moderate distance to where you've got some kind of building and you want to put power in there and then the question which several people actually asked is what do you do about the earthing in the remote building so this is the sort of deal where you've got your say ground or your garden or whatever and in one position of course you have your house and let's draw the house in here and as it has doors and windows and things inside and of course your house already has X Rissa T provided because obviously it's built that way and helps may even have a roof on top and then of course at some point you want to put another building in your garden and it's probably going to be a shed or garage or car hole or whatever else you want to actually call it so here's the additional building and in the UK this is a pretty common situation and there's even a fashion now to put garden officers on the like in there so medias of course to get power from somewhere as in your house to the building over here and Civic videos will be situated in your garden or the back of your house or somewhere like that so in terms of distance we're not talking huge it's and they sort of tens of meters away probably certainly not talking about miles away and of course it sometimes it can be very much closer to the building now so that's getting power to there also since various methods you can use and the most common would be an armored cable typically buried underground or some coated overhead or whatever that's all looking like this time is the method of earthing that you're going to use and the first thing to consider is what kind of anything you've got in your house and this is going to be determined by whatever the electricity supplier has given you it's not something you can only make a choice on yourself and in the UK as we've seen in previous videos there are three possible options we have a TT system TM cs4 TN yes and it will be one of those three things there the other types are not permitted for use in the UK and domestic dwellings so it will be one of those three options now let's do the easiest one first so if your house is a TT system and this means you've got an earth rod in there electrodes in the ground then the supply is not providing with earth so you're only earth is that throughout the stuck in the ground somewhere if that's what you've got in the house then that is what you can have to use in your outbuildings well there's no choice in the matter because you don't have either of these so if you have the earth rot in the ground here for your house then UK was going to come along and going to have to put another one in for your outbuilding over here like that and that's all you can do because say that's the only choice you have so you've got a TT in your main building that's where you have to use in your remote building as well so earth rot in the ground and just like in house all of the circuits will have to have our cities on them regardless of whether they need them for other reasons it goes with an earth rod any fault current when there's a line to a fault or you're fairly small so you need an RCD to ensure that the circuits will disconnect and that's pretty much all there is to that one so it's just a question of putting another earth reading at the remote building and then installing the appropriate things inside now TT installations in their actual buildings are fairly uncommon in the UK it's generally one of the other two so let's have a look at those ones which are something more usual now for a TN s so this this is typically it comes in the property there will be three conductors which will be the line neutral and earth and this kcs for separate means at the neutral nerve are separate as they come into the building and then should be separate all the way back to the transformer typically found in older properties and that'll be where you have to with a LED cable coming in or LED covered cable to cause inside the liner neutral and the earth is on the lead outer covering and that's typically found in older properties and the other system which is normally found in newer buildings is the TM C now that incoming cable only has two conductors inside which are the line and a combined neutral and earth and tend up to if we're going to get a new building menu because the lead cable isn't typically used or your mail anymore and it's not see what you get with a twin conductor coming in cheaper to supply and therefore use far more often now in both cases here the electricity company is providing you with an earth connection and certainly in terms of how you can use it then you will also see connect to that now one possible option you could do with an outside building is to ignore this connection completely install your own earth rot in the ground and go down the route that we saw on the previous page there however there's very little reason to do this there are a few specific areas which might require that with in most cases you are going to want to use the earth connection provided from the literacy supplier mainly because it's going to be far more reliable and some stick just rammed into the middle of your garden and but you might now give me four years of metal use what you've got now in terms of your actual outbuilding what you do next upend on what is contained within and just draw the building in there the main question you need to ask is just this outside building contain any extraneous conductive parts and just like in your main house any extraneous connected parts are basically things are the conductive which typically means made of metal extraneous means I come in from outside of the building and the kiss case it'll be the actual building itself and parts opposite means it's a part of some piece of equipment now most common pieces that you're going to find coming in will be things like water pipes possibly gas pipes are those that it less likely in an outside building or bassy any other metallic service that comes into the building and another thing to consider here is that the if the building itself is made of metal their metal cross also will be considered an extraneous conductive part because if this building will say a greenhouse made out of aluminium it's only resting on the ground and in some cases a bit to it bury in the ground so the whole frame that then would be an extraneous conductive part because it's bringing in a potential from outsiders on the ground into the building now if your building does not have any and this is probably the most common situation so it's just a garage that sort of standing there made out of blocks or bricks or whatever or it's just a wooden shared or some of those garden offices which in many cases are just wouldn't shared with a couple of lights and above whatever stuck inside then if there aren't any all you need to do is bring in the three wires from the house so we're going to have your line coming in here and of course the neutral coming in there and neither say the earth coming in as well and of course as long as you sized properly for whatever load you have in the building then that's absolutely fine so all it is simply the cable coming in three conductors and then you can connect your whatever equipment it is inside and this would apply regardless of which type of system it was whether it was the C has or the TMC s provided that there are no extraneous conductive parts in there now the cable coming in could be a three core cable and which case it's obviously three cores there it's going to use armored cable then you could use a two core cable steel wire armored and in case of that one the two cause would be the line and the neutral and for the earth you could use the armor of the cable now being on the armor is not made of copper it's made of steel so you wouldn't need to make sure that the armor is of suitable size to carry any fault current if there was some kind of problem but as it turns out set it in virtually every situation up to normal sizes that you can be using in your house we are a bit of a two core steel wire armored cable is big enough to use as the earth connection so you can just have your two cores inside connected as the earth provided course use the proper glands and things on both ends or you could just use the three core in which case they are would just be connected to us anyhow and you'd use one of the inner cause as the earth as the third one but either way that's absolutely fine and there's no real special applications or things to consider other than the usual ones about the size of the cables the load and the distance and so on now in the case of your building outside having extraneous convective part so here's the buildings and let's just say there was a water pipe that came into the building from underground because you had say a tap inside may have like a single something in there so there's your water tap in there no single watching scene or something like that then this is nearly a metallic item and just like metallic geysers in your house you will need to install bonding to this item here now it's going to bring your wires in you're going to have your cables coming in so here's the neutral coming in there and of course the line coming in and the earth that comes in from the main house you will need to connect main bonding from this to the water pipe in this case and if it was a gas one you'd also connect it to that if this was a metal frame the building then of course you would also connect on to that as well and then the question is how big does this Earth need here in the cable need to be now in the case of the TNS system even though size of bonding is generally six square millimeters although it's quite often 10 anyhow on many new installations so providing that this here is of a suitable size this would have to be at least six millimeters in size to ensure that it could be used as a bonding conductor and note that this is separate from the car to the earth I mean this could be quite thin while here it was just supplying say one socket or something this could say be like to 2.5 square millimetres and the earth can maybe similar size but in its Utley case as we've got main bonding required here this would have to be a suitable size so at least six millimeters for TNS if it's a TM CS system I'm spending more modern properties the minimum size for bonding there is 10 the square millimeters so in this wire here would have to be at least 10 square millimeters just for the fact you could have to be used for the bonding the other conductors probably don't need to be because there are going to be suitable for the load in the actual L building but the earth wire doesn't need to be considerably bigger than you would at first imagined because essentially what you're doing is taking the from the building in this case either one of these and you're extending this outside and therefore you're treating this as part of your house if it's got extraneous connected parts they need to be bonded just the same as the water pipe in the house or the gas pipe in the house or oil lines or whatever else you've got now unlike on the previous one if you're going to use that armored cable it is by far the most likely choice if you go to use a three core then that kind of means that the minimum size you can actually at least six square millimeters and in most cases probably 10 this also means that the line in boot will be 10 as well because all the conductors in there are the same size and you seem to need that because the earth wire here it's going to use as the bonding connector as well also cause you need that minimum of 10 square millimeters if you're going to use a two core cable this is where it goes a bit wrong because previously we could use the armor as the earth connection but in this case we can't because also we need to be the bonding conductor so we would actually need a minimum of at least 10 square millimetres equivalent on the armor and I think most armored cables have at least 10 centimeters of metal around the outside unfortunately it's usually made of steel which of course is not as conductive as copper and to get the same kind of effective from A to Z is copper you have to invent the surface area that's around eight times bigger and in virtually every case to core cable is not equivalent to ten millimeters of copper even on some quite large sizes so in this case you would have to use a three core and most of the time it's going to have to be a ten millimeters to allow for the fact that the third core you're using is the earth it also being used as a bonding conductor I'm gonna collect needs to be around the ten millimeter size and just as before you would still connect the armor to the earth as well except Lee was going to use the third quarter side as the conductor at least ten square millimeters now if you found that it was far too expensive to use this kind of stuff here then another option would be to ignore the earth connection coming in from the main building here and then just install your own earth rod here in the ground and then I will turn it into a TT system and that's always an option but it's a fairly poor option in most cases because they might even have to put this in the ground that's extra expense and cost in some types of locations you may need to have several electrodes or some kind of grid or something in the ground depending on the local conditions and of course that means you can have to have our CDs and things in here which wasn't sure it would trip in the event of some kind of fall to ground so a lot of extra costs and bother I know though using somewhat larger cables here may cost more in the end it's probably the easiest option and then while the earth here is going to be far more reliable and some piece of metal just jabbed into the middle of somebody's garden so tell me then if you've got a TT system in your house then the only choice outside of course is the same because that's all you've got so that's the easiest situation if you have TNS or TNCs in your house then in most cases you can just use those outside as well and then you'll end it with the same thing in your outbuilding the only thing to consider here then is if you have extraneous conductive parts and in this case you'll need to have bonding to those various parts which usually means that the earth conductor and also the bonding conductor is at least 10 millimeters 1000 millimeters in size so for these ones it doesn't actually matter it's just the size that the probit load whatever is size 4 there are extraneous connected parts then it will have to be at least 10 square millimeters in size and of course we'll need to connect those to the parts there and typically that's going to be the same things you would have say in your house so water pipes gas pipes and basically any kind of metal frame of a building plan say things like greenhouses or from those shipping containers and stuck in your garden use it storage or whatever so that's outbuildings and the varieties in most situations you can just use the earthing system from the main building and of course if it's a TT system well you've got no choice you have to put an electrode in empty our building but for T n type systems which is by far the most common and you can just use the same system as in the house it's going to use the two core arm and cable then the armor on that is in almost all circumstances easily big enough to be used as the earthing conductor so it's long a neutral inside and the armor as the earth conductor however if you're going to be having extraneous conductive parts in the outbuilding such as a water pipe that will require name bonding and you can't use the armor of the cable for that because in that case it's not big enough needs to be at least 10 square millimeters or the equivalent size in other metals and of course some steel enemies are looking for at least 80 plus square millimetres of steel around the outside which simply isn't the case on almost all sizes of armored cable but reasonably straightforward and of course if you have experience connected parts outside then there's need to be bonded in the same way as all those in the house and that'll get rid of them and therefore you wouldn't need to now I put links in the description of this video to a whole load of other videos which cover things like main bonding and earthing systems and so on so have a look there I'll see lots more information available one that until next time thanks for watching you

  1. Hi John great video as usual however i have a query. If a 6mm 3 core SWA was used couldn't you join the armour with one of the core wires to increase the eath rating? And if so would this meat the 10mm requirement?

  2. I would like to know what earth arrangement would be best so that a fault causing the RCD to trip in the shed does not also trip the RCD in the main consumer unit?

  3. John, I see in table 51 , 18th. A positive earth is coloured blue. Is this in any system nowadays ?
    I think I remember a positive earthed generator in the RAF , maybe in old cars or bikes at one time.

  4. in a TT system, one should not have more than one earthing point to avoid ground loops which depending on the fault can cause power to flow from one ground point to the other, which is why the electrical code in my country requires that all earth wires go to a earth bus bar within the DB, before a single earth wire goes to the earthing rod. No looping of earth wires is allowed.

    So by right, the outhouse should also have an earthing wire going back to the panel in the main building, if it was a TT system

  5. Particular care is required where conductive pipes and such items as telecommunication cable sheaths , covered walkways, etc may be continuous between separate buildings and thus establish a parallel earth/neutral path

  6. earth and neutral combined….this is confusing for the casual viewer (me) if the neutral is damaged in some way does this leave the whole kit a kaboodle "live"?

  7. Would the 'spare' core of a 3 core 6mm SWA combined with the armour be acceptable as equivalent to 10mm earthing? Great Video by the way as with all your content!

  8. Great video (as usual) thanks John.I came across this only recently so I think that the 18th
    edition change on bonding insulated section pipes may have overtaken the
    I can see that, for the TNC-S case with an open circuit PEN,
    the outbuilding feeder earth connection plus the 10sqmm connection to the water
    pipe could carry the entire installation neutral load current. I suppose this
    would only be so if the house water and gas pipe earth resistances were high
    and the outbuilding water pipe earth resistance was low.
    Wouldn’t it be easier to insert a piece of (insulating) hose
    pipe and avoid having to connect the water pipe to the protective equipotential
    bonding?I would be a bit reluctant to supplement the 6sqmm armoured cable
    earth core and about 23sqmm of steel armour in parallel. However, I don’t know
    if this arrangement would equal the current carrying capability of the 10sqmm
    bonding conductor? If the d.c. resistance of the armour is about 6.7ohms/km and
    the d.c. resistance of the 6sqmm copper is 3ohms/km with a current carrying
    capacity of about 42A, could we assume the armour will have a current capacity
    of about 3/6.7 x 42 = 18A? This would give a total rating of 42+18=60A – which
    approximates to the 60A rating of the 10sqmm single core.
    A bit shaky perhaps – A bit of hose pipe may be safer.Regards

  9. Hi John I’m doing an old outhouse up and need electricity in it the outside is 2 metres away from electric can you use power from a wall socket to a consumer board and do it in armoured cable or do i need to get power from mains board all its doing is running a frezzer and garden lights thanks again

  10. If you do not have Extraneous conductive parts, so you are only bonding socket and consumer unit boxes. Is it ok to use standard earthing sized cable?

  11. If I have a TT connection at my house, couldn't I use a normal 3 core cable to connect the earth from the TT connection and use it as the earth in my shed? Or would that be a stupid idea?

  12. If the water supply coming in to the outbuilding is MDPE (plastic) is there still a need for bonding and therefore an Earth conductor with 6/10mm cross-sectional area?

  13. man i miss the m.e.n. (multiple earth neutral) systems of nz and australia, what sort of earthing system have i got… hmmm, m.e.n., what cable shall i run?? single core neutral screen, and chuck down a stake, easy as. 😀 i detest friggin swa worst cable ever!!!

  14. Hi JW. On a TN-C-S (PME) system (and a TN-S) if you are exporting the DNO's earth externally to out-buildings etc are you not required to obtain their permission, after all it's their earth and they assume it ends at the intake and rightly so? I would assume this is not good practice in any scenario to export a TN earth, whereas creating a TT system at the outbuilding I thought ticks all the boxes. Similar scenario to EV Charging Points really. Great videos by the way, love your work, keep it up.

  15. Hi JW,
    If the outbuilding is less than a meter away from the main building would you still need another earth electrode (assuming the main building is TT, and the cable supplying the outbuilding is RCD protected at the main building end)? The circuit into the out building would be supplied via 3 core 2.5mm SWA (from a 16amp MCB) and be supplying a 2.5mm radial circuit of not more than a few meters in length.

  16. John, I am setting up a battery system with an inverter in a metal shed for some stables would it be a good idea to put in an earthing system? and as I am dealing with horses and the danger of power to the horse from the earthing?

  17. I solved the problem of getting power to my shed for lighting by fitting a 12V solar system £100 for a 40w panel & charge controller, £15 for two 12v 10w LED floodlights and a used car battery £0.00. Six years later and it's still working fine. It would have been too much hassle to correctly run a mains supply to the shed which is mostly galvanised steel.

  18. Great videos. Really informative. Would i be able to use the 10mm2 earth cable separate for a hot tub outside? Or does that definitely need to be set up as a tt system? Im on TN-C-S

  19. Hi jw just wanted to ask my kitchen sockets and upstair sockets are in 1 ring …but i want know is it possible to seperate a boile rsocket line from this ring to a circuit of its own and how would you do it .

  20. Hi John what if there was a fault on DNO side regarding a loss of Neutral and earth on a TN-C-S earth system. Wouldn't a TT be safer especially if you had a water supply in the shed or the shed was constructed with metal frame work.

  21. Great explanation, JW,
    I have a question regarding the earthing an outbuilding with a secondary earth electrode on the same property.
    If you had this arrangement, is there any chance of circulating earth currents occurring from either electrode causing RCD's to trip in either the house or the out-building?
    Over here in New Zealand, we use the MEN (Multiple-Earthed-Neutral) system of supply, which is a variation on the TN-C-S system used in the UK.
    If you are running a supply to any other out-building, we normally remove the Neutral-Earth link in the out-building switchboard to prevent nuisance tripping of the RCD's in the out-building, as opposed to driving a second electrode.

  22. Hi John,

    I made an installation in a wood cabin at my uncles property.
    For gas and water we used PE plastic pipes for the underground runs, and switched to copper pipe after coming inside the cabin.
    Should these copper pipes be bonded to earth as extraneous conductive parts?
    Or since they are 100% inside the cabin are they potentialfree?
    We had an elektrician connect up 3phase run for sauna heater, he has not mentioned bonding at that time.
    So this is just curiosity.
    Nice video, thx

  23. Thanks John for another useful video.
    i think this is a grey area as the regs focus on caravan sites, outbuildings are not detailed as much. If i understand correctly the reason that caravan installations are regulated as TT systems is because the ESQC regs say it should be so, the reason is because of the danger of imported earth voltages. Fuel installations likewise
    the local authority i work for sent me to inspect and test an installation where a power socket unit was installed outside at ground level. this had been installed as a TT system with 30ma rcd protection from a kiosk which had a TNC-S supply available.
    personally i agree with you, if you have an earth use it, but ensure periodic inspection and testing takes place frequently.
    the disadvantage of TT systems is that they can change impedance values due to changing ground condition caused by weather conditions.
    would you have installed a power socket arrangement as TT or a TNC-S?

  24. Thanks John really useful video. I am installing a feed to the garage, I have bought an earth rod and also have 4mm 3 core armoured cable supply. After watching your video I realise I have no extraneous conductive parts and the 4mm earth supply would be suffice. My question is should I still install the earth rod as I've already purchased it so there will be no additional cost or is the 4mm house earth a better option?

  25. Hello JW.
    I've been asked about 2 similar installations which I would appreciate your input on; both are rural houses with TT earthing. The first house has a dual RCD CU fitted and there are 2 small sheds very close together (5mtrs apart) which are about 12mtrs from the house (the cable run is about 20mtrs). Neither of the sheds have extraneous metalwork.

    The first shed will have one inside light and one outside and one double socket inside as well as an outside socket; this shed has a 3 core 1.5mm SWA from the house CU area to the shed already installed (I would have chosen larger but it should be OK) and another SWA to the second shed which just requires one light and a double socket. These SWA cables have not been connected yet but they will be terminated in steel boxes with the proper glands etc and T+E (2.5mm) tails (in plastic conduit) run to the CUs each end. The owner of the house has bought a metal garage CU for the first shed fitted with 1 x RCD, 1 x 16 amp MCB and 1 x 6 amp MCB.

    It is proposed that the SWA at the house end is going to be connected to a 16 amp MCB which will be on one of the house RCDs (both are 30mA) that split the house wiring, this is going to connect to the RCD in the first shed CU and – via the 2 MCBs – supply lights and sockets (radially) and the SWA to the second shed was also going to be connected to the 16 amp MCB in the first shed CU. In the second shed the SWA will connect directly to the socket (via termination box) with the light running from a fused spur (3 amp) off the socket. The second shed was going to get its earth from the first shed via the SWA 3rd core and armour.

    The second house is similar situation so these questions are relevant to both installations.

    Can the SWA between the house and 1st shed have its armour connected both to the house earth and the shed earth so linking both stakes? If it can only be connected one end which should it be?

    This has caused some discussion between my commercial and domestic electrical friends! One says that if the SWA is connected both ends it will cause nuisance tripping and says it should be connected only to the shed end earth whilst another says it should be connected only at the supply end (house). I think it can be connected to both ends as both are TT installations (house and shed) and 2 stakes can only improve the earthing. There is no danger – as there would be with a PME house and TT shed – of the street loosing a negative and earthing on mass to the shed rod!

    Is the doubling up of RCDs likely to cause problems beyond both the shed and a house RCD tripping at the same time if the shed has a fault? Inconvenient but not a real problem. It is not desirable to change the rating of a house RCD to 100mA – S type for safety reasons.

    Is the shed RCD even necessary? These garage CUs always come with one but would the space in the CU be better used for another 16 amp MCB for the second shed. Although that gives a load beyond the SWA it is still connected to just on one 16 amp MCB at the house end. There must be an earth trip on the SWA at the house end (even if it is shared) for safety in case of a breach of the armour by something sharp and metal… like a water board digger; that's what happened to an SWA at a neighbours house! I don't think there is room for a third RCD in the house CU on this installation although a 16 amp RCBO might be able to be fitted and powered directly.

    The second house has a main workshop and 5mtrs away another small shed with a 25mtr cable run from the house CU to the main workshop. The house is small with an older (TT) installation with just a simple CU and trip, although this may have to be changed to a metal dual 30mA RCD CU when the work is done and if so this could probably have room for a 3rd S type 100mA RCD just for the out-buildings; which makes sense of also having a 30mA RCD in the workshop CU.

    It is proposed that the workshop will have a 6mm SWA 2 core from the house CU off a 30 amp MCB and a decent sized CU (12 way or more) will be installed in the workshop for several circuits, a ring, lights and some radials for specific low wattage equipment; the workshop will have it's own earth stake. The small shed just needs a light and no sockets and will have a 1.5mm SWA linking to the main workshop CU on it's own 6 amp MCB. The workshop is all timber and has no extraneous metalwork but the small shed is metal clad.

    As all cabling is going into 40mm underground ducts to facilitate possible future changes, so a larger (10mm) earth could be run between the sheds, but is it necessary for one plastic bulkhead light? All wiring within the shed (and workshop) will be run in plastic conduit and the switch could be an external plastic type so effectively it would be all double insulated.

    Just how a shed completely clad in old corrugated steel sheets (walls and roof but it is a timber frame) could be bonded is a subject within itself. The shed is going to be dry lined inside with no metal visible or accessible but the external steel sheets are galvanised and now worn and rusty and probably don't make great electrical contact with each other where they overlap; would a bond cable have to be bolted to each sheet in turn? There's probably over 50 sheets! I suppose it will have to be done because of the danger of the cladding not being bonded and a trailing extension from the workshop being used outside for an earthed device/tool used within touching distance of the cladding.

    Any advice would be appreciated as I'm getting conflicting information from different electricians I know particularly about earthing the SWA both ends or which end; I would prefer to see the stakes linked so the houses and sheds are on the same earth equipotential zone, just in case of trailing extensions for garden equipment etc as the buildings and houses on both installations are reasonably close together (the steel clad shed is physically only about 7mtrs from the house). On both installations the earth stakes will be large (8ft long) at both ends (house and sheds) so hopefully they will test well… (If not we'll bury a steam roller!)

    I've done commercial electrical work (factory, offices, theatre lighting) but not for a long while as I had been doing commercial comms for 20yrs before getting too ill and old, but I'm trying to choose which friendly electrician to get in on these projects for friends. Apologies for this turning out so long!

    Many thanks, Peter

  26. If you only need modest power requirements for your outbuilding, say lights and a pond pump, then treat the outbuilding as an appliance.
    Simply connect to the house supply via a standard plug in a convenient socket. Your outdoor wiring is no longer a permanent installation and is treated as an extension lead. Obviously, the connecting cable should be suitably stout to handle the max possible load rating of the plug, even if only as a fault condition. I would still use a small consumer unit in the outbuilding regardless, and protect the connecting cable.

    Not what you are teaching here, John, I know, and thank you for it, but jogged my memory and I believe is worth keeping in mind.

Leave a Reply

Your email address will not be published. Required fields are marked *