Volume 8 – Mechanical and Electrical Services - Malaysia Geoportal

Chapter 7 ELECTRICAL SERVICES
APPENDIX 7E SAMPLE CALCULATION FOR GENERATOR POWER RATING REQUIREMENT
Calculating the generator power rating requirement for the pumping station with the following
loads:-
Starting
Running
Item KVA KW KVA KW
Total lighting and small 17.5
Power loads (G.L.) 20.13
28x0.415x1.732x0.85
=17.1KW .
Start the 1 st pump
10hpx5.3kVA/hp 53
53kVAx0.85pf 45
Add G.L.load 20.13 17.5 20.13 17.5
Total 73.13 62.5 20.13 17.5
Run the 1st pump
At 0.88 efficiency
10hpx0.746KW/hpx1/0.88 10.0 8.5
Add G.L .load 20.13 17.5
Total 30.13 26.0
Start the 2nd pump
10hpx5.3kVA/hp 53
53kVAx0.85pf 45
Run 1 st pump and G.L .load 30.13 26 30.13 26
Total 83.13 71 30.13 26
Run the 2 pump
At 0.88 efficiency
10hpx2x0.746KW/hpx1/0.88 19.95 16.96
Add G.L .load 20.13 17.5
Total 40.08 34.46
Start the 3rd pump
10hpx5.3kVA/hp 53
53kVAx0.85pf 45
Run 2 pump and G.L. load 40.08 34.46 40.08 34.46
Total 93.08 79.46 40.08 34.46
Run the 3 pump
At 0.88 efficiency
10hpx3x0.746KW/hpx1/0.88 29.92 25.43
Add G.L .load 20.13 17.5
Total 50.05 42.93
The critical generator loads are the KVA and KW for starting the third pump. During the start, the
first two running motors can act momentarily for half a cycle as generators and, hence, reduce the
demand on the generator, but the effect lasts much less than a second and is difficult to assess, so
the effect is ignored, especially as it may take up to 6 second to start the 3 rd pump.
The calculated KVA required is 93.08 KVA. Assuming that the generator efficiency as 0.85, the aging
factor for 5 years is 0.8 and additional spare capacity required is 20%.
The generator size is = 93.08x 1.2 KVA
0.85x0.8
= 164.26 KVA
Hence the generator size selected shall be 175 KVA
March 2009 7A-5