How to size amp wiring the right way
Powerful car amplifiers pull a lot of current from a 12-volt electrical system. Undersized wire wastes power as heat, makes the amp sound weak when the bass hits and, in the worst case, can cause a fire. Choosing wire and fuse correctly takes only a few steps, and this calculator does the math for you.
1. Current draw
An amplifier converts electrical power into sound with some efficiency loss. Divide the amplifier’s RMS output by its efficiency to get the input power, then divide by the system voltage to get amperes. Class D amplifiers are typically 75–85% efficient, while Class AB designs are closer to 50–65%, so they draw more current for the same output.
Current (A) = input power ÷ voltage
2. Fuse size
The fuse protects the wire, not the amplifier. It must be larger than the maximum current the amp draws, otherwise it will blow during loud peaks, but small enough to protect the wire. We add 25% to the calculated current and choose the next standard fuse rating. Place the main fuse within about 18 inches (45 cm) of the battery terminal.
3. Wire gauge and voltage drop
Every wire has resistance. The thinner and longer it is, the more voltage is lost along the way. A drop of 3% on a 13.8 V system is about 0.4 V, which is a good limit for audio. We test each standard gauge against both the voltage drop and its safe current rating, which must be at least the fuse size. Thicker wire is a better investment than a bigger amp when the system is struggling.
Drop % = voltage drop ÷ system voltage × 100
| AWG | Ω per 1000 ft | Typical safe current (copper) |
|---|---|---|
| 8 | 0.628 | ~50 A |
| 6 | 0.395 | ~70 A |
| 4 | 0.249 | ~100 A |
| 2 | 0.156 | ~150 A |
| 1/0 | 0.098 | ~200 A |
| 4/0 | 0.049 | ~350 A |
Cheaper copper-clad aluminum (CCA) wire has about 60% higher resistance, so use one or two sizes thicker than copper. Use a short, thick ground wire (same gauge as the power wire) of under 3 feet to clean bare metal on the chassis.
4. Match amplifier and speakers
For the best sound, the amplifier’s power per channel should be about 75% to 150% of the speaker’s RMS rating. Too little power makes loud passages clip, which can damage tweeters; far too much invites over-driving the speaker. The calculator compares your per-channel power with the speaker rating.
5. Engine-off listening and the alternator
With the engine off, music draws on the battery alone. Because most starter batteries should not be discharged below about 50%, only half of the rated Ah is usable. At average listening levels, music uses a fraction of an amp’s maximum current, which is what the duty factor represents. For large systems, an upgraded alternator, “Big 3” cables (alternator-to-battery, battery-to-chassis, engine-to-chassis) or a second battery help keep the voltage stable.