C. Altitude Derating For transformers located at higher altitudes, they must be derated to accommodate the decrease in cooling capability due to the thinning atmosphere. The rule of thumb for transformers installed above 1,000 meters (~3,200 feet), is the transformer should be derated 0.5% for every 100 meters above 1,000 meters. 𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫𝑫 𝒌𝒌𝒌𝒌𝒌𝒌 = 𝑹𝑹 𝑹𝑹𝑹𝑹𝑹𝑹𝑹𝑹 𝒌𝒌𝒌𝒌𝒌𝒌 𝒙𝒙 𝟏𝟏𝟏𝟏𝟏𝟏 − ඌ𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨𝑨 𝑨𝑨𝑨𝑨 𝑰𝑰𝑰𝑰 𝑰𝑰 𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰𝑰 𝑰𝑰 −𝟏𝟏 , 𝟎𝟎𝟎𝟎𝟎𝟎 𝟏𝟏𝟏𝟏𝟏𝟏 𝒙𝒙 𝟎𝟎 . 𝟓𝟓 % ඐ ൨ Where: Derated kVA = Maximum kVA loading at installed altitude Rated kVA = Nameplate full load rating Altitude Installed = the installation altitude in meters when over 1,000m
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