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Kenny.s

Hi, first post so please accept my apologies for going straight in with a question.

Sorry if this is a bit long winded but I thought knowing the full story may help.

We bought an old semi detached cottage about 7 months ago which is at the end of a 320m overhead LV supply. Initially this consisted of a single phase 30mm open conductor for about 260m dropping to 16mm between the 2nd last pole and our neighbours property. All told there are 5 properties fed from this supply.
Needless to say the volt drop we experienced was quite significant.
A call to the DNO resulted in 3 engineers coming out to check the lines etc no issues found other than the 25kva Tx had a 200A fuse in it!

Voltage monitors where fitted and confirmed that the supply was way below what it should be. I got a letter and a phone call to say that it would be upgraded. At that time I expressed an interest in having a 3phase connection to my planned workshop and if needs be I would be happy to contribute towards the cost of further upgrading the equipment to ensure I got a good solid supply. I was assured by the engineer assigned to the job that I would be able to have a 3 phase connection on completion of the upgrade but I would obviously have to pay for the new service cable etc.

Fast forward a few weeks and they had scheduled the upgrade, I was quite impressed by how quickly they had acted on this and had hoped it would be a straightforward job.
They installed a 3phase 50kva Tx and 50mm ABC, dropping to 35mm for the last span. The first 3 houses share a phase, my neighbour has a phase to himself and so do I. I assume they did this because we're the furthest from the TX?

No sooner had they left I applied some load on and low and behold the volt drop was still quite significant.
The resulting phone calls over the next few days revealed that they hadn't done any calculations, the guy in charge had basically taken a guess! By they're own standards 50mm was too small for this job.
Several months passed before they finally arranged to come and put up the 95mm. The volt drop measured at my cutout is now approx 0.31v/A whilst not great its bearable.

I recently sent in the application for the 3 phase connection asking for 42kva as advised by the engineer (now a different guy) I thought this seemed a bit high given its only a 50kva TX but he assured me it would be fine.
Next day I got a call saying that I couldn't have 42 or I would need to pay for a 100kva TX. I said to reduce it to 30 or even 25. I got the quote through yesterday stating I could only have 16kva evenly split across the 3 phases!! That's 2kva less that I'm allocated just now.

It would therefor seem that they have cut this to the bone when the did the upgrade if they can't even give me another 7kva, its almost as though they're trying to claw back some load?

Basically after that big story, what I'm asking is, how do they apply diversity when calculating how big a supply TX needs to be?

Many Thanks,

Kenny.
 
Diversity can be applied to much higher levels the further upstream you go in the supply network. At the point of generation the diversity applied is massive. The DNO will have their own diversity figures they work to and they'll be far higher that those applied within a single domestic premises. I'm suspecting your voltdrop issues are due to cable lengths rather than high loads.
 
Thanks for the reply Marvo, The volt drop is now much less than it was and I could live with it but its why they are wanting to reduce the capacity of my supply in exchange for a 3phase connection that I can't understand.
I suspect the TX is already under capacity even once a large amount of diversity is applied hence why they're trying to reduce my supply?
 
I was talking to a DNO worker about the supply cables, where they ran etc. he told me it was common to have a diversified load of 5 amps for each house, and many cables were less than 10mm to the customers cut out.
Clearly it isnt a problem to the vast majority of users, but, like the OP, if you have some heavy drawing equipment, you may have to pay for the upgrade, which will be far more than you expect - it isnt just your local supply, they may have to upgrade back to the substation, or, if on a pole, the transformer, which, as you have found, will require a rather large financial input from you.
 
Yeah, I'm not sure why they're offering you such a small 3 phase supply. The problem with small 3 phase supplies is you can't run any large 230v appliances such as a shower. Without working it out exactly 16KVA is only going to give you just over 20Amps per phase which would make life awkward with a domestic supply in a country where a standard socket circuit is a 32A ring.

Sounds to me like they've got their local infrastructure stretched to the max already and are reluctant to increase capacity consumer side without further upgrading back to the MV supply. You may be able to force their hand into upgrading at their expense if you monitor your voltage and it still regularly drops below the allowable limits especially during peak demand times.
 

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