How to calculate the size of Diesel Generator for my office ?

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In most of the interviews for an electrical engineer, one question is regularly put in and most of us are like “ok it is pretty easy but I don’t know how to start the calculations”.  I faced the question a couple of times and my answer would be followed in these steps

 

First of all these are the requirements which we need to know before we start the calculations

  1. Starting kVA and starting current
  2. Running kVA and running current
  3. Load in kW, P.F., type of load, diversity factor
  4. Future expansion

 

For eg:–

 

If we have following loads in a premises the required DG kVA will be = Starting KVA  X Future Expansion X Average Use of Equipments

*We will account for the power consumption on the basis of starting current as it is always higher than running current.

 

  1. 4 No’s of 1Ph, 230V, 80Watt CFL Bulbs, Diversity Factor is 0.8; Starting & Running P.F is 0.8.
  2. 2 No’s of 1Ph, 230V, 3000Watt Air Condition, Diversity Factor is 1, Starting & Running P.F is 0.8.
  3. 2 No’s of 1Ph, 230V, 500Watt Halogen Lights Diversity Factor is 0.8, Starting & Running P.F is 0.8.
  4. 1 No’s of 1Ph,230V, 10KW Motor with Y-D Starter, Diversity Factor is 0.8, Starting P.F is 0.7 & Running P.F is 0.8
  5. 1 No’s of 3Ph,430V, 130KW Motor with Soft Starter , Diversity Factor is 0.8,Starting P.F is 0.7 & Running P.F is 0.8

 

Let’s Calculate

  1. 4 No’s of 1Ph, 230V, 80Watt CFL Bulbs, Diversity Factor is 0.8; Starting & Running P.F is 0.8.

 

Full Load KW of CFL Bulb=(No X Watt X Diversity Factor) /1000

Full Load KW of CFL Bulb=(4x80x0.8)/1000=0.256KW

Full Load KVA of CFL Bulb=KW / P.F

Full Load KVA of CFL Bulb=0.256 / 0.8= 0.32 KVA————(1)

 

Full Load current of CFL Bulb=(No X Watt X Diversity Factor) / (Volt x P.F)

Full Load current of CFL Bulb=(4x80x0.8) / (230×0.8) =2 Amp————(2)

 

Type of Load is Linear                       (*refer table 1 given below for linear and non linear loads)

 

Starting KVA of CFL Bulb=1 X (KW / Starting P.F)

Starting KVA of CFL Bulb=0.32 / 0.8=0.4KVA—————(3)

 

Starting Current=100% of Full Load Current.

Starting Current=1 X 2= 2 Amp.————–(4)

 

 

  1. 2 No’s of 1Ph, 230V, 3000Watt Air Condition, Diversity Factor is 1, Starting & Running P.F is 0.8.

 

Total Full Load KW of A.C=(No X Watt X Diversity Factor) /1000

Total Full Load KW of A.C =(2x3000x0.8)/1000=4.8KW

Total Full Load KVA of A.C =KW / P.F

Total Full Load KVA of A.C =4.8 / 0.8=6KVA————(1)

 

Total Full Load current of A.C =(No X Watt X Diversity Factor) / (Volt x P.F)

Total Full Load current of A.C =(2x3000x0.8) / (230×0.8) =26 Amp————(2)

 

Type of Load=Non Linear

 

Starting KVA of A.C=1.6 X (KW / Starting P.F)

Starting KVA of A.C =1.6 X (4.8 / 0.8)=9.6KVA—————(3)

 

Starting Current=160% of Full Load Current.

Starting Current=1.6 X 26= 42 Amp. ————–(4)

 

 

  1. 2 No’s of 1Ph, 230V, 500Watt Halogen Lights Diversity Factor is 0.8 Starting & Running P.F is 0.8.

 

Full Load KW of Halogen Bulb=(No X Watt X Diversity Factor) /1000

Full Load KW of Halogen Bulb=(2x500x0.8)/1000=0.8KW

Full Load KVA of Halogen Bulb=KW / P.F

Full Load KVA of Halogen Bulb=0.8 / 0.8=1KVA————(1)

 

Full Load current of Halogen Bulb=(No X Watt X Diversity Factor) / (Volt x P.F)

Full Load current of Halogen Bulb=(2x500x0.8) / (230×0.8) =4 Amp————(2)

 

Type of Load=Non Linear

 

Starting KVA of Halogen Bulb =1.6 X (KW / Starting P.F) / Starting P.F

Starting KVA of Halogen Bulb =1.6 X (0.8 / 0.8)=1.6KVA———————-(3)

 

Starting Current=160% of Full Load Current.

Starting Current=1.6 X 4= 7 Amp .————–(4)

 

 

  1. 1 No’s of 1Ph,230V, 10KW Motor with Y-D Starter, Diversity Factor is 0.8,Starting P.F is 0.7 & Running P.F is 0.8

 

Full Load KW of Motor=(No X Watt X Diversity Factor) /1000

Full Load KW of Motor =(1x10000x0.8)/1000=8KW

Full Load KVA of Motor =KW / P.F

Full Load KVA of Motor =8 / 0.8=10KVA————(1)

 

Full Load current of Motor =(No X Watt X Diversity Factor) / (Volt x P.F)

Full Load current of Motor =(1x10000x0.8) / (230×0.8) =43 Amp—-(2)

 

Type of Starter=Star-Delta

 

Starting KVA of Motor =4 X (KW / Starting P.F)

Starting KVA of Motor=4X (8 / 0.7)=45.7KVA————————(3)

 

Starting Current=4 X Full Load Current

Starting Current=4 X 11.4 = 174 Amp.————–(4)

 

 

  1. 1 No’s of 3Ph,430V, 130KW Motor with Soft Starter , Diversity Factor is 0.8,Starting P.F is 0.7 & Running P.F is 0.8

 

Full Load KW of Motor=(No X Watt X Diversity Factor) /1000

Full Load KW of Motor =(1x120000x0.8)/1000=96KW

Full Load KVA of Motor =KW / P.F

Full Load KVA of Motor =96 / 0.8=120KVA————(1)

 

Full Load current of Motor =(No X Watt X Diversity Factor) / (Volt x P.F)

Full Load current of Motor =(1x120000x0.8) / (1.732x430x0.8) =167 Amp—-(2)

 

Type of Starter=Auto Transformer

 

Starting KVA of Motor =3 X (KW / Starting P.F)

Starting KVA of Motor=3 X (96 / 0.7)=411.4KVA—————(3)

 

Starting Current=3 X Full Load Current

Starting Current=3 X 167 = 501 Amp.————–(4)

 

Total Load Calculation:

 

Total Starting KVA = Add all —–(3)s

Total Starting KVA =0.4+9.6+1.6+45.7+411.4 =468.7 KVA

 

Total Starting Current = Add all —–(4)s

Total Starting Current =2+42+7+174+501= 725 Amp

 

Total Running KVA = Add all —–(1)s

 Total Running KVA = 4+6+1+10+120= 137KVA

 

Total Running Current= Add all —–(2)s

Total Running Current=2+26+4+43+167= 242 Amp

 

Size of Diesel Generator= Starting KVA X Future Expansion X Average Use of Equipments

Size of Diesel Generator=468.7 X 1.1 X 0.8

Size of Diesel Generator= 412 KVA

 

NOTE

Table 1

Type of Load Equipment Starting Current
Linear Load General Equipment 100% of Full Load Current
Non-Linear Load UPS, Inverter, Computer, Ballast 160% of Full Load Current

Table 2

Type of Starter Starting Current
DOL 6 X Full Load Current
Star-Delta 4 X Full Load Current
Auto Transformer 3 X Full Load Current
Soft Starter 2 X Full Load Current
VFD 1.5 X Full Load Current

 

 

 

 

 

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