3 phase load distribution

Discussion in 'Electricians' Talk' started by Alexa1, Oct 23, 2013.

  1. Alexa1

    Alexa1 New Member

    Hi all,
    I would like to get your advice in how to distribute loads safely and best way in a TP & N CU before my electrician starts the new wiring.
    Basically we are moving our hair dressing business into a new premises where there is a new TP & N fused at 80 A, with TN-S Earthing arrangement, the shop is approx 19 x 5.5 metre floor area.
    All our loads are single phase but the existing meter is three phase, and I guess I will be more comfortable using 3 phase in order not to run out of amps but i know it will more expensive installation! I'm thinking distributing the load across 3 phase as follows:
    Phase 1: Water tank (protected by MCB’s)
    3 X 4 kW single phase water tank heater elements. All separate circuit, each element will be terminated to a local 20 amp DP switch. Also possibly one had dryer being also fed from this phase.
    Phase 2: 13 amp sockets (protected by RCBO’s)
    3 ring main socket circuits, to take 4 X Electric heaters maximum 2 kW each on a 13 amp plug and up to 8 hair dryers (800 watts each)
    Phase 3: lighting (protected by mcb’s), 3 to 4 Lighting circuits
    Could you advise if this is the way you spread the load across 3 phase supply?
    Many thanks
  2. Lectrician

    Lectrician Screwfix Select

    That's not really spreading the loads. The immersions should be spread across the phases, as should the lighting etc.

    Let your sparky sort it out. It's what you pay him for ;-)

    If the DB is already in place (you imply it is), it is not going to be any more expensive.
  3. scruffydog

    scruffydog New Member

    As Lectrician states let your sparky sort it out, spreading similar equipment out across all phases means if for any reason one phase has a problem you can hopefully keep working until it's fixed.
  4. Alexa1

    Alexa1 New Member

    Thanks guys for your replies, my electrician has been today and said we can get our DNO uprate the fuses from 80 amps to 100 amps and then only use single phase. There is no any DB in the shop yet, so I can then decide to go for a single phase CU. Just to add on to my original thought about 3 phase distribution I appreciate the way I proposed wasn't a balanced loads, but am I thinking correctly in order to have a safer installation it's best to keep the phases physically separate so that you eliminate the Phase to Phase faults?? having an unbalanced load do we pay any penalties?? and i guess my third question would be if I do go for a three phase consumer unit and just use one phase or two phase can do that?
  5. Lectrician

    Lectrician Screwfix Select

    Nothing wrong with using three phase in adjacent accessories, or indeed inside the same enclosure. It is really not any less safe if things are done correctly.

    You don't get penalties for unbalanced loads if you're well under your agreed capacity. If you're using your full agreed capacity, they would want to see it balanced, and if there were issues, they would ask you to balance. They wouldn't fine you. Balanced could be 30A, 50A, 45A. However, 80A, 80A, 5A would not be good.

    Most TP DBs have single phasing kits available, or all three phases could just be henleyed to one.
    Last edited: Oct 24, 2013
  6. Alexa1

    Alexa1 New Member

    Many Thanks Lectrician, well explained.
  7. Alexa1

    Alexa1 New Member

    Now I'm thinking to go back to TP board!! I'm only worried having 400 volts (415 volts) within the installation or shouldn't I worry about that??? Can I ask a general question...., if you had a scenario where you had the option of using a single phase board or a three phase board and you are not taking cost into equation, but thinking possible future expansion if TP used against the presence of 400 volts, which board would you fit????
  8. Lectrician

    Lectrician Screwfix Select

    I would go for TP.

    There is no problems with having 400v around your premises. Every time you go in tesco, pay your fuel, eat in a restaurant, sleep in a hotel, you are likely surrounded by 400v. It is extremely unlikely to come into contact with TWO separate phases at the same time.

    The only issue you may find with TP is that the RCD protection will be costlier. To comply, it is just your sockets that really need RCD protection, so just a few RCBOs
  9. Alexa1

    Alexa1 New Member

    Lectrician once again many thanks for your reply, I'm convinced now TP is the way forward also gives me extra capacity for future should I need it.
  10. sinewave

    sinewave Screwfix Select

    Concur with Lec, as usual! :cool:
  11. Alexa1

    Alexa1 New Member

    I'm thinking to have Denmanns BS Basics branded 3 phase DB fitted in our shop with their rcbo's (controlgear rcbo), can anyone give me any feedback on the quality of BS Basics range DB's and Denmanns rcbo's please?
  12. Lectrician

    Lectrician Screwfix Select


    MerlinGerin(Schneider) or MEM for me.

    At a push, Square D, Wylex, Crabtree.

    The Denmans stuff would be my very last choice.

    All electricians have their own preference, and you're best to let him chose and supply. Why would you want to buy it?
  13. Alexa1

    Alexa1 New Member

    Thanks Lec, It was only when he told me rcbo's supplied by Denmanns for their DB it only cost approx £14 but the others are a lot more! but since then we have found kA rating on them is only 6 kA compared to more expensive ones that are 10 KA, so more than likely especially after your comments I will ask him to supply and fit MerlinGerlin.
  14. unphased

    unphased Screwfix Select

    Alexa I use Denmans Curve boards in domestic properties but I would be reluctant to use them in a commercial environment as they take more of a hammering. Merlin Gerin is generally best choice but don't discount Crabtree, Wylex, Eaton or Hager if you can get a better price. Ask your sparky which one he would prefer to fit. Merlin Gerin are going to be the highest price. You get what you pay for. :)
  15. Bazza-spark

    Bazza-spark Well-Known Member

    Alexa, there is nothing wrong with a rating of 6kA as long as your prospective fault current doesn't exceed that value. It would be worth getting your electrician to measure what the potential pfc is before ordering so that you can get the correct breakers.

    Kind regards

  16. Alexa1

    Alexa1 New Member

    Thank you all for your kind replies, I've been advised our Pfc is calculated at just under 4 KA so as BS says probably 6KA rated rcbo's would be suitable. When Unphased said 'in commercial environment BD takes more hammering' I'm sure you must be referring to electrical stress? and if so, if a DB is built and tested to BS standards shouldn't it take all that stress? The reason I'm still debating my saga is that my electrician knows me for years and has said he will fit MK DB but when populated Denmanns BS Basics range is 1/3 of the price and to BS standard!! So he doesn't mind trying it provided that if I get problems I'm not going to blame him! I'm thinking a DB is hidden behind a closed door and if it safely protects my installation same as the others then why not. On the other hand the installed reference plants on MK, Merlin .... Is a lot more than Denmanns DB with many years proven technology.... So it appears for a rest of mind it's best to cough up and pay for the name!?
  17. Alexa1

    Alexa1 New Member

    Oh Just for the record our Earthing arrangement is definitely PME and NOT TN-S as stated in my first posting.
  18. CraigMcK

    CraigMcK Well-Known Member

    Buying one make of DB and fitting another make of devices almost certainly is not in line with BSEN 61439-3, MK will not warrant any failures since you did not use the assembly as they designed and you have no idea how well another make of MCB's will work in someone else's board. So if you are concerned about working in line with standards, this is not the way to go
  19. Lectrician

    Lectrician Screwfix Select

    Not sure MK even do a TP board these days?

    Nothing wrong with having 6ka on installs with greater PFC if there is BS1361 or BS88 fuses backing these up (DNO fuse). Very common in densly populated areas with huge TX's.
  20. unphased

    unphased Screwfix Select

    That's the problem, Lec. There is misunderstanding of that fact. Where installations are protected by those fuses the breaking capacity of the mcb is deemed to be 16kA.
    Last edited: Nov 1, 2013

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