Note: This is the third part of a series of articles in the estimation of pant electrical loads. Check these for Part 1 and Part 2

In Part 2, a key single line diagram and an electrical load list were prepared. In the electrical load list, the size of the minimum rating of the power transformer was estimated providing a growth margin of 25%. The growth margin could be adjusted depending on the client's requirements. In this case, the chosen power transformer rating is 250 kVA. The question that may pop-up in this case will be, why only 250 kVA, why not select a higher rating? Below are the standard rating of power transformers according to IEC 60076-7.

Standard Power Transformer Ratings
kVA Rating In (A)
400 V (Nominal Secondary)
100 141
160 225
250 352
315 444
400 563
500 704
630 887
800 1127
1000 1408
1250 1760
1600 2253
2000 2816
2500 3520
3150 4436

As an Electrical Design Engineer, future proofing the equipment is always a good consideration during design stage. Economic considerations however will always prevail at the final selection of the equipment. Higher rating will increase the cost of equipment, power transformers, protection equipment, cables, real estate requirements, etc. Thus, unless it is stipulated in the Basis of Design, the next higher standard rating of equipment from the result of the load calculation will be selected.

The selection of protection devices will again be dependent on the project requirement. In this case, the primary protection is a pole mounted power fuse and the power transformer secondary main protection is an air circuit breaker. These has already been laid out in the key single line diagram previously prepared for the project.

Below is a complete Single Line Diagram complete with protection devices.

Single Line and Protection Diagram


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