The IEEE Transformer Loading Guide equations use the top oil temperatu
re rise over ambient to determine the winding hottest spot temperature
during an overload. Recent investigations by the author and others ha
ve shown that during overloads there is a time lag between the top oil
temperature rise and the oil temperature rise in the winding cooling
ducts. This phenomena results in winding hottest spot temperatures gre
ater than predicted by the current (1981) IEEE Loading Guide equations
. Accurate predictions of the winding hottest spot temperature require
s the use of the temperature of the oil entering and exiting the windi
ng cooling ducts. Low flammability liquids such as silicone or high te
mperature hydrocarbons are not covered in the present Loading Guide. R
esistance change with temperature and liquid viscosity are not conside
red in current Loading Guide equations. Loading Guide equations also a
ssume a constant ambient air temperature during a load cycle. Improved
loading equations based on analysis and testing are presented in this
paper. The improved equations consider type of liquid, cooling mode,
winding duct oil temperature rise, resistance and viscosity changes, a
nd ambient temperature and load changes during a load cycle. A PC BASI
C computer program to perform the calculations was developed and will
be included in the next edition of the IEEE Loading Guide for Mineral
Oil Immersed Transformers.