Diversity and Demand factor..etc

DEMAND FACTOR, DIVERSITY FACTOR AND TRANSFORMER SIZING 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 SIZING CALCULATION

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)