U.S. Code of Federal Regulations
Regulations most recently checked for updates: Oct 26, 2020
(a) In this subchapter, an ambient temperature of 40 °C (104 °F) is assumed except as otherwise stated.
(b) A 50 °C (122 °F) ambient temperature is assumed for all rotating electrical machinery in boiler rooms, engine rooms, auxiliary machinery rooms, and weather decks, unless it can be shown that a 45 °C (113 °F) ambient temperature will not be exceeded in these spaces.
(c) A 45 °C (113 °F) ambient temperature is assumed for cable and all other non-rotating electrical equipment in boiler rooms, in engine rooms, in auxiliary machinery rooms, and on weather decks. For installations using UL 489 (incorporated by reference; see 46 CFR 110.10-1) SA marine type circuit breakers, the ambient temperature for that component is assumed to be 40 °C (104 °F). For installations using Navy type circuit breakers, the ambient temperature for that component is assumed to be 50 °C (122 °F).
(d) Unless otherwise indicated in this subchapter, a 55 °C (131 °F) ambient temperature is assumed for all control and instrumentation equipment.
(e) If electrical equipment is utilized in a space in which the equipment's rated ambient temperature is below the assumed ambient temperature of the space, its load must be derated. The assumed ambient temperature of the space plus the equipment's actual temperature rise at its derated load must not exceed the equipment's total rated temperature (equipment's rated ambient temperature plus its rated temperature rise).