There are a number of references:
Small Motors:
NEMA MG-1-1998 Rev 2 - 12.63 MACHINE WITH MOISTURE RESISTANT WINDINGS-CONFORMANCE TEST A (Small Motors)
Large Motors:
NEMA MG-1-1998 Rev 2 - 20.18 MACHINE WITH SEALED WINDINGS-CONFORMANCE TESTS
IEEE 429 (withdrawn?) – Thermal Evaluation of Sealed Insulation Systems for form-wound 6900 and below