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''Electrical Design from TRANSFORMERS to Distribution Boxes'' (Conforming Indian Standards)
ELECTRICAL DESIGN
Electrical design / installation takes as example is with a connected load of 2 numbers of 100 HP motor, 2 numbers of 25 HP motor, 6 numbers of 10 HP motor & 5 numbers of 5 HP at Sub Switch Board-I (SSB-I), SSB-II with a connected load(CL) of 95HP & SSB-III with a CL of 50 HP and Lighting load of 10 KW. A standby generator required for all connected loads. Assume Diversity factor as 1.5.'
STEP:-1
SELECTION OF TRANSFORMER
Total Connected load = (2X100)+135+95+50+10
= 480 HP + 10 KW
Diversity Factor(DF) = 1.5(assumed)
Maximum Demand (MD) = Total CL / DF
= 490/1.5=326.6KVA.
Provide 20% spare capacity for future expansion
Transformer rating = MD+(MD*20/100)= 391.92 KVA.
Nearest standard rating of transformer = 400KVA. (selected)
Check:- Highest Capacity motor = 100HP Transformer rating required for this motor with assisted start
Transformer rating = Highest Capacity Motor X (100/40)
Transformed rating required = 100 X (100/40)=250KVA. Therefor 400 KVA transformer is sufficient.
STANDARD RATING OF TRANSFORMERS
100KVA, 160KVA, 200KVA, 250KVA, 315KVA, 400KVA, 500KVA, 630KVA, 750KVA, 1000KVA. 1250KVA, 1600KVA ETC..
STEP:-2
Selection of Generator
Total CL = 490KVA D.F = 1.5 M.D= 490/1.5=326.6KVA
Provide 20% spare capacity for future expansion
Therefore generator rating required = 391.92
Next standard size of Generator = 400KVA.
Check:- Generator rating required to start the largest motor
Largest motor = 100HP Generator rating = 100*(100/33.3)=300KVA. Therefor 400KVA generator is sufficient.
STANDARD RATING OF GENERATORS' 10KVA, 15KVA, 25KVA, 33.5KVA, 50KVA, 63KVA, 82.5KVA, 100KVA. 125KVA, 160KVA, 180KVA,200KVA, 250KVA, 320KVA, 400KVA, 500KVA, 625KVA etc....
For design consider KVA=KW=HP(Only for design)
For direct changeover at MSB/SSB generator rating shall corresponding to the present & anticipated MD.
When generator is used to run a Unit Load generator rating shall be 120% of the load.
