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Waveform/wavecon cable sizing

Just had a quote in from Western Power for a new 100a 3 phase supply. There is approximately 100m between the pole mounted transformer and the cut out and the DNO has specified 185mm wavecon cable.. this seems like an overkill to me, the pole transformer is 50Kva and it seems like 95mm would be more appropriate!  Not sure what voltage drop on waveform cable is like but compared with regular SWA 95mm still seems plenty big enough.. Thoughts on this would be appreciated! Thanks




  • A generous cable size is good practice to reduce voltage drop at the load end, and in particular to avoid obtrusive lamp flicker when motors or large heaters such as electric showers are switched.

    Also as this is a DNO cable, THEY are paying for the losses between transformer and meter.

    If you, or a future customer use a substantial and long hour load the cost of losses are significant.

    Finally, they may wish to use this new cable to supply other customers in future.


    Hopefully they are replacing the transformer ? DNOs take an optimistic view of transformer ratings, but adding a three phase 100 amp load of about 72 KVA to an existing 50 KVA transformer might be a bit over optimistic, since it may be presumed that significant load is already connected.
  • Thanks for that. As you say I had a feeling it would be due to voltage drop. The 50kva transformer is actually replacing a small 15kva single phase one and the cable will only ever supply one customer. Although a 50kva transformer is still being used for a 72kva 100a supply so I guess the DNOs don't mind a bit of overload!
  • A 50 KVA transformer is probably sufficient for one 3 phase 100 amp supply, despite this being about 72 KVA . It is unlikely that the full 72 KVA would often be demanded other than briefly.


    A 100 amp three phase supply would allow for three electric showers AND 3 EV chargers AND plenty of lighting and small power circuits.

    And if significant electric heating is involved, then a 50 KVA transformer could probably be loaded to 72 KVA long term in UK winter conditions. 72 KVA of air conditioning on the hottest day of the year in a warm climate might end badly.

    My misgivings were presuming an EXISTING 50 KVA transformer with significant load already existing.
  • Former Community Member
    0 Former Community Member
    Although a 50kva transformer is still being used for a 72kva 100a supply so I guess the DNOs don't mind a bit of overload!


    What kVA did you apply for?


    Did you declare any disturbing loads?


    72 kVA is in CT metering land


    Regards


    BOD
  • I didn't actually make the application but I believe 69kva was asked for in line with wanting a 100a per phase supply. A fast EV charger was the only declared load.
  • Former Community Member
    0 Former Community Member
    I didn't actually make the application


    Just had a quote in from Western Power for a new 100a 3 phase supply



    The actual quote should detail what load WPD intend supplying along with contestable and non-contestable works.


    Regards


    BOD


  • I must admit I haven't seen the full quote just the costs of materials, I will check it out with thte client ?
  • Former Community Member
    0 Former Community Member
    I understand WPD policy is to only use:


    35 mm2 single core

    35 mm2 three core

    95 mm2 three core

    185 mm2 three core

    300 mm2 three core


    All aluminium cores with copper wave-con outer.


    The 95 mm2 is only used for a service cable where there is NO possibility of a second cut-out required, say for a granny flat, second house on the plot etc. 


    185 mm2 is used if there is or likely to be a second service or there is a second service.


    I'd wonder why the client has not shown you the entire quote.......... but that's me having been "burned" many times!


    As an aside, I think there could be some fun with my most recent application on behalf of a client. I CAT and genny'd the site to locate the existing service to adjacent industrial plot and as client had heard rumours of "free electricity", I got him to expose the cable and it is WPD, not SWA. Submitted application and WPD returned plan seeking confirmation that we'd do digging, of which I expected a short run. Hmm, their plan doesn't show this cable, only the main outside the site. Of course the plans could be wrong but again rumour has it that the connection was made by an employee at the weekend a long time ago............................


    Regards


    BOD
  • Thats really helpful, thanks BOD!


    As for not seeing the whole quote, I will ask for all of it, I think the client just wanted to see what could be done to save costs as the quote has come back at over £25K!!!!
  • Even if they will agree to a smaller cable, the saving will be modest.

    The actual cable cost is probably a fairly small part of the total.

    Transformer cost and labour to install and connect same will be unchanged.

    Trenching will cost the same.

    Labour to install the cable is probably the same, or only slightly reduced by smaller cable being easier to handle.

    Management time and trouble in planning the work will be the same.

    Cost of meter and fitting same unaltered.