What's the current size of the wires are you looking to replace and what voltage drop are you experiencing?
For example: a 100 amp alternator is not going to provide more than 100 amps if you increase the wire size. It will run at maximum efficiency. However, if you have a 100 amp alternator and wiring that can only carry 75 amps, then an upgrade to match the wiring to the alternator output would be worthwhile.
Another example: if you have a 100 amp alternator, 3' of 4/0 wire and running at 12v DC, your overall voltage drop is .029v (0.25%). I highly doubt an increase in wire size is going to be noticeable without a multimeter.
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