What wire and fuse does your amp actually need?

The part of a build that gets skipped most and does the most damage — dimming lights, cooked batteries, melted wire. Enter your RMS and the run length: get the gauge that survives it, the fuse that protects it, and whether your charging system can keep up.

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.

Your amplifier

Picking an amp fills in RMS power and class below from this site's real amp database. You can always override either one by hand.
Use the RMS rating at the impedance you're actually running, not the peak/max number on the box. If you don't know your final impedance, run the Wiring Calculator first — it computes the load at your amp's terminals from your coil wiring.
Efficiency decides how much current the amp pulls from the battery to make its rated output. A class AB amp pulls substantially more current than a class D amp for the same power. Class D runs about 85% into 4 Ω and falls to roughly 65–75% at 1 Ω (Rockford Fosgate's published figures); class AB is estimated at 55%, a figure no measurement here backs.
Only class D changes with this. Picking an amp above sets it for you. A 1 Ω rating draws about a fifth more current than the same watts at 4 Ω, so it gets a bigger wire and fuse. If you don't know the load, leave it at 4 Ω or run the Wiring Calculator first.
Wire and fuse are always sized at 12.6 V, the higher current, because a fuse has to hold at the worst voltage the system sees. This setting only changes how the voltage drop is shown. Newer cars with smart charging can run anywhere from 12.5 to 15 V, Victron says. 13.8 V stays the default here.

Your wiring run

Measure the route the wire actually takes — along the firewall, under the sill, around the trunk — not the straight-line distance. The 17 ft filled in here is the power-wire length in an amp wiring kit (Rockford Fosgate's kits ship 17 ft), not a measurement of any car; yours may be shorter or longer.
Leave at 0 for a normal short chassis ground — this tool does not secretly double your run the way a two-wire DC calculator would. Enter a real length only if you're running a long ground back to the battery instead of grounding to chassis nearby. Amp makers cap the chassis ground's length: JL Audio 36 in or less, Rockford Fosgate under 30 in, Kicker 24 in or less and the same gauge as the power wire.
The ground carries the same current as the power wire, so size it like the power wire. Bolt it to bare chassis metal near the amp: sand or scrape off the paint and sound deadener (JL Audio, Rockford Fosgate, DD Audio), put a star washer under the bolt (JL Audio, Rockford Fosgate), and never use a seat-belt bolt (Rockford Fosgate, DD Audio). Add or upgrade the battery-negative-to-chassis cable too (JL Audio, Rockford Fosgate, Kicker). To check a ground, measure the voltage across it under load (see "Measure before you buy cable" below).

This matters more than people expect. A lot of budget "4 gauge" wire kits are CCA. It has about 62% of copper's conductivity, so for the same gauge it drops more voltage and carries roughly 78% of the current — two different penalties, and this calculator applies both. On voltage drop alone, CCA needs about two gauge sizes larger to do the same job; the 78% current limit is a separate check. If your wire wasn't clearly sold as OFC, assume it's CCA and pick that option — and note that a lot of CCA cable is also undersized against its printed gauge, which is a separate problem this tool can't see. CTA-2015, the car-audio cabling standard, says it exists so buyers can confirm their wire is made with "copper or another metal that performs as well".

Your charging system (optional)

Usually printed on the alternator case or in the service manual. Leave blank and this tool simply won't guess at your charging headroom. That number is the maker's rating at a high alternator speed, not what it makes at a stoplight. At idle the same alternator makes less, and less again when hot: the one stock car alternator in the published data makes about half its rating at idle, heavy-duty units sold for idle output about three quarters (see the note at the foot of the page). If the maker prints an idle figure for yours, use that.
Everything the car already runs before your stereo: ECU, fuel pump, lights, HVAC, infotainment. About 35–50 A with the engine idling and nothing else on (Larry Carley, AA1Car); more with the radiator fan, blower or lights. 50 A is a reasonable default.

Result

Enter your amp and wiring run above, then hit Calculate.

4 gauge CCA is not 4 gauge copper

The material changes the answer as much as the gauge does. Pick CCA above and this calculator applies both penalties for you.

CCA 4 gauge carries what copper 6 gauge carries Chassis ampacity by conductor, from the table this calculator uses Two different penalties, and they are not the same number 62% of copper's conductivity sets the voltage drop 78% of copper's current rating sets the heat limit ~2 gauge sizes larger to match copper Amps each wire carries before it gets too hot 1/0 AWG copper 245 A 2 AWG copper 181 A 4 AWG copper 135 A 4 AWG CCA 105 A 6 AWG copper 101 A 8 AWG copper 73 A 0 245 = largest The two orange bars are the point: 4 gauge CCA and 6 gauge copper carry almost the same current. Buy CCA at the gauge printed on the box and you have quietly bought a smaller wire. CAVEAT A lot of CCA is also undersized against its printed gauge — thinner conductor than the label claims, on top of the material penalty. Nothing in this calculator can see that. If it was not clearly sold as OFC, assume CCA. calculator/electrical-calculator.js — CHASSIS_AMPACITY x CCA_AMPACITY_FACTOR 0.78 — 2026-09-15

Two numbers, two different jobs

Most calculators give you one current figure. That's the mistake — sizing everything off a single number gets you wrong answers in both directions.

Continuous drawsizes wire & fuse
Music-average drawsizes charging headroom

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.

The battery fuse protects the wire

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. The fuse on the amp itself is a different job: DD Audio says its amp fuse protects both the amp and the car's electrical system from fault conditions.

Mount the main fuse within 18 inches of the battery, measured along the wire. 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). Disconnect the battery negative before you start, and fit the fuse last, once the wire is connected at the amp.

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.

Two myths worth killing

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 — so less voltage is lost between the alternator, the battery and your amp. Kicker's words for what it buys: "the maximum performance your alternator is able to deliver" — not more than that. JL Audio asks for it once your main fuse is above 60 A; Kicker at 1,000 W or more. Both say 4 AWG at least, and Kicker adds that 2 AWG or 1/0 is better. Keep the factory wires and add the new ones alongside, remove the paint at each contact and bolt it down (Kicker). Kicker calls the engine-block wire optional once the battery-to-chassis cable is upgraded. Measure first (the card below): the Big 3 only helps if the wiring is actually losing voltage. If your alternator is 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. The makers describe a second battery as engine-off listening time and support for competition-level systems (Kicker, Odyssey), and it cannot fix a charging deficit. As Kicker puts it: "If your car can't charge one battery, it will never be able to charge two batteries." Adding one to solve dimming lights treats the symptom.

Measure before you buy cable

A voltmeter tells you whether the charging wiring is losing voltage, and where. Do it before you buy a Big 3 kit.

Run the engine at 1,800–2,000 rpm with every light and accessory on — headlights, blower, defrost — and the stereo playing. Measure from the alternator's output stud to the battery's positive post, then from the alternator's case to the battery's negative post. Delco Remy, testing at the alternator's full rated output, allows 0.5 V for the two together. The repair-shop figures from Larry Carley (AA1Car) are 0.5 V or less on the positive side (0.2 V ideally) and 0.2 V or less on the negative side. Check your amp's ground the same way while it plays, but across the connection itself: from the amp's ground terminal to the bare metal it is bolted to. The repair trade allows about 0.1 V across a ground and 0.2 V across a wire or cable (Fluke).

Over the limit? Fix the joints first — paint off, bare metal, bolted down (Kicker) — then add cable. A steady reading is not the whole story: JL Audio warns that dips can be too brief for a DC voltmeter to catch.

If you add a second battery

This calculator does not model a second battery. The rules the battery and isolator makers print:

  • Batteries wired in parallel should match in age, size and type (Optima), and never mix AGM, flooded and lithium (Odyssey).
  • Take the load from opposite ends of the pair, so both batteries share the work (Odyssey).
  • A diode isolator drops 0.3–0.45 V. Victron says the result can be batteries that "are not charged to the full 100% and age prematurely", unless the alternator's output is raised to make up for it. A voltage-sensitive relay has "virtually no voltage loss" (Victron).
  • Fuse the link between batteries to suit its wire (Victron), and check the charging voltage at each battery's own terminals (Optima).

Lithium (LiFePO4) is a different battery, and this tool does not model it. Makers cap its charging voltage at 14.6–14.7 V (Dakota Lithium, Antigravity). Cold limits charging: Battery University advises against charging it below freezing, and Dakota's own battery cuts charging off at 23°F (−5°C), so follow your battery's limit. Makers also limit how much charging current it takes, from 15 A (Antigravity) to 80 A from an alternator (Dakota) — read yours before wiring it to the car.