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Power Density - Single or Three Phase?

Simon87
Hello all,


Quick question regarding initial load assessments using power densities.


Keeping my numbers simple, lets say we take a small power density of 20W/m2, over a GIA of 1000m2, this gives a max demand of 20kW. Now i have always taken this to be a single phase load, and so if the space is being served via a three phase power supply, would it be correct to split this 20kW across the three phases (assuming perfect balance across phases). i.e. the small power maximum demand for the space would be 6.7kW.


Any help would be appreciated.


Thanks in advance,


Simon.


  • 0
    Power is usually quoted as a total, unlike current which is usually per phase. So 20kW could be (approx) 87A single phase or 29A per phase 3-phase (or even 44.5A per phase split phase) - as you say presuming you can achieve an equal split between phases. (Often that's not entirely convenient and you end up with not quite an even split - but it should total up the same).

      - Andy.
  • 0
    Hello Andy, and thanks for your response.


    The problem is they dont total up to th same. If we are looking to size the incoming supply there is a big difference if the max demand is single or three phase. If we take the outcome of the earlier example and assume the small power load is all three phase, we would need to apply for a 21 kVA TP&N supply (assuming .95 PF), whereas if we assume the load is all single phase (and will be split across all three phases) this would mean we only need a 7 kVA TP&N supply. Depending on the building type there may be some three phase equipment that would fall into the banner of small power, but the majorty of small power is likely to be single phase (in hindsight the example would have worked better with lighting!)


    The figures ive used are only hyperthetical, but if we consider the same method for load analysis on a 5,000m2 office building, this would have a huge difference in the size of supply required.
  • 0
    Wait to see what the DNO types here have to say, but I would have expected that kVA (like kW) ratings for a supply would be totals across all the supplied phases.


    So a 7kVA 3-phase supply would only provide about 10A/phase and be completely inadequate for supplying a 21kVA load no matter how it was arranged.


    I think most DNOs describe a 60A single phase supply as 13.8kVA and 60A 3-phase as around 41kVA.


       - Andy.

  • 0
    What is the reqd MPR (kva)? 

    Any 3 phase equipment?


    MPR (kva) 35mm2 single phase Al split concentric/concentric 100 amp cut out max 100 amp fuse.

    1--------23


    MPR (kva) 35 mmsq2 three phase Al concentric 100 amp cut out max 100 amp fuse.

    24-------69


    These are small size services. For budding distribution engineers the rule of thumb for calculation is:

                                                           I=4xkva dividedx3

    So 3 phase, 4x69 dividedx3 = 92ampxphase. 

    These are balanced loads, on site measurements are different.

    So Mr Jewsbury is correct in saying the load is across 3 phases.

    Regards, UKPNZap

                                                               

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