Cable, fuse
and power supply

How these are worked out. The alternator is sized on the average draw — music pulls roughly half of continuous full power — plus the car's own draw — and that is not fixed: a modern car pulls more than an old one, because electric steering, an electric radiator fan and dozens of control units eat far more than LED lights save. The battery, by contrast, is not chosen on Ah but on internal resistance: that is what decides how far the voltage sags on a bass note. Ah only tells you how long it lasts with the engine off, which is a different question.

The highlighted row is the thinnest one that can carry the fuse. The voltage drop sits beside it so you can go thicker if you want — but the difference between two neighbouring sizes is typically under 0.1 dB.

The size applies to both cables. The recommendation is the cross-section of one cable, and the ground cable must be at least the same. The current is identical all the way round, so a thin ground eats exactly the voltage you saved by running a thick positive. If several amplifiers share the cable, add their RMS power together and calculate on the total.

The fuse protects the cable — not the amplifier. It has to be large enough that the system does not blow it in normal use, and small enough that the cable does not burn first. It belongs within 30–45 cm of the battery positive terminal; every bit of cable before the fuse is unprotected, and if that shorts to the bodywork there is nothing to break the current. The fuse sizes are pooled across MEGA, ANL and MIDI/AMI — no single series carries them all, and MIDI/AMI stops at 200 A.

Two batteries — one up front and one in the boot? The gauge does not change. The rear battery buffers short peaks, but it has to be recharged through that same long cable, so the average current is unchanged — and the cable has to carry its own fuse regardless. So still enter the full length from the front battery to the amplifier: that is the run the voltage drops across under sustained load, and measuring only from the rear battery gives you a far too optimistic figure. What the rear battery buys you is stability during the bass note itself, because the current peak now comes from half a metre away instead of four. Two things do change: the cable is fed from both ends and can short from either side, so it needs a fuse within 30–45 cm of each battery's positive terminal — not just the front one. And the rear battery needs a ground cable to the chassis at least as thick as the positive: the charging current has to return the same way, and four metres of spot-welded body panel is a poor conductor. Use the same type and age of battery at both ends, or the stronger one will constantly charge the weaker.

CCA is not just cheaper copper. The core is aluminium with a thin copper skin, and it conducts only about 62% as well. That gives 1.6 times the resistance — but only about 21% less current capacity. The two figures differing is not a mistake: heating in the cable goes as the current squared, so the limit only falls by the square root of the resistance difference. Net, CCA typically has to go one to two sizes up to do the same job. Bear in mind too that cheap CCA often has thick insulation and a thinner conductor than the label promises — measure it.

How the thickness is chosen. Above all the cable has to carry its own fuse, so current capacity decides. Voltage drop is calculated as a check — resistance is ρ · length / section, with ρ = 0.0172 Ω·mm²/m for copper at 20 °C, and positive plus ground added together — but it only forces a thicker cable once the drop passes 3.5%. That limit is set so the recommendations line up with the charts used in the trade.

Do the lights dim on the bass? The cable is rarely the cause. A bass note is supplied by the battery, not the alternator — it cannot react that fast — and a tired battery or a poor ground connection typically sags more than the whole cable run does. Measure the voltage at the amplifier terminals and at the battery posts while it plays: if both sag by the same amount it is the battery; if there is a large difference it is the cable or the ground between them. Check the ground first — bare metal, no paint, short path. Distinguish the two kinds of sag as well: if the voltage only drops during the bass note and recovers between notes, that is internal resistance. If it falls slowly across a whole song, the alternator cannot keep up with the draw. And a capacitor will not fix it: a 1 farad can hold 200 A for about 5 milliseconds before it has itself dropped a volt. One swing of a 40 Hz tone lasts 25 milliseconds — and a bass note is many swings. To manage even a tenth of a second it would have to be around 40 farad. Where it does seem to help, it is usually the heavy cable run in order to fit it.

About the current capacity. The figures follow the chassis wiring column of the common AWG table — one cable in free air, not in a bundle — converted from AWG to mm². They are a ceiling under favourable conditions, not a guarantee: run the cable in a bundle, through a hot engine bay or under carpet, and it will carry less. When in doubt, go one size up.