While researching the basis of the 40oC max operating temperature outlined in the normative requirements for industrial control panels (UL508, UL 508A, NFPA 70, NFPA 79) the question struck me as to why is 40oC the magic number? Why not 35, 41, 45, 50 oC maximum operating temperature? After doing some research into peer-reviewed documents, I am walking away scratching my head.... and am hoping someone within the brotherhood of electrical engineers knows why 40oC is the basis and can point me in the direction of peer-reviewed publications, books, or other documents so I can learn what is so magic about this value. Any assistance is appreciated.