What size fuse for a 3000 watt amp?

About 260 amps of draw at 14.4 volts for a 3,000 watt RMS class D amplifier, so a 300 amp ANL main fuse on 1/0 gauge oxygen-free copper for a short run, or two 1/0 runs each fused at 150 amps for a longer one. At this power the alternator and the big 3 wiring matter as much as the fuse: a stock alternator cannot feed 260 amps, so check the alternator size calculator before buying the amp.

What size fuse for the amp power wire?

The main fuse protects the wire, not the amplifier, so it is sized to the wire: around 100 to 125 amps for 4 gauge, 150 amps for 2 gauge and 250 to 300 amps for 1/0 gauge copper on runs under about 15 feet. Put it within 18 inches of the battery, then let the amplifier's own onboard fuses protect the amp. If the calculator says the amp draws more than the wire's fuse rating, the wire is too small.

What size fuse do I need for a 1000 watt amp?

A 1,000 watt RMS class D amplifier draws about 87 amps at 14.4 volts, so the main fuse near the battery should be 100 amps, on 2 gauge or 1/0 power wire. 4 gauge copper is rated 85 amps, below the draw, so it is too small for this amp at full power. If the 1,000 watts is a peak or max rating, the real RMS draw is lower; size from the RMS figure.

What size fuse for a 1200 watt amp?

At 1,200 watts RMS a class D amplifier draws about 104 amps, which calls for a 125 amp main fuse on 1/0 gauge wire. That is more than 2 gauge can protect at its published 115 amp ampacity. For a class A/B amplifier of the same power the draw is closer to 150 amps, so the wire and fuse step up again.

Are these figures published or are they rules of thumb?

Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.

Why does this calculator sometimes refuse to give an answer?

Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.

Do I still need an installer if I use this?

Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Electrical calculator

Amp Fuse Size Calculator: What Size Fuse for Your Amp

Updated 2026-09-26. Researched from published standards and manufacturer specifications.

Quick answer

A 1,000 watt RMS class D amp draws about 87 amps at 14.4 volts, so it needs a 100 amp main fuse on 2 gauge or 1/0 wire; 4 gauge (rated 85 amps) is too small. A 1,200 watt amp draws about 104 amps and takes a 125 amp fuse on 1/0. Size the fuse to the wire, and mount it within 18 inches of the battery.

Main fuse by amplifier RMS power (class D, 80 percent efficiency, 14.4 V)
Amp RMS power	Current draw	Main fuse	Smallest power wire
500 W	43 A	50 A	8 gauge
800 W	69 A	80 A	4 gauge
1,000 W	87 A	100 A	2 gauge
1,200 W	104 A	125 A	1/0 gauge
1,500 W	130 A	150 A	1/0 gauge
2,000 W	174 A	175 A	2/0 gauge

Published figure Source: Wire ampacity from NFPA 70 (National Electrical Code) Table 310.16, 75 C column; current draw from the class D efficiency convention; both computed by the calculator below. Class A/B amplifiers run near 55 percent efficiency and draw roughly 45 percent more current for the same power. Check the amplifier manual: many list their own recommended fuse, and the main fuse must never exceed what the wire can carry.

Result

Main fuse 100 A Published figure

Current draw at rated power 86.8 A Convention

Conductor ampacity 150 A Published figure

Assumed efficiency 80 % Convention

Fit this fuse within 18 inches of the battery positive terminal. A fuse at the amplifier end protects nothing: the whole length of cable between the battery and the fuse is unprotected.

The formula

I = P_rms / (efficiency x volts), then fuse at or below the conductor ampacity

THE FUSE PROTECTS THE WIRE, NOT THE AMPLIFIER. Its job is to open before the cable can start a fire, which is why it is sized to the conductor. The amplifier has its own onboard fuses and they protect the amplifier. Fit the main fuse within 18 inches of the battery positive terminal.

The figures above are computed with these exact inputs: Amplifier RMS power 1000 W, Power wire size 1/0, Amplifier class d.

What this calculator assumes

Some of these are published physics or manufacturer figures and some are conventions. They are not the same kind of number, so each one is labeled.

Assumption	Kind	Where it comes from
Class D Efficiency: 0.8	Convention	A planning efficiency for a class D mobile amplifier at typical listening levels. Real efficiency varies with load impedance, output level and the amplifier itself, and no standards body publishes a single figure
NEC Table 310.16	Published figure	NEC Table 310.16, 75 C copper
Overcurrent protection sizes to the conductor	Published figure	Overcurrent protection sizes to the conductor, not the load. The same principle as NEC 240.4: a fuse protects the wire it is connected to
Fuse Distance18in: 18	Published figure	The 18 inch main fuse distance is the figure published in mobile electronics installation practice and mirrored by ABYC E-11, which requires overcurrent protection within 7 inches of the battery for boats and allows longer runs only in sheathed conduit

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That fuse size is the number the Power & Wiring Plan logs next to the wire gauge it protects, mounted within 18 inches of the battery.

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Frequently asked questions

What size fuse for a 3000 watt amp?: About 260 amps of draw at 14.4 volts for a 3,000 watt RMS class D amplifier, so a 300 amp ANL main fuse on 1/0 gauge oxygen-free copper for a short run, or two 1/0 runs each fused at 150 amps for a longer one. At this power the alternator and the big 3 wiring matter as much as the fuse: a stock alternator cannot feed 260 amps, so check the alternator size calculator before buying the amp.
What size fuse for the amp power wire?: The main fuse protects the wire, not the amplifier, so it is sized to the wire: around 100 to 125 amps for 4 gauge, 150 amps for 2 gauge and 250 to 300 amps for 1/0 gauge copper on runs under about 15 feet. Put it within 18 inches of the battery, then let the amplifier's own onboard fuses protect the amp. If the calculator says the amp draws more than the wire's fuse rating, the wire is too small.
What size fuse do I need for a 1000 watt amp?: A 1,000 watt RMS class D amplifier draws about 87 amps at 14.4 volts, so the main fuse near the battery should be 100 amps, on 2 gauge or 1/0 power wire. 4 gauge copper is rated 85 amps, below the draw, so it is too small for this amp at full power. If the 1,000 watts is a peak or max rating, the real RMS draw is lower; size from the RMS figure.
What size fuse for a 1200 watt amp?: At 1,200 watts RMS a class D amplifier draws about 104 amps, which calls for a 125 amp main fuse on 1/0 gauge wire. That is more than 2 gauge can protect at its published 115 amp ampacity. For a class A/B amplifier of the same power the draw is closer to 150 amps, so the wire and fuse step up again.
How does the fuse size calculation work?: I = P_rms / (efficiency x volts), then fuse at or below the conductor ampacity. THE FUSE PROTECTS THE WIRE, NOT THE AMPLIFIER. Its job is to open before the cable can start a fire, which is why it is sized to the conductor. The amplifier has its own onboard fuses and they protect the amplifier. Fit the main fuse within 18 inches of the battery positive terminal.
Are these figures published or are they rules of thumb?: Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.
Why does this calculator sometimes refuse to give an answer?: Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.
Do I still need an installer if I use this?: Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Researched guidance, not professional advice. These results are planning figures from published relations and manufacturer specifications. Fit the main fuse within 18 inches of the battery positive terminal, because that fuse protects the wire and the wire is what starts a fire. Disconnect the battery before running power wire, use a grommet through every piece of sheet metal, and remember that sustained high sound pressure causes permanent hearing damage.