TT Supply Multiple Buildings, Where to Start...

[CaptainB]

Hi All,

I'm not a sparky so go easy. However, I did study electrical and electronic engineering way back so I'm not completely clueless.

We bought a place last year and inherited a pretty shoddy (in my opinion) electrical system. We have a TT supply which comes to the house. The tails split after the meter, one way goes to our domestic CU, the other into a stand-alone RCD. A sub main comes out of the RCD with 25mm SWA and strung across the road (I'm guessing no permission sought!) to an outbuilding (insulated and heated etc) where it splits again into a CU for this outbuilding and a CU for the attached holiday cottage. The CU in this outbuilding also then feeds 2 SWA cables via an RCD to a garage/workshop and a shepherd hut which is currently under construction. The whole system only has two earth electrodes, one at the house and one at the holiday cottage. I think I need more and would like to put one at the garage and one at the shepherd hut too, possibly even one at the outbuilding. Still with me? I've drawn up a diagram - a picture being worth a thousand words and all that - but I can't post it yet as I don't have enough posts. I'll try and link to it here.

Clearly the supply is being stretched, especially in winter as the holiday cottage has electric heaters, shower and immersion. I also think that there's been little regard for forward planning and that all this has been cobbled together on the hoof. Having surveyed the existing kit I am thinking that my first move should be to have a new supply installed so that I can seperate the house from everything else and do away with the overhead SWA. I have set the wheels in motion for this but now I have to make sense of everything else.

I am getting a couple of local sparkys in to give me their opinions but they are backed up for weeks at the moment so I thought I'd give it some proper consideration myself and have some idea of what they are likely to propose.

My concerns are:

• It seems to me that there are multiple RCDs in line and it's anyone's guess as to which will trip first. Is it an issue to have multiple RCDs like this? Something popped in the garage the other day and the RCDs in the garage and the outbuilding didn't trip, the stand-alone one in the house tripped which knocked out everything on the sub-main - a bit annoying.
  
• Do the SWA runs to the garage and shepherd hut need to be RCD protected given that there are RCDs in the CUs in both buildings.
  
• There's no S-Type RCD anywhere in the installation. I believe that there should there be, am I right? I do have a LUGS unit in the house CU, whats a LUGS unit?
  
• As I mentioned earlier, I think there's a need for more earth electrodes.
  
• Is there still too much load on the new supply? I know I have to do a diversity calculation but with electric heaters, oven/grill/induction hob, 2 electric showers, immersion heater, occasional woodworking machinery use and all the other stuff is it still likely to be too much draw on the supply?
  
• I'd like to replace the old CU in the house and liked the idea of all RCBOs. Is there any problem using RCBOs on a TT supply if there's a double pole S-Type RCD fitted?
  

Any answers, ideas or suggestions would be most welcome to get me moving in the right direction.

Cheers!

 

[ProDave]

Forget the earthing, that is the least of your problems.  Total load worries me more.  I counted THREE electric showers on your drawing.  That is going to grossly overload your one incoming supply if 2 or more are used at once, and particularly with a holiday let, that is very likely.

I would be looking to get the DNO to provide a new electricity supply to the holiday cottage (and get rid of the overhead wire over the road).  If you can avoid electric showers I would. An unvented hot water tank and thermal mixer shower gives a far better shower in any event, and a much lower electricity demand even if heated by the immersion heater.

What did the EICR say before you bought the properties?

What does the electrician that's going to wire the shepherds hut say?

CaptainB's drawing:

 

[Bruspark]

Yep   Separate supply to the other side of the road and get the overall design checked and updated as suggested.   

 

[CaptainB]

Forget the earthing, that is the least of your problems.  Total load worries me more.  I counted THREE electric showers on your drawing.  That is going to grossly overload your one incoming supply if 2 or more are used at once, and particularly with a holiday let, that is very likely.

I would be looking to get the DNO to provide a new electricity supply to the holiday cottage (and get rid of the overhead wire over the road).  If you can avoid electric showers I would. An unvented hot water tank and thermal mixer shower gives a far better shower in any event, and a much lower electricity demand even if heated by the immersion heater.

What did the EICR say before you bought the properties?

What does the electrician that's going to wire the shepherds hut say?

Thanks for that ProDave and apologies for my delay in getting back to you.

Yes, the total load has been a big concern. The holiday cottage (which sleeps up to 4 people, though it's usually only 2) has two bathrooms, one of which has an electric shower and one has a shower fed from a bog-standard vented cylinder with an immersion heater. We have thought about doing away with the bathroom which has the electric shower altogether and in addition to that we thought we'd probably replace the cylinder with an unvented type for greater efficiency and better flow rate (the existing header tank is only 2ft or so above the cylinder so the flow is pretty dismal!).

I don't recall seeing an EICR when we bought the property. To be honest, not being an electrician and with so much else going on at the time, I didn't pay too much attention to the electrical system other than to note that the house would probably need it's CU updating sooner rather than later and that everything else had what appeared to be 17th edition CUs. With everything looking modern I think I mentally dismissed the electrics as being something I didn't particularly need to worry about.

Other than to oversee the laying of the SWA to the shepherd hut the electrician hasn't been back and I haven't really discussed the big picture with him. Re the shepherd hut, at the time we were unsure if we were going to utilise a gas water heater or electric shower/water heater so a 10mm2 cable was run to keep our options open. I've been looking at small storage water heaters (30-50 litres). These seem to offer the best solution in that there's no huge current demand and they can be piped up to the shower, basin and sink. I know that the amount of water on hand isn't huge but it should suffice for a 5-6 minute shower.

I am going to call one of our electricians tomorrow and see if I can get him to come over for a consultation. It will be interesting to see what he suggests.