How to do 3-Phase load Calculation?

In previous article, we have calculated single phase electrical load. Any electrical load which is more than 5KW, we need to have 3-Phase load for it. For three phase system we use following formula for 3-Phase load calculation.

P = Sqrt(3) x V x I x P.F

Here, P stands for Power, while V for Voltage, I for Current and P.F is used for Power Factor.

Using above mentioned formula anyone can calculate three phase load. But before this 3-phase load calculation, we need to know the consumption of the load which will be fed.

Table 1.
S.NoAppliancesWattageQunatityTotal Power
1AC2000W48KW
2Three Phase Motor10HP17.46KW
3Lighting Load5000W15KW
4Boiler15000W115KW
5Power Load12KW112KW
6Machines Load33KW133KW
Total Connected Load80KW
Table 1: Details of Electrical Load

After calculation of connected load, We have to calculate demand load.

Demand Load:

 The demand load is the maximum load that is running during a specific period of time.

As we know that all connected appliances never runs at a same time so we use diversity factor of 80% of the connected load for the selection of Circuit Breaker and Cable Sizing.

How to calculate the size of Circuit Breaker?

Once we know connected load, just multiply load in KW into 1.8 you will find the current. For instance, In this case, our load is 80KW.

Just multiply it into 1.8

Current= 80 x 1.8 = 144A

So, we will look for available very next higher rating circuit breakers. In this case, the higher nearest rating available circuit breaker is 150A. As result of this 150A Circuit Breaker will be sufficient for this load.

Selection of Cable:

Now we will look into manual of Pakistan Cables named as Low Voltage Cables Book which is designed as per BS-7671 Standard. Here you will have to see table which fulfill your requirement whether you need a single core cable or multicore cables.

Furthermore, In this section, you should know installation method of cables so on basis of installation method you can select suitable rating cable. In this case our installation method is perforated cable tray so we will have to deploy 4C 50 Sq.mm Cu./PVC/PVC cables which has capacity more than 144A .

In this way we can calculate current, breaker size, Cable size etc.

If you feel any confusion, you can comment below.

Asif Malik

A young Electrical Engineer having more than 5 years of experience in Electrical Power System, Construction and Execution of different projects, Project Management, Electrical Inspection. He is a article Writer as well on the different topic related to Electrical Engineering and Information Technology.

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