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

- Starting kVA and starting current
- Running kVA and running current
- Load in kW, P.F., type of load, diversity factor
- 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.

- 4 No’s of 1Ph, 230V, 80Watt CFL Bulbs, Diversity Factor is 0.8; Starting & Running P.F is 0.8.
- 2 No’s of 1Ph, 230V, 3000Watt Air Condition, Diversity Factor is 1, Starting & Running P.F is 0.8.
- 2 No’s of 1Ph, 230V, 500Watt Halogen Lights Diversity Factor is 0.8, Starting & Running P.F is 0.8.
- 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
- 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

**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)**

**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)**

**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 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 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 |

## One thought on “How to calculate the size of Diesel Generator for my office ?”

Thanks very interesting blog!