View Full Version : Wire gauge for amp install

07-23-2007, 08:03 PM
I'm installing a system and trying to determine what size power cable I need. I'm pushing 800W RMS, 700 to my mids and highs and 100 to the sub. The guy at the car audio shop recommended 4 awg, which is what I was planning on to begin with, but reading on some of these threads, I've seen people recommend 2 awg or 1/0 for anything over 500W - what's the need for such thick wire? :confused:

I found this site with a guide:

Also, at what point do I need to upgrade the alternator? I have a prestolite (I believe it's 55A, but I couldn't find it written on it). My amp is ~50% efficient, which would bring me to ~130A. Can that damage my alternator, or would it just not be able to maintain charge on the batteries? I'm planning on upgrading to dual batteries and a 1 Farad cap.

Please advise.

07-23-2007, 09:01 PM
What is the total if you add all of the amp's fuses? That will give you a good idea of what the cable needs to be able to handle. The sum of the fuses is worst case scenario but it's a good rule of thumb for sizing conductors. What make and model of amp are you using?

You want to use an amp that could draw approximately 2.4 times what your alternator can produce without any other accessories or the motor's current draw and you're asking if you need a bigger one? Just curious, but at what point would it seem obvious to you? Have you ever touched jumper cables that had been used to start a car with a dead battery?

High current causes a lot of heat, which drops voltage, creating more heat, which drops the voltage, etc. Thick wire conducts more current than thin wire. If you want your amp to do what you're buying it for, you'll need to wire it correctly, or you could have stereo problems, at best. At worst, you can have a fire. Whatever you use for the positive, you'll need to use the same for the negative and beef up the charging lead, use a distribution block on each and when you install the ring terminals on the cables, crimp and solder them. If the head unit is fairly close to the amp's location, run a ground lead from the back of the head unit's case to the amp's ground block. That will minimize the chance of having noise problems.

BTW, if you want to hear the bass, you're going to have to run more power to the sub(s). Human ears are a lot more sensitive to upper midrange, not bass. If your mids put out 93db @ 2.83V/1M (standard test voltage and distance, although at 4Ω, it's 2 Watts, not 1 Watt) and your sub(s) put out 89db @2.83V/1M, the power imbalance will make the mids/tweets too loud for this application unless the woofer is a lot more efficient. Remember, you're not inside and that makes a huge difference. Look up 'transfer function' to find out more about this.

07-23-2007, 09:51 PM
High current causes a lot of heat, which drops voltage, creating more heat, which drops the voltage, etc.

hmmm ... I lot of heat causes a voltage drop - I'd have to disagree with you. How about current and resistance.

The setup ... I have a 2-channel Punch 500 driving 4 Polk Momo 6.5" components on the tower. I have them 2 per channel in parallel, so they are at 125W RMS per speaker, with a 2Ω load on the amp. I have a 5-channel Punch 600 running the 4 - 6.5" internal speakers at 50W RMS each and a 10" sub at 100W RMS. Both amps are ~50% efficient. I'm not concerned about the sub for now - I'm using what I have for now until I decide to upgrade.

With 4 ga. wire, I would have a ~0.4V drop worst case and I'm sure I'm not going to be drawing anywhere close to that ever.

I've seen plenty of people on this forum post that they are pushing 2000-3000W of power off a 90A alternator. So what happens to the alternator? I assume it's designed to put out up to it's rated current and if more is needed it is sourced from the battery and will not blow the alternator.

07-23-2007, 10:15 PM
It's part of physics- when a conductor's temperature increases, it's resistance also increases. The voltage drops as a result.

The Power formula is P=IE and if P is going to be anywhere near constant, as E drops, I absolutely must increase (and we all thought we would never use algebra, eh?). It's the same reason starters fail when the battery voltage drops from cranking for too long. Voltage is electrical pressure, current is the volume of electron flow. Excessive current in a conductor that's too small causes temperature increase due to friction. The cross sectional area and length of of the conductor allow it to carry the current. A short conductor of a specific gauge will handle more current than a longer one of the same gauge.

Knowing that you have Punch amps, you need heavy cable. Punch amps don't have heavily regulated power supplies and because of that, the peak current draw is high. That's how they get their name- they're about as punchy as you can get. Old ones or new ones? The old ones used V-FETs and they have a really sweet sound.

Re: cable size and alternators- why take a chance of being stranded at the worst possible time? If a 2000W system is running on a 90A alternator, it's not possible to develop 2000W. No amp that I know of puts out its rated power at 12V. It's always at 14.4, or close to that. 2000W@100% efficiency draws 138.888A. What it's rated for and what it puts out are totally dependent on the power available, not what someone wants it to put out or what a sales brochure says. Idling at a stop light, a high power system does not put out its rated power, unless the regulator is adjusted for higher than normal voltage.

Not a good idea, regardless of whether a second, third or fourth battery is used- it's still straining the alternator. I would use more than 4AWG, if I were you. That's good for 60A @16 feet. Trying to make an alternator run at it's maximum for too long will cook it. Crank the snot out of a car stereo and then put your hand on the alternator. Do it with the hood open, if you want. Just don't keep your hand there for long.

I ran three 12V install shops, have done over 7000 automotive audio/security installs and have had almost no comebacks, and definitely none due to inadequate charging system/power supply. MECP First class certified, twice. Currently CEDIA certified. Oh, yeah- I was a MC tech for five years, too.