It appears that your fusible link box protects (A) the high beam relay, (B) low beam relay, (F) lighting relay (gauge clusters) and lastly, (F again) the main relay which feeds your ECM and various sensors. There's more interconnectivity than I've stated, however, that's it in a nutshell. You don't need to upgrade any of that. I'm looking at the same junky "typical wiring diagram" you are. According to this, you have a white wire running from fusible link F to the + POS side of the battery. That's the only wire I could see which might need the upgrade. Normally, you'll have a big hot cable running from the alternator to a lug on a large amp fuse (100 amp or greater) located on the end of your under the hood fuse panel. From the other lug on the big fuse, another hot cable runs to the battery. When that fuse goes belly up, your alternator will no longer charge the battery. Looks to me like the white wire serves that purpose. I like Isuzus and their ruggedness, however, some of their engineering (primarily electrical and fuel/emissions) leaves a lot to be desired. What amp alternator did you upgrade to? It would have to be pushing 300 to be calling for a 250 amp fuse. <img src="/forums/images/graemlins/lol.gif" alt="" /> Like the other reply stated, your electrical needs, relays, starter, etc. are only going to draw the current necessary for them to fully function if everything is in order. The wire recommended to be a 6 or 4 gauge; which one is it? The white wire to the battery?