Work out what your amplifier actually pulls, what wire gauge survives it, what fuse protects that wire, and whether your charging system can feed the whole thing. This is the part of a build that gets skipped most often and causes the most real damage — dimming lights, cooked batteries, melted wire.
Undersized power wire is a fire risk, not a performance compromise. Every number here is a starting point calculated from published conductor data — not a substitute for an installer who can look at your actual vehicle, your actual wire, and your actual terminals. If the result surprises you, get a second opinion before you cut anything.
This matters more than people expect. A lot of budget "4 gauge" wire kits are CCA, which carries roughly 62% of copper's current for the same gauge. In practice CCA needs about two gauge sizes larger to do the same job. If your wire wasn't clearly sold as OFC, assume it's CCA and pick that option.
Most calculators give you one current figure. That's the mistake — sizing everything off a single number gets you wrong answers in both directions.
Wire and fusing have to survive the worst case, so they're sized on full continuous draw. But real music with bass in it averages roughly a third of that, so sizing an alternator to the continuous figure massively overbuys. That ⅓ figure is a long-standing rule of thumb, not a measured constant — it moves with genre, gain structure, and how hard you drive the system.
This is the single most misunderstood part of a power install, so it's worth stating flatly. The main fuse exists so that if your power wire ever shorts to chassis anywhere along its run, the cable doesn't become a heating element.
Mount the main fuse within 12–18 inches of the battery positive terminal. Any unfused wire between the battery and the fuse is unprotected — that short stretch is the entire reason the distance matters. The fuse rating must sit at or below the wire's ampacity (or the wire, not the fuse, becomes the weak link) and at or above your continuous draw (or it'll nuisance-blow).
Honest note on ampacity: published AWG ampacity tables disagree with each other by 2–3× depending on whether you read the "chassis wiring" or "power transmission" column. This tool uses the chassis-wiring convention, which is the correct one for car audio — short runs, single conductor, free air. It's a convention, not a physical constant, and it shifts with insulation temperature rating and how tightly wire is bundled.
The "Big 3" upgrade does not increase your alternator's output. It upgrades three cables — alternator positive to battery positive, battery negative to chassis, and engine block to chassis — which lowers resistance in the charging path you already have. That's a real, worthwhile improvement, and it is not the same thing as making more current. If your alternator is genuinely too small, the Big 3 will not fix it.
A second battery does not add charging capacity. The alternator is still the only thing generating current in the system. A second battery adds a buffer — genuinely useful for bass transients and for listening with the engine off — but it cannot fix a charging deficit. Adding one to solve dimming lights treats the symptom.