Usual Temperature and Altitude Service Conditions-Equipment conforming to this Standard shall be suitable for operation at its standard rating, provided that:
(a) The temperature of the cooling air (ambient temperature) does not exceed 40C if the equipment has copper-to-copper contacts or equivalent.
(b) The temperature of the cooling air (ambient temperature) does not exceed 55C if the equipment has all contacts silver surfaced or equivalent, and in addition has all conducting joints, including terminal connections, either (1) Separately held mechanically, and either soldered or silver surfaced, or (2) Brazed, welded or silver soldered and provided that such operation does not result in temperatures in excess of 90C for class O, 105C for class A or 130C for class ? insulations.
( c ) The altitude does not exceed 3300 feet (1000 meters).