Transformer Efficiency and Temperature Rise

It is best to obtain the actual load and no-load losses in watts from the transformer manufacturer, but sometimes those data are not available. In that case, temperature rise is a rough indicator of transformer efficiency. For example, a transformer with an 80C temperature rise uses 13-21% less operating energy than a 150C rise unit.
A more efficient transformer generates less waste heat in the first place, but transformer temperature rise results from not only how much heat is generated but also how much heat is removed. Be careful that a unit carrying a low temperature rise figure is not also inefficient, using fans to remove the excess heat.
Lower Temperature Rise Means Increased Overload Capability
A lower-temperature-rise transformer results in a transformer with higher overload capability. For example, an 80C rise dry-type unit using 220C insulation has 70C reserve capacity compared to a 150C unit. This allows the 80C unit to operate with an overload capability of 15-30% without affecting the transformer life expectancy. Also, a cooler running transformer means a more reliable unit and more up-time.
Designing a Transformer with Lower Temperature Rise
Transformers with lower temperature rise often use windings with lower resistance. The low resistance per unit length of copper allows lower temperature rise transformers to be built without unnecessarily building a bigger transformer. For example, an aluminum-wound transformer coil requires conductors with approximately 66 per cent more cross-sectional area than a copper-wound transformer coil to obtain the same currentcarrying capacity.
High Efficiency and Conditioned Spaces
High-efficiency, low-temperature-rise (80C rise dry-type or 55 C liquid-filled) transformers are frequently found in confined spaces, like inside electrical rooms, underground vaults, and air-conditioned spaces in buildings. High efficiency means less waste heat generated, thus lower ventilation and airconditioning requirements. Selecting such a transformer, properly sized to the load requirements, assures greatest efficiency, longer life and increased overload capability.