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How to derate a transformer for use at high altitudes?

At altitudes greater than 3300 feet (1000 meters), the density of the air is less than at lower elevations.  This reduces the ability of the air surrounding the transformer to cool, so the temperature rise of the transformer is increased.  Therefore, when a transformer is being installed at altitudes greater than 3300 feet above sea level, is is necessary to derate the nameplate kVA by 0.3% for each 330 feed (100 meters) in excess of 3300 feet (1000 meters).