How to do 3-Phase Electrical load Calculation?

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

    \[      \boxed{P= \sqrt{3}* V*I* P.F}\]

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

Using above mentioned formula you can do three phase electrical load calculation. But before this 3 phase load calculation, we need to know the consumption of the each load which will be fed.

Table 1.
S.NoDescriptionWattageQunatityTotal Power
2Three Phase Motor10HP17.46KW
3Lighting Load5000W15KW
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% to get demand load.

However, for the selection of Circuit Breaker and Cable Sizing,

Figure: Sample Load Sheet for clear understanding.

Read Also: How to Calculate Area based load sheet?

How to perform Breaker Sizing?

Once we know connected load, For circuit breaker sizing, we have to do current calculation, 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

After ampere calculation, Now, you may be confused that from where does the factor 1.8 come?

Don’t worry, I am just explaining ,

using above mentioned 3 phase power calculation formula,

Current Calculations
Current Calculations

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

Selection of Cable:

Now we will look into manual of cables manufacturer as we use 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 of electrical load calculation, 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 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.

Electrical Load Sheet Format:

Electrical load format has been given below for preparation of the load sheet.

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