Installed apparent power (kVA)

The installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA.

The installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA.

The apparent-power demand of a load (which might be a single appliance) is obtained from its nominal power rating (corrected if necessary, as noted above for motors, etc.) and the application of the following coefficients:

η = the per-unit efficiency = output kW / input kW
cos φ = the power factor = kW / kVA

The apparent-power kVA demand of the load

[math]\displaystyle{ Pa=\frac{Pn}{\eta\cos\varphi} }[/math]

From this value, the full-load current Ia(A) taken by the load will be:

• [math]\displaystyle{ \mbox{Ia}=\frac{\mbox{Pa}\times {10^3} }{\mbox{V} } }[/math]

for single phase-to-neutral connected load

• [math]\displaystyle{ \mbox{Ia}=\frac{\mbox{Pa}\times {10^3} }{\sqrt3\times\mbox{U} } }[/math]

for three-phase balanced load where:

V = phase-to-neutral voltage (volts)
U = phase-to-phase voltage (volts)

It may be noted that, strictly speaking, the total kVA of apparent power is not the arithmetical sum of the calculated kVA ratings of individual loads (unless all loads are at the same power factor).

It is common practice however, to make a simple arithmetical summation, the result of which will give a kVA value that exceeds the true value by an acceptable “design margin”.

When some or all of the load characteristics are not known, the values shown in Figure A10 may be used to give a very approximate estimate of VA demands (individual loads are generally too small to be expressed in kVA or kW). The estimates for lighting loads are based on floor areas of 500 m².

Fluorescent lighting (corrected to cosφ = 0.86)
Type of application	Estimated (VA/m2) fluorescent tube with industrial reflector	Average lighting level (lux =lm/m2)
Roads and highways storage areas, intermittent work	7	150
Heavy-duty works: fabrication and assembly of very large work pieces	14	300
Day-to-day work: office work	24	500
Fine work: drawing offices high-precision assembly workshops	41	800
Power circuits
Type of application	Estimated (VA/m2)
Pumping station compressed air	3 to 6
Ventilation of premises	23
Electrical convection heaters: private houses flats and apartments	115 to 146 90
Offices	25
Dispatching workshop	50
Assembly workshop	70
Machine shop	300
Painting workshop	350
Heat-treatment plant	700

[a] example: 65 W tube (ballast not included), flux 5,100 lumens (Im), luminous efficiency of the tube = 78.5 Im / W.

Fig. A10 – Estimation of installed apparent power