30 amps is a bunch, for anything other than a starter on a race car. Weatherpacks are made for up to 10-12 gauge which should handle 30A fine. Is the motor a problem? Never seen one even half that. I've never had a hint of a problem with the Weatherpack stuff..

Is that one fan or multiples? On multiples, I use a separate relay and wiring for each motor.

If the results don't match the theory, change the theory.