your good put in a 40

 

Make sure your relay is rated at what ever fuse you are going to run, those fans will probably draw slightly more than 30 amps so make sure you have the wire and relay to hold it. I however had a brand new fan that was blown before I put it in the truck so it is possible to have a bad fan motor.

 

the current draw will be determined by the resistance in the fans and you have to use wire and fuses to match. if you use smaller wire all you will do is create a fire hazzard.

i am with the other guys, try a 40amp fuse. the fans motors are like a dead short right as they start up and probably really draw 60amps plus but the fuses can take a little of that till the fans get spining and the current draw goes down.

 

I would ohm out the fan motors. It sounds like one may be bad. I have a 25 amp on each fan I think. If the fuse is popping when you put it in, there has to be a dead short. If it starts to turn then blows, like everyone else said try a bigger fuse, or it may be a bad winding in one of the fan motors. good luck