What are your needs? Do a key word search using "MCC heat" for a list of threads (in this forum and others) -- possibly there are some ideas there (you might try some other key words to see if other threads show up..)
There are also a number of resources for estimating the heat gain for electrical equipment: I use a book, "HVAC Equations, Data, and Rules of Thumb" by Arthur A. Bell, Jr. for estimating A/C needs for MCC rooms --
rittal have a section of their catalogue devoted to this...I think they have a bit of software available too, on the electronic version of the catalogue if my memory serves
Suggestion: It is necessary to know watt dissipation of various items located in the MCC compartments, e.g. contactor and relays coils wattages, thermal overloads heat dissipation, control transformer wattage, contact resistances, etc.
The electrical enclosure mfrs. have these formulas and discussions in their catalogs (Hoffman & Hammond, amongst others). You will need to know the wattage dissipated inside the JB, and its surface area. The formula will be able to give a temperature rise above ambient (TRAA). Looking at the Hoffman, they supply a linear relation depicted in a chart, that can be summarized as:
TRAA(degs F) = 4.375 degs. F / sq.ft. (1)
So if you have a 48" X 36" X 16" enclosure that has 300 watts being dissipated within it, this would mean that: