How to do load calculation

how to do load calculation for office or apartments

As an electrical design engineer they should know why load calculation is very important for before doing any electrical works. in general the electrical load calculation is done before they sizing electrical supply for domestic and industrial purpose. here we will see about why we should we need   load calculation and how to calculate the load calculation .

Why should we need load calculation:

There are three important factors behind the load calculation,

1.Sizing of incomer source

  1. a) Ministry supply  b) Generator
  2. Prepare the load distribution
  3. Selection of protection device

we will see one by one briefly below,

  1. Sizing of incomer source

Generally any industry or domestic construction required electrical power for their equipment’s or  appliances. so that purpose they have to calculate electrical load first, then only they can sizing the incomer supply. there are two option available in any country for incomer supply.

  1. government supply
  2. self power producing by generator

For example:

we need power supply for our office and we are choosing with generator supply. Now we need to find generator sizing?

In this case we have to prepare load calculation first,

Load calculation :

for making load calculation we should need load details that is what are the load we are going to use or connect to our power supply.let us consider below load details for our office and we will do load calculation.

S.No Electrical appliances Watts quantity Total watts
1. Air Conditioner, 2 TON, Single Phase 2400 5 12000
2. Tube Lights 72 20 1440
3. Personal Computer(PC) 1600 15 24000
4. Printer 500 3 1500
5. Geezer 1500 3 4500
6. Microwave Oven 1400 1 1400
7. Elect. Kettle 2400 1 2400


as per the load details we can calculate total loads required,

so our total load is = 47240 watts

load = 47.24KW

Generator Sizing:

Based on our total load we can easily sizing the generator,

according to IEC standard, the generator capacity should be 125% of total load .

(Note: its include 25% for future purpose)

so our total load is 47.24KW and 125% of total load is 60KW.

so we need 60KW or 75KVA generator for our connecting load.

Load distribution

You have to distribute your load in balanced way otherwise its leads to voltage drop and reduce the efficiency of generator. so prevent this problem we should calculate load calculation

For example:

We have 60 kw total load and we have 3 phase 415v 50Hz power supply. Then how to we distribute the load


Connecting load R PHASE Y PHASE B PHASE
LOAD 1(10KW) 10
LOAD 2 (10KW) 10
LOAD 3 (10KW) 10
LOAD 4 (10KW) 10
LOAD 5 (10KW) 10
LOAD 6 (10KW) 10


Selection of protective device for incomer

If you know the total load and load distribution then you can easily select the protective device

There are 5 main protective device for incomer.






Based on load we can select our main incomer ELCB and MCCB

For example: total load is 60KW then ELCB rating should be

4pole 200Amp.


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