Your best bet for splicing an FKM material is to use an angled cut. Glue using a viton based adhesive, and heat the joint up to allow the adhesive to cure. This will provide a much better join than using glue - especially on a FKM material. I believe alot of the supply houses (mc master, etc)...
The IEEE rating usually points specifically to motor enclosures, as the definitions are called out in the NEMA motor standard.
Alot of pump users are aware of this standard, even on the other areas of the pump, not just the motors. They will use these ratings on more than just the motor...
