Should a Transformer be used in full capacity?

Transformers and their importance

Transformers have become as important to us as oxygen is to our bodies. Without oxygen, a human body cannot survive, and without electrical power, the world cannot. From getting up in the morning till hitting bed, we are entirely dependent on power in one way or another, whether directly or indirectly.

The first thing which strikes our mind after power is the substation where transformers are at their core as their heart and soul. Without transformers, the power can neither be transmitted nor distributed to the point of the utility to be finally used in various applications. Such is the importance of transformers in our daily lives, which is ruled by electrical power. Even a single minute power outrage is unbearable and life-halting for us whether we are at the office or home.

Life span of a transformer

I am sure you must have been across these life movers at some point of time in your life. Generally, these transformers are designed and manufactured for a life span of 30 years and can easily serve you much more if installed and used as per the instructions and guidelines without overloading them and doing maintenance at the desired intervals of time. These transformers are static devices and otherwise demand very low maintenance, once installed.

As discussed, these transformers have such a life span that to procure them and install them is once a lifetime affair if it’s selected appropriately considering the application, present load, and future expansion load. 30 years is such a long period that everything undergoes a sea change from its application to load consumption, therefore one has to be extra careful while selecting the rating so that on one hand, it should not be underloaded and on the other hand it should not be overloaded.

Why under-loading and over-loading, both are not recommendable?

Transformers are procured in such a way so that they should neither be underloaded nor overloaded. If they are underloaded their no-load losses would be higher and if overloaded it would drastically deteriorate their life-span, limiting it to few years after they have been to be overloading, besides of making them hazardous to the surrounding area by making them vulnerable to explode because of the high temperature being generated in them. Once exploded, it’s a real catastrophe as it’s the tank is filled with the volatile oil, which has low igniting temperature and it’s near impossible to control it once it catches fire. So, it’s a tricky question to decide the optimum rating of the transformer to full-fill both needs, as neither to be under-loaded and nor to be over-loaded. So, let’s take you to the optimum loading of the transformer.

What is the optimum loading of the transformer?

The distribution transformers work optimally when they are loaded at 50-60% of their capacity. This parameter is normally met when the core losses and the copper losses of the transformer become equal. As the load on the transformers continuously keeps on changing, therefore, they are designed to give their best efficiency at 50% loading.

The power transformers on other hand are designed to operate at 100% load, which is at their full rating capacity, as they are continually in operation and what matters most are the power losses rather than the no-load losses. These transformers are designed to have their core losses and the power losses the same at their full loading.

If the transformers are loaded for more than this range then the negative effect on the terms of relative terms may not seem to matter much, but in absolute terms, it leads to significant money-wastage. Therefore, it’s important to consider the load and the application both before finalizing the final rating of the transformer, otherwise one would keep on getting penalized for the entire life of the transformer for his onetime wrong decision.

What are these no-load losses and power-losses?

The no-load losses are considered as the losses which are incurred on behalf of charging the transformer and which broadly remains the same without being affected by the actual loading of the transformer. These losses happen in the core of the transformer and provide the magnetizing field to let the transformer to work and to meet the requirement of the eddy current losses that flow in their iron plates.

The Power losses or copper losses are the losses that take place in the transformer because of the actual current flowing in the transformer in their primary and the secondary windings, which results in producing heat in the windings due to I2R losses. As these losses are dependent on the current flowing in the windings, therefore the actual loading of the transformer matters a lot.


The above decision implies that the distribution transformers should never be loaded to their full capacity, as they are designed to work at their best under 50-60% of their rated capacity, otherwise, they are designed to work best at their full 100% loading.

In case the loading of the transformer is not as it is intended for, it would unnecessarily result in losses. As a result, one would be paying significant money on overcoming their losses on the one hand, and on the other hand, would be reducing their operating lifetime. But, in any case, the transformer should never be loaded beyond it’s rated capacity.