Multiple external sheds/buildings

[DaveSpils]

Hi

 

I need to provide power to 3 external buildings - a small flat and 2 barns - without the need for 3 separate SWA cables. In the main house, I already have a 100A 2 pole isolator switch between the meter and a Henley block. From the Henley block tails supply the power to the CU for the main house. I plan to have a 16mm2 SWA also from the same Henley block to feed power to the flat. In the flat, the SWA will enter a Henley block. From this Henley block, I will then have a feed to the CU of the flat and the other feed to the first barn. Same in the first barn where it has a Henly block and again 2 exits from it to the CU in the barn and the other providing a feed to the second barn. In the 2nd barn, the SWA simply goes directly to its CU.

 

So, there is a "continuous" 16mm2 SWA cable that provides power to all 3 buildings with a feed into each CU per building from a Henley block (I have estimated the expected total load to be lower than 75A, which is the operating limit for 16mm2 underground). I would like to know if this sounds acceptable? I may even add an additional 2 pole isolator switch in each building to isolate each without affecting any other building downstream.

 

The other query I have is earthing. The main house is a TN-C-S system. The flat is brick and the barns are wood (all on concrete bases). There is a water supply to the buildings and therefore 10mm2 bonding will be used. I assume it is fine to use an "exported PME" - to use the earth provided by the supplier (the earth in the house), rather than using a TT earth local to the buildings.

 

Sorry if not clear, I can provide a drawing if required.

 

Thanks

 

Dave

[DaveSpils]

[Jeremy Harris]

Welcome.

 

What's protecting the 16mm² SWA from over-current?  It needs a fuse at the isolator, as you're only permitted a maximum 3m run of 25mm² tails from the meter without additional protection.

 

Exporting the PE can be OK, depends on the distance and the risk from a PE - N fault on the incoming supply (which will now include the run of SWA).  For example, I ran a ~15m length of 25mm²  3 core SWA from our meter cabinet to the house, with the SWA terminated at the house end at a metal enclosure, with 25mm² tails from that enclosure to the CU (I just double sleeved the SWA cores to create tails, rather than make an additional connection).  The PE is imported via both the CPC in the 25mm² and the armour.

 

For our detached garage supply, and my car charge point, I ran 2 core SWA from an external CU next to the meter box, with the armour isolated at one end.  Those two circuits are both wired as TT installations, with the appropriate RCD protection. 

 

The final check would be to see what the voltage drop is going to be at each end point, for the typical max demand that's predicted.  If the cable run is long you may need to increase the size of it from 16mm² to 25mm², perhaps more.

[ProDave]

What worries me about this is the TOTAL load.

 

You estimate the total load of the outbuildings to be 75A.  That only leaves 25A for the house it is all fed from, so not much at all.

[Carrerahill]

10 minutes ago, ProDave said:

What worries me about this is the TOTAL load.

 

You estimate the total load of the outbuildings to be 75A.  That only leaves 25A for the house it is all fed from, so not much at all.

Exactly my thoughts.

 

The OP is now into the realms of a substantial sub-mains here, something that shouldn't really be thrown together, this needs careful consideration and design before implementation. 

 

 

Edited April 7, 2020 by Carrerahill

[DaveSpils]

Thanks for the replies.

 

@Jeremy Harris The "Meter --> Isolator --> Henley --> CU" are all contained in the cupboard within the house so we are talking "cm" not "m" in length of the tails. The isolator was put in by an electrician to provide a SWA to a charge point for an electric car we once had (7KW supply). We no longer have this and will disconnect that SWA from the Henley and use it (potentially!) for a SWA to the flat - this is where the proposed 16mm2 SWA starts (coming out of the first Henley block in the diagram). But you are correct, there is nothing protecting the 16mm2 from overload, other than the RCDs within each CU of the 3 buildings (I assume the SUM of the RCDs should be less than 75A for 16mm2). However, in air, the maximum current is 99A (75A under ground) for a 16mm2 - which it will be between the house and flat (flat to the barns is underground). Still, same applies, it needs to be protected so it is impossible to draw more current than it is designed to deal with. What do you recommend?

 

The distance is quite far. The flat is about 15m away (in air); barn1 is about another 50m (mostly underground) and the final barn a further 15m (underground) - so these are the distances between each Henley block in my diagram and the final Henley block and CU in Barn2. So the total length from house to barn2 is about 80m of cable. May be 25mm2 is the correct size for this length taking into account the voltage drop.

 

@ProDave In terms of demand, it could be quite high if all 3 buildings were at maximum draw - but this will not happen. Barn 2 is low load (lights and sockets) but Barn 1 and the Flat are potentially high load (in addition to the main house). However, it needs to be safe. The cable that supplies the power to the house I assume has a very high working maximum current - but what is that typically? So long as that is larger than what I need (with all buildings at their maximum), then I need to ensure each feed has the correct SWA with appropriate protection against overload.

 

The flat is currently fed with 10mm2 cable from a MCB in the house (40A). It has never tripped despite there being an induction hob and oven (each can draw up to 30A maximum), plus small fridge, kettle and TV and the obvious (lights and available sockets). However, I assumed it would be better to take the feed from the Henley block and not from the 40A MCB within the CU of the house (please correct me if I am wrong!). The CU in the flat can draw up to 63A (theoretically) before the RCD trips but it will trip in the house way before then at 40A - which it never has. Taking the feed from a Henley block instead means the flat could draw up to 63A and the existing cable (10mm2) would still be adequate but I plan to change to 16mm2.

 

Barn 1 will be used for my home brewing hobby and will include 2 water heating elements rated at a total of 5KW, so ~20A in total, plus lighting, fridge and sockets available for miscellaneous. The elements will only be on when brewing (2-3 days per month at most) and most of the time, only 1 element will be in use (~3KW). Otherwise, very low load.

 

Barn 2 will be very low load - lights and sockets for miscellaneous use.

 

Also, sounds like a TT setup is advised but would I need this for all 3 buildings? Currently, the flat uses the earth from the house and never had any issues. But Barn 1 and Barn 2 are much further down the garden and therefore does this mean a TT is advisable for these? 2 foot down in my garden and you hit bedrock - is this an issue for the earthing rod? I assume that if a TT is used, only 2 core SWA is needed (would still connect the SWA to the earth from the house but isolate at the other end).

 

Dave

[Jeremy Harris]

The over-current protection for the 16mm² SWA needs to be at the supply end, so a DP fused switch with a 60 A fuse, like this: https://www.tlc-direct.co.uk/Products/CGFS100.html  This is required to protect the cable. 

 

RCDs don't provide over current protection, their job is only to provide residual current protection should there be an earth leakage fault.  An RCBO can provide combined over current protection and residual current protection.

 

One way to calculate the total demand is to assess the loads within each circuit, apply the diversity rule, and then see what the total may be.  An acceptable alternative is to measure the load over a representative period of time, using a peak hold clampmeter, and then use that as a guide to the total load.  The diversity guidance is contained within the OSG, if you list the appliances and their maximum ratings, together with the circuits and their rated current, one of us can have a stab at doing the sums.  The diversity rule works on the basis that it's unlikely for some appliances to draw their maximum rated current for long periods, together with the fact that it takes time for any overload protection device to open, if the overload is only modest (can take hours for a modestly overloaded fuse to blow, for example).  Some circuits/appliances cannot have diversity applied, as they draw their rated power for long periods of time.  Examples are water heaters, car charge points, electric heating etc. 

 

The choice as to whether to export the PE from the incoming supply to a distant outbuilding depends on the distance, the protection provided to the cable and whether or not there is an enhanced risk of shock from exposed conductive parts at the remote end.  For example, a car charge point will often be wired as a TT installation, with an earth electrode close to the charge point, as this ensures that the exposed bodywork should remain close to the local ground potential if there was a fault.  My garage is wired like this, as it has a concrete floor and metalclad outlets, so I wanted to be confident that the local ground would be close to the same potential of the exposed conductive parts in the event of a fault upstream from the installation.

 

The voltage drop for 50m of 16mm² 3 core SWA buried in the ground will be around 7.5 V at 60 A.  This is just about acceptable, but 60 A is close to the limit for 16mm², which seems to be about 67 A (according to the TLC voltage drop calculator: https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html ).  SWA current ratings do vary a bit from one supplier to another, though, for some reason (often related to the temperature rating of the cable).  Often the limit will be the lower maximum temperature that the fittings at the end of the cables will tolerate.

[DaveSpils]

Does this diagram make more sense?

 

 

1. Remove my current 100A isolator and get a new fused 100A isolator (as advised) and position between the Meter and Henley block in the house.

2. I know the flat has never consumed more than 40A and so happy with the current set up of the 6mm2 3-core SWA supplying power to the flat, which also uses the earth from the main house (exported PME). So, I know long-term, 40A has never been reached. In air, 6mm2 SWA good for 56A so I see no issue with this.

3. From the same Henley block in the main house, run 25mm2 2-core SWA to a Henley block in Barn 1 (3 core not needed).

4. From this Henley block, go the the CU in Barn 1 and CU in Barn 2.

5. Both barns use a TT earth.

 

The Total Load (main house + flat + barns) cannot exceed 100A due to the fused isolator and if it does, ALL power will be shut off. This is the limit of the CU in the main house also and protects all the SWA cables.

 

If the voltage drop is not too severe, I could even use 2-core 16mm2 as the Total Load for the barns will be way below capacity of this cable. But, I would need a second fused isolator (63A or 80A) to protect this segment. Could I position this isolator in Barn1 just before the Henley block (shown by the black circle with a question mark)?

 

Additional question - typically, what is the maximum current that can be drawn into the property before the main fuse is blown? I assume this is pretty standard across most "normal" properties.

 

Thanks

 

Dave

 

 

 

Edited April 7, 2020 by DaveSpils
keeps adding other images!

[PeterW]

It’s worth isolating on every end of an SWA. At about £16 each, a double pole isolator is not expensive and really important. 
 

 

[Jeremy Harris]

You can't easily terminate SWA in a Henley, needs a metal enclosure to accept the cable gland, as the cores of the SWA must either be enclosed or have an additional insulating sleeve fitted over them so they are double insulated if exposed (as they probably would be when going into a Henley).   You can do as I did and fit an SWA gland to a metal enclosure, then double insulate the SWA live cores (line and neutral) and run those to a CU just like meter tails (meter tails come pre-double insulated).  This saves additional interconnection points, which is generally a good thing.

 

You need to do the max load calcs, both to size the cables, etc and to determine whether the total load is within the incomer rating.  This means adding up all the circuit loads, applying diversity where applicable, and then demonstrating that the total load won't exceed any of the ratings.  This isn't optional - whoever signs this off (and it's notifiable work under Part P) will be signing to state they have assessed the total load.

 

You may need to ask the DNO for the incomer rating.  Incomer fuse heads are now often marked "100 A", but that isn't necessarily the rating of the fuse.  It's not at all uncommon to find a 100 A carrier fitted with a 60 A or 80 A fuse, I've even seen a 40 A fuse fitted once.  You cannot cut the seal and check the fuse, that has to be done by the DNO, or, perhaps, a meter fitter from the supplier, if they are there to replace/fit a new meter.

[DaveSpils]

@PeterW Many thanks. Do you mean at the position already highlighted (black circle with ?) and also one just before the CU in Barn2?

[DaveSpils]

@Jeremy Harris Many thanks for this. I incorporated a Henley block as a way to "split" the SWA in 2 directions (to the CU and continue to the next building). I understand how I can connect it to a metal box using  a cable gland but how do you take 2 feeds from this, one for the CU and the other to go to the next building? I may be misunderstanding this - sorry if I am!

[Jeremy Harris]

Just now, DaveSpils said:

@Jeremy Harris Many thanks for this. I incorporated a Henley block as a way to "split" the SWA in 2 directions (to the CU and continue to the next building). I understand how I can connect it to a metal box using  a cable gland but how do you take 2 feeds from this, one for the CU and the other to go to the next building? I may be misunderstanding this - sorry if I am!

 

 

You could add a Henley inside an enclosure, then run either tails or other runs of SWA from that.  You need the enclosure so that the SWA cable glands have something to fit to, and to protect the single insulated SWA cores from being able to be touched.

[DaveSpils]

@Jeremy Harris could I ask a favour of you please? Do you have a picture/example of an adaptable box with a Henley block (or similar) showing one incoming SWA and 2 outgoing SWAs? I have searched for one and cannot find any. Does the Henley block get fixed inside the box or is it loose? Is there another solution to this?

 

thanks

 

Dave

[Jeremy Harris]

8 hours ago, DaveSpils said:

@Jeremy Harris could I ask a favour of you please? Do you have a picture/example of an adaptable box with a Henley block (or similar) showing one incoming SWA and 2 outgoing SWAs? I have searched for one and cannot find any. Does the Henley block get fixed inside the box or is it loose? Is there another solution to this?

 

thanks

 

Dave

 

 

I don't have a photo to hand, but I've done it in the past and it's easy enough.  just fit glands in the box for the SWA and fix a couple of Henleys and an earth block inside, screwed through the back to the base board.  Needs one of the larger adaptable boxes so there's enough room, but easy enough to do.  I always tend to use piranha nuts on SWA glands now, as they are a lot easier than faffing around with brass banjos. 

[Jeremy Harris]

Just found this photo showing a bit of 25mm² SWA terminating in a small 4 x 4 adaptable box, with tails fed out to the CU.  There's no Henleys inside, just an earth block to terminate the armour and cable CPC and allow a 10mm² CPC to be fed out to the CU, along with the sleeved live cores from the SWA.  Worth noting that there's a slot cut between the two grommets where the tails come out, to break the magnetic circuit that would otherwise be formed by the steel of the box.   The meter in this photo is a private one, not the suppliers.

 

 

[DaveSpils]

Thanks for the picture. I understand that at Barn 2 would look similar to this as the SWA is terminated. I just cannot envisage what it would look like inside the adaptable box for where I have a Henley Block. Is there a fixing for a Henley Block inside the adaptable box? Sorry to labour the point. Here is what I think is needed:

 

I just looked at the Piranha nuts too - much easier.

[Jeremy Harris]

Best to use 3 core SWA if exporting the PE, IMHO.  That way there's a high degree of integrity in the connection, as it's not reliant on the galvanised steel wires always providing a very low impedance connection.  If the installations are going to be wired with earth electrodes, as TT, then the CPC from the CU in that diagram would go to the earth electrode and the armour CPC would be isolated from the installation.

 

Inside the box (would need to be a reasonably big one) you'd just have two Henleys (one for line, one for neutral) plus an earth block.  The earth block needs enough ways to accept the various CPCs, so for the set up above it would need 5 ways.  If really stuck for space you could use a double decker Henley, but personally I'm not a great fan of them, and much prefer to keep the line and neutral terminations separate.

[DaveSpils]

Yes, you are of course correct with respect to the earthing. My diagram was a little generalized... If you recall from my earlier diagram, I plan to leave the flat as is fed by a 6mm2 3-core SWA and therefore will use the CPC from the consumer unit in the main house to provide the earth for the flat (as it currently does). However, the feed to the barns (because of their distance), I planned to use 16mm2 (or 25mm2??) 2-core SWA and use a TT earth for those. Therefore, the SWA for the barns will be isolated from the TT earth as it will be connected all the way back to the CPC in the consumer unit in the main house.

 

Does that make sense?

Dave

 

 

 

[Jeremy Harris]

It depends very much on the total load, both on the installation at the end of each sub-main, and on the incomer.  This needs to be determined and stated on the lodged EIC by the competent person undertaking, inspecting, testing and signing off the work, so before getting materials a few things must be done.  Firstly, the actual rating of the incoming supply needs to be determined.  This could be as high as 100 A, but may be as low as 40 A.  The DNO will do this on request (they have to do it, as the fuse is their property).

 

Next the total load at each installation (in this case the house, flat, barn 1 and barn 2) needs to be determined, or measured in a representative way.  Bear in mind that fuses don't rupture at all quickly for modest overloads, so you cannot use a fuse rating to determine the maximum load (for example, a BS88 100 A  fuse can take about 10 minutes to rupture at 200 A, or maybe 5 hours to rupture at 150 A).

 

For an existing installation it may be possible to do a 24 hour peak load measurement to get a feel for the true load, which is OK if done at a time likely to be representative.  The other way to do it is to calculate the load, using the expected circuit loads, allowing for diversity where applicable.  For example, here's a sample calculation for a fairly typical domestic installation:

 

2 off ring final circuits, with 32 A over current protection

2 off lighting circuits, with 6 A over current protection

1 off cooker circuit, with 40 A over current protection, supplying a cooker rated at 7 kW maximum, with no additional 13 A outlet

1 off immersion heater circuit, with 20 A over current protection, supplying a 3 kW immersion heater load.

 

The two ring finals are calculated as presenting a load of 100% of the total up to 10 A, plus 50% of anything over 10 A, so would be 10 A + (50% x (32 A - 10 A)) = 21 A each, 42 A in total

The two light lighting circuits are calculated as presenting a load of 66% of the total, so 6 A x 66% = 3.96 A each, ~8 A in total

The cooker circuit (7 kW / 230 V = ~30.4 A), is calculated as presenting a load of 10 A + (30% x (30.4 A - 10A)) = ~16 A in total

The immersion heater circuit presents a total load of 3000 / 230 = ~13 A in total (diversity cannot be applied to water heaters or electric heating)

 

Adding up the total from the above calculations gives a total load from this installation of ~79 A

 

There are ways to reduce the total load by using load priority devices, or by restricting the maximum load on some circuits.  I suspect you may well have to consider doing this if your total load looks as if it will exceed the rating of the incoming supply.  The competent person that must do this work and sign it off (this cannot be DIY'ed, I'm afraid)  may be able to suggest ways to keep things within limits and safe, if this is practical to do.  It's possible to reduce the rating of ring final circuits, perhaps, if the loads are only ever likely to be fairly low, or it may be possible to wire one or more of the sub-mains with some form of load limiting or priority load relay system.