Our industry has excellent international standards (IEC) and excellent software. Both provide us with high accuracy calculations. However, despite the precise methods, can we be sure of the results of the calculations? Of course not! At least because we don’t always understand which source data to use in calculations. If we do not have accurate initial data, then regardless of the quality of our methodology (IEC standard or software), we will never get an accurate result.

One of the examples I suggest thinking about is a very simple one. Let’s say we have a two-circuit cable line that feeds two transformers. An electric power of the load is S. The question arises – what power should be used as a source data for cable thermal calculations and cable cross-section selection?

There are at least three different options (“cases”) for the cable line thermal calculations and core cross-section selection, and these cases differ from each other in terms of power up to 2.8 times (!):
1️⃣ Power of 0.5S.
2️⃣ Power of 1.0S.
3️⃣ Power of 1.4S.

Cases 1 and 2 are quite understandable. As for case 3, it requires comments since the load power is S, and in this sense, the cable line power of 1.4S raises questions.

It should be remembered that the cable and transformer are elements of the same power supply system, and therefore their ability to transmit electric power must match each other (rated power of cable and transformer must match each other). The transformer’s rated power in normal operation is usually 0.7S (although in fact the transformer only passes 0.5S, that is, it works with some underloading). If one line circuit (CL-2) is disconnected, it turns out that another one (CL-1) must pass a power equal to the rated power of two transformers, that is 1.4S – this is exactly the case 3.

We have not yet talked about the fact that the power transformer operates in the air and the cable operates underground, and when coordinating their current ampacity, we need to think a hundred times about what ambient temperature to use. This is a separate issue.

The idea of today’s post is clear – before calculating the cable line (using very accurate standards and software), please make sure that you have been given normal initial data, and not a pig in a poke.

If only one power value (or current value) was given as the source data, then most likely such data cannot be trusted. There should be several power values (or current values) at once, and explanations should be given about the number of operating circuits, environmental parameters, and overload requirements. As a result, it turns out that the preparation of initial data for calculations according to IEC 60287 can be much more difficult and responsible than these calculations themselves.