This method of calculation is only applicable if the following conditions are fulfilled:

1.       There is an approximately even distribution of power losses inside the enclosure.

2.       The installed equipment is so arranged that air circulation is but little impeded.

3.       The equipment installed is designed for direct current or alternating current up to and including 60Hz with the total of supply currents not exceeding 3150A

4.       Conductors carrying high currents and structural parts are so arranged that eddy-current losses are negligible.

5.       For enclosures with ventilation openings, the cross section of the air outlet openings is advised to be at least 1.1 times the cross-section of the air inlet openings.

6.        There are no more than three horizontal partitions in the PTTA or a section of it.

7.       Where enclosures with external ventilating openings have compartments, the surface of the ventilating openings in each horizontal partition shall be at least 50% of the horizontal cross-section of the compartment.

8.       For enclosures having more than one section with vertical partitions the temperature rise of the air inside the enclosure shall be determined separately for each section.