Estimation of actual maximum kVA demand

All individual loads are not necessarily operating at full rated nominal power nor necessarily at the same time.

Factors ku and ks allow the determination of the maximum power and apparent-power demands actually required to dimension the installation.

Factor of maximum utilization (ku)

In normal operating conditions the power consumption of a load is sometimes less than that indicated as its nominal power rating, a fairly common occurrence that justifies the application of an utilization factor (ku) in the estimation of realistic values.
This factor must be applied to each individual load, with particular attention to electric motors, which are very rarely operated at full load.
In an industrial installation this factor may be estimated on an average at 0.75 for motors.
For incandescent-lighting loads, the factor always equals 1.
For socket-outlet circuits, the factors depend entirely on the type of appliances being supplied from the sockets concerned.

Factor of simultaneity (ks)

It is a matter of common experience that the simultaneous operation of all installed loads of a given installation never occurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimating purposes by the use of a simultaneity factor (ks).
The factor ks is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board). The determination of these factors is the responsibility of the designer, since it requires a detailed knowledge of the installation and the conditions in which the individual circuits are to be exploited. For this reason, it is not possible to give precise values for general application.

Factor of simultaneity for an apartment block

Some typical values for this case are given in Figure A10, and are applicable to domestic consumers supplied at 230/400 V (3-phase 4-wires). In the case of consumers using electrical heat-storage units for space heating, a factor of 0.8 is recommended, regardless of the number of consumers.

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Number of downstream consumers	Factor of simultaneity (ks)
2 to 4	1
5 to 9	0.78
10 to 14	0.63
15 to 19	0.53
20 to 24	0.49
25 to 29	0.46
30 to 34	0.44
35 to 39	0.42
40 to 49	0.41
50 and more	0.40

 

 

 

 

 

 

 

 

     

Fig. A10: Simultaneity factors in an apartment block

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Example (see Fig. A11):
5 storeys apartment building with 25 consumers, each having 6 kVA of installed load.
The total installed load for the building is: 36 + 24 + 30 + 36 + 24 = 150 kVA
The apparent-power supply required for the building is: 150 x 0.46 = 69 kVA
From Figure A10, it is possible to determine the magnitude of currents in different sections of the common main feeder supplying all floors. For vertical rising mains fed at ground level, the cross-sectional area of the conductors can evidently be progressively reduced from the lower floors towards the upper floors.
These changes of conductor size are conventionally spaced by at least 3-floor intervals.
In the example, the current entering the rising main at ground level is:

[math]\displaystyle{ \frac{150\times0.46\times10^3}{400\sqrt3}=100A }[/math]

the current entering the third floor is:

[math]\displaystyle{ \frac{\left(36+24\right)\times0.63\times10^3}{400\sqrt3}=55A }[/math]

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Fig. A11: Application of the factor of simultaneity (ks) to an apartment block of 5 storeys

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Factor of simultaneity for distribution switchboards

Figure A12 shows hypothetical values of ks for a distribution board supplying a number of circuits for which there is no indication of the manner in which the total load divides between them.
If the circuits are mainly for lighting loads, it is prudent to adopt ks values close to unity.

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Number of circuits	Factor of simultaneity (ks)
2 and 3	0.9
4 and 5	0.8
6 to 9	0.7
10 and more	0.6
Assemblies partially tested in every case choose	1.0

Fig. A12: Factor of simultaneity for distribution boards (IEC 61439)

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Factor of simultaneity according to circuit function

ks factors which may be used for circuits supplying commonly-occurring loads, are shown in Figure A13.

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Circuit function		Factor of simultaneity (ks)
Lighting		1
Heating and air conditioning		1
Socket-outlets		0.1 to 0.2 (1)
Lifts and catering hoist (2)	For the most powerful motor For the second most powerful motor For all motors	1 0.75 0.60

(1) In certain cases, notably in industrial installations, this factor can be higher.
(2) The current to take into consideration is equal to the nominal current of the motor, increased by a third of its starting current.
Fig. A13: Factor of simultaneity according to circuit function

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