What kind of transformer are you specifying?
In general, transformer BIL is selected based in the following technical and economical factors such as:
o System voltage (both primary and secondary).
o Type of application: Distribution, Instrument, power transformers, etc.
o Cooling/Insulation medium: Fluid immersed (oil, silicon, etc) vs. dry type insulation.
Important distinction should be made between the requirements for internal versus external transformer BIL.
o External BIL: Bushing typically is selected at full BIL rating. However, uprating BIL may be required to compensate for dielectric losses for adverse environmental conditions such as pollution level or high altitude (above 1000 m) among other factors.
o Internal Insulation level: Winding BIL is selected based on connection configuration. Ungrounded typically required full BIL while grounded systems may require reduced BIL level (typ. 1 or 2 steps bellow standard full BIL level) if appropriate MOSA surge arrester is installed close to the transformer per standards such as IEEE std C62, IEC as shown below.
The enclose site provide some guides how to select the appropriate BIL for transformers based in ANSI guidelines.
