Wiring my engine from the key.

I've been having problems with my mower lately, not starting from the key so I rewired it using my steps below.

5 prongs on the ignition switch- SBLMG

S- post on starter solenoid
B- Wire straight to the battery with a 20 amp fuse
L- Red Stator Wire and Grey Wire from Carb
M- Black Wire from Coil/ Magneto
G- Ground

Mower starts and Run fine using my steps above. But the "B" Wire Straight to the battery and 20 amp fuse was very hot and then melted at the fuse. My Question is WHY?

Wiring my engine from the key.