You want delay fuse instead of fast-acting fuse. Look for delay on product description.
Since the motor has a high amp draw when first starting, then that means surge of heat traveling over wire.
Fuse trips when sensing heat.
Delay fuse will accept high heat for longer period of time before tripping.
You have 1 Hp 6K321 motor from WW Grainger. You can contact grainger.http://www.grainger.com/Grainger/DAYTON-General-Purpose-Motor-6K321?Pid=search
I recommend that you use 12 gauge wire and 20 amp fuse. Over-sizing wire will save heat loss and prevent damage to motor, and heat damage fuse box.
If you use smaller 14 gauge wire and 15 amp fuse, the motor will heat the wire, especially if length of wire is longer than 100 feet from fuse.
The motor has thermal protection, so motor is not overheating.
The wire is overheating.http://waterheatertimer.org/Color-codewire.htmlhttp://waterheatertimer.org/See-inside-main-breaker-box.html