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How much vertical clearance is required for stacking two ventilated transformers?

Clearances of Transformers to objects above

Product Line:
LV Transformers

Applies to Ventilated Transformers by SquareD/Schneider Electric

Transformers installed in limited height clearance situations

Both Transformers are required to be in an ambient not to exceed 40degC with a 24 hour average of 30degC. If the average exceeds 30C because of the heat generated by the lower unit, the derating is 0.4% for every degree above 30C per ANSI C57 12.96 (this basic formula is only good up to an average of 50degC.) So if the average ambient of the upper Transformer becomes 40degC, it would have to be derated 4%.  SquareD/Schneider Electric recommends a minimum vertical clearance of 3` (36``) to minimize the heating of the upper Transformer`s cooling air by the lower unit. 
(NOTE: this information does not apply to EX transformers.  Presently, the clearance required between stacked EX has not been determined).
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