Oil-Immersed Transformers and regulator,

This volume is intended for use as a guide in determining the amount of overload that oil-immersed transformers and regulators can carry under various operating conditions and with varying degrees of insulation deterioration.
WATER COOLING.- Do not spray water on transformers or radiators except in dire emergency. As water evaporates, minerals from the water will adhere to cooling surfaces, the minerals are almost impossible to remove. This will further reduce cooling and transformer life.The hottest-spot winding temperature is the principal factor in determining life due to loading. The temperature cannot be measured directly because of the hazards in placing a temperature detector at the proper location because of voltage. Standard allowances have,therefore, been obtained from tests made in the laboratory. The hottest-spot copper temperature is the sum of the temperature of the cooling medium, the average temperature rise of the copper, and the hottest-spot allowance. The hottest-spot allowance at rated load is 10 °C for transformers with 55 °C average winding temperature rise by resistance and 150C for transformers with 65 0C average winding temperature rise by resistance