8/10/2006

Diversity and Demand factor..etc

DEMAND FACTOR, DIVERSITY FACTOR AND TRANSFORMER SIZE CALCULATION


a) Demand factor - the ratio of the load that a piece of equipment actually draws when it is in operation to the load it could draw (which we call full load).

For example, an oversized motor - 20 HP - drives a constant 15 HP load whenever it is on. The motor demand factor is then 15/20 = 75 %.

Demand factor= Maximum demand/ Connected load.

This is generally confused with diversity factor. Demand factors should be ideally applied to individual loads and diversity factor to a group of loads. When you talk about ‘diversity’, there is naturally more than one (many, in real case scenarios) loads involved.

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  Modified for further clarity and as per reader's comments: 06-09/2011
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b) Diversity factor -Gives an indication about the simultaneous maximum demad of a system considering various loads operating at a time and the highest ' coincident' Maximum demand of all those loads.

Diversity factor =
Sum of individual maximum demands of various sub-loads / Maximum demand of the total system.
This will be less than 1.

Note: Reciprocal of the above ratio ( will be more than 1) also is used in some other countries. 

c) Where to use which factor!

Demand factor can be applied to calculate the size of the sub-main which is feeding a Sub panel or a fixed load like a motor etc.

If the panel (Say SSB 1) total load is 250 kVA , considering a Demand factor of 0.8, we can size the feeder cable for 250 x 0.8= 200 kVA, which is equal to 280 Ampere on 415 V 3 phase supply.

Diverity factor can be used to estimate the total load required for a facility or to size the Trnasformer required.

Simultaneous maximum demand = (Sum individual maximum demands of different loads) x Div. Factor

( Note: if a value greater than 1 is used, divide with that factor)

Assuming there are 4 such SSB’s as per the chart below,

PANEL       LOAD        Demand Factor%       Max Demand
SSB 1        250 kVA            80                               200
SSB2         200 kVA            80                               160
SSB3        150 kVA             75                               112.5
SSB4        400 kVA             85                               340

                                                                      Total 812.50 kVA

The Sum of individual maximum demands is 812.5 kVA as given above.

To calculate the Main incoming capacity and Transformer size, we can use the Diversity factor which will take into account the fact that all the four SSB’s above won’t attain their individual maximum simultaneously.

Assuming a Diversity factor of 0.7, the simultaneous maximum demand would be

812.5 x 0.7 = 568.75 kVA

Transformer could be sized after considering a loading factor of 80 % and future requirements of another 20 % extra.

Transformer size= ( 568.75 / 0.8 ) x 1.2 = 853 kVA ( Say 1000 kVA , the nearest available standard size)

10 comments:

  1. Hi
    Thanks for the information.
    Will you please provide the information about, in design of Electrical Panel what is the the relation between Total Connected Load, Max Demand of panel & busbar Current rating selection.
    in short how to design panel when I have TCL & MD.

    ReplyDelete
  2. My understanding is that diversity factor is defined as ratio of sum of individual consumer maximum demands to system maximum demand at the point of common coupling of these consumers and not as quoted in your note:-
    Diversity factor =
    Sum of individual maximum demands/ Total connected loads
    = 240 / 300 = 0.8
    This is the demand factor.
    The diversity factor is usually > 1.0. The reciprocal factor < 1.0 is the coincidence factor.

    Although the general conclusion in your note is correct there appears to be some confusion in the usage of these terms and their definition in the literature.
    The ratio of average load to maximum demand over a given period or the load factor can also be used in the estimation of maximum demand.

    HT

    ReplyDelete
  3. IEEE Std 141-1993 sez: demand factor = the ratio of the maximum coincident demand to the total connected load. diversity factor = the ratio of the sum of the individual non-coincident maximum demands to the maximum demand. Maximum demand is the greatest of all demands that have occurred during a 'specified period of time' and coincident demand is any demand that occurs simultaneously with any other demand, also the sum of any set of coincident demands. This business about 'specified period of time' keeps things a bit fuzzy.

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  4. Hi,

    I was a lecturer of electrical engineering for almost 13 years. My understanding of diversity factor is always greater than 1. Diversity factor can be defined as the sum of the individual maximum demand connected in the system divided by the maximum demand of the entire system. The entire system has its own maximum demand to serve, whereas individual connected load has also its maximum demand to consume or serve.Diversity which is distinguished from diversity factor refers to the percent of time that the machine is running at its demand ( which is always less than or equal to 1).

    Bernard Bullas, Jr
    IIEE Life Member
    IET UK-Member
    Engineers Australia- Member

    ReplyDelete
  5. good day,

    is it possible to have a demand of 9KW for a 5MVA Power Transformer with 7 units of identical 200-kW pump motors connected?

    ReplyDelete
  6. Sir,
    I am not an electrical engineer, but interested to know. As you said diversity factor is always >=1, but in ur example as well in others no where i found it is more than/=1?
    could u pls explain?

    pavan

    ReplyDelete
  7. Anonymous9/05/2011

    Hi San,

    Normally british standard uses Div factors less than 1 wheras US and some European contries use a value more than 1.However the formula remains same, only thing one is the reciprocal of the other.


    Dinesh

    ReplyDelete
  8. hi sir..

    i'm new in electrical designing..your guide is very useful for me as i dont have any senior engineer in my company to teach me for practical design. I would like to find an PRACTICAL example for complete calculation on how to design building electrical starting from cable sizing & protection from LV side until MV side.
    I wish that someone can share with me the example of that complete calculation based on experience since most of info i found in google are mostly theory and not the practical calculation for complete electrical system.if i found the calculation, what can i really found just only certain part of calculation, not the complete one.

    i really need that calculation desperately..my email is hafyzz@gmail.com

    i am really glad if someone can provide this one.

    ReplyDelete
  9. I like this note so much. I'm an electrical student. Pls can you send this note and more to my email. This is my add. olagab95@yahoo.com

    ReplyDelete
  10. Very nice post, thanks for sharing the information. Keep up the good work.

    electrician woburn ma

    ReplyDelete