IEEE C57.91:1981 (R1991)

New IEEE Standard - Superseded.
General recommendations for loading 65 degrees Centigrade-rise mineral-oil-immersed overhead and pad-mounted distribution transformers are covered. Recommendations for 55 degrees Centigrade-rise transformers are included because a substantial percentage of these transformers are still in service. The recommendations are based on winding hottest-spot calculations only. Other factors that may limit the permissible load to lower values are listed. A method to establish winding temperature and loss of life by nomographs is given.

This guide covers general recommendations for loading 65 "C rise mineral-oilimmersed overhead and pad-mounted distribution transformers described in American National Standards and rated 500 kVA and less. (For transformers rated 501 kVA and larger, see ANSI/IEEE C57.92-1981, [31') Recommendations for 55 "C rise transformers are included in this guide because a substantial percentage of these transformers are still in service.