I have a 1991 Costic amp that has small output connector holes (about 1/16in diameter) for + - terminals. I'm hoping to connect subwoofeers with largest wire possible, but am I missing some hardware terminal tip/trick to use medium-large gauge wire?
There is no reason to use large gauge speaker wire unless you simply want to for looks.
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Support the12volt.comTrue. The size of those terminals should be a clue for you that the amp manufacturer knew what the maximum amperage flow could possibly be to the speakers. They indicate the wire size that will handle the current with ease.
I have an old pair of home speakers from the '70's that had such small speaker wire holes I couldn't insert a normal 16 ga. wire. So I drilled them out. Maybe you could look into that possibility if you want to run fat wire. And if you can't do that, you can clip off the wire strands that won't fit. Once installed nobody would know, and you're not dealing with a danger factor in this case by tampering with the wires because of the known low current output. You would just end up with the fat wire look.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.