The Canadian Electrical code was changed due to this problem.
Following is quoted from the "CSA CE Code Handbook 2012" (C22.1HB-12)
"Rule 26-240 Transformers — General
Subrule (3) recognizes that the temperatures in the enclosure of an air-cooled (dry-type) transformer can exceed 30 °C, and the surface temperatures of the core-and-coil assemblies can exceed 90 °C. If the conductors enter the transformer enclosure through the top, conductor insulation and electrical equipment problems can occur from overheating. Conductors used to connect air-cooled (dry-type) transformers are not allowed to enter the transformer enclosure through the top, unless the transformer is marked to permit top entry."
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