Tech / Tech Articles

Alt Text: 5 Things You Should Know Before Using a High-Output Alternator


PWM-47796_HH_mlSo you’re considering swapping out your stock alternator.

There are plenty of good reasons to make the leap to a high-output alternator, but you’ll need to do a little bit of homework first. Luckily, we’ve got smart friends to help us with our studies, so you can ace the topic. In conjunction with the alternator experts at Powermaster and MSD, we’ve compiled the five things you need to know before upgrading to a high-amp, or high-output alternator.

This starts with the most basic of questions:

Do You Really Need a High-Output Alternator?

If you’ve got a basic, stock vehicle, chances are you don’t need a high-output alternator. Most factory alternators are rated at 65 to 100 amps and are capable of handling your vehicle’s basic necessities, such as headlights, gauges, fuel pumps, A/C, etc. These alternators also typically come with a 10 to 15 percent reserve to handle additional accessories.

However, many of our readers don’t have a stock vehicle. For example, you may have a custom-built street rod with a unique combination of accessories. Or you may have a high-end stereo system or a race vehicle with an array of on-board electronics. As the electrical load of all these accessories add up, you may find yourself in need of a higher-amperage alternator.

But how do you know?

There are a few ways to figure out whether you need to upgrade your alternator. A few telltale signs are dim headlights, poor stereo system performance, or an alternator that simply wears out quickly. You can also check your electrical load using an ammeter. Simply connect the ammeter in series with the battery’s ground terminal (with the engine turned off), switch each electrical component on and off, and note their amperage draws. Add up the total electrical draw and compare with your alternator’s rated output. The output should be 50 percent greater than the draw.

One final way to estimate your vehicle’s electrical load is to check the accessory fuses. The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your vehicle’s electrical load.

What Amperage Do You Need?

That depends on the current draw, along with any future accessories you plan to add. For that reason, we’ve supplied a list of some common accessories and their amp draw:

Accessory: Amp Draw:
Air Conditioner 20-21
Audio Power Amplifiers 10-70
Back-up Lamps 3-4
Cigarette Lighter 10-12
CD/Tuner with amp 7-14
CD/Player/Tuner without amp 2.5-5
Clock 0.3
Dome Light 1-2
Electric Cooling Fans 6-15
Head Lamp Dimmer 2
Head Lamp (Low Beam) 8-10
Head Lamp (High Beam) 13-15
Heater Defroster 6-15
Horn 10-20
Ignition 1.5-4
Ignition (Racing) 8-36
Instrument Panel 0.7-1.5
Lamp, Gauges 1.5-3.5
Lamps, License Plate 1.5-2
Lamps, Parking 1.5-2
Lamps, Side Marker 1.3-3
Lamps, Tail 5-7
Nitrous Oxide Solenoid 5-8
Power Windows Defroster 1-30
Power Seats 25-50
Power Windows 20-30
Power Antenna 6-10
Pumps, Electric Fuel 3-8
Starter Solenoid 10-12
Voltage Regulators (1 Wire) 0.3-0.5

How Much is Too Much?

You can never have too much amperage when it comes to alternators; therefore, you never have to worry about choosing an alternator with too high of a rated output. Here’s why:

Amperage is basically the amount of electrical current your alternator can supply. And it basically operates off of supply and demand. That is, your alternator will only supply the amount of amperage a particular component demands—and no more. So high-output alternators will not harm your components or charging system, no matter how high you go with the amps.

prf-30709_xlWhat Gauge Wire Do You Need?

A performance alternator really doesn’t require much in the way of modifications. However, Powermaster and other alternator manufacturers do recommend you replace both the ground straps and charge wire. Keep in mind the factory cables weren’t designed to handle the juice of a higher-output alternator, and can restrict the flow of electricity.

In the case of the charge wire, you really can’t go too large. However, here is a chart that matches cable gauge size to total amperage:

Amps Up to 4′ 4′-7′ 7′-10′ 10′-13′ 13′-16′ 16′-19′ 19′-22′ 22′-28′
0-20 14 12 12 10 10 8 8 8
20-35 12 10 8 8 8 6 6 4
35-50 10 8 8 6 6 4 4 4
50-65 8 8 6 4 4 4 4 2
65-85 6 6 4 4 4 2 2 0
85-105 6 6 4 2 2 2 2 0
105-125 4 4 4 2 2 2 2 0
125-150 2 2 2 2 2 0 0 0

What is Pulley Ratio (and Why Should You Care)?

In short, pulley ratio is a comparison between the crankshaft pulley diameter and alternator pulley diameter. This ratio is derived by dividing the crank pulley diameter by the alternator pulley. For example, a 6-inch crank pulley with 2-inch alternator pulley will yield a 3:1 pulley ratio.

The ratio has a direct effect on how fast the alternator spins.

In order to understand the importance of pulley ratio, you first need to understand the “power curve” involved with alternator output. Although the alternator’s output is dependent upon engine speed, it follows a unique curve. At idle, small changes in the alternator’s speed can make a big difference, so the pulley ratio becomes very important.

Powermaster supplies its alternators with pulleys matched to the alternator’s power curve. The company follows this common rule of thumb:

  • Street use = 3:1 ratio or slightly higher
  • Drag racing = 1.75:1 ratio
  • Circle track = 1:1 ratio

So why should you care?

Because differing ratios can affect performance, you should take care to maintain the same pulley ratio if you decide to use dress-up pulley sets. A mismatched pulley ratio and alternator can lead to big problems, especially at idle where alternator performance is critical. That’s because these high-amp units typically lose output under 2,400 rotor rpm. Rotor rpm are a factor of pulley ratio multiplied by engine speed. So, if you have a pulley ratio of 2:1 multiplied by an engine speed of 870, you’ll get a rotor rpm of 1,827.

At 1,827 rpm, you’ll see a significant drop in alternator output.

Again, the ideal ratio depends on your application (street, drag racing, circle track racing), but you need to understand the effects of altering pulley ratio.

With all this in mind, you’re ready to choose the right alternator for your application.

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  1. bryan danks says:

    ALong these same lines what if your vehicle uses 16 volt battery or batteries

  2. I have a 1984 elcamino.what modifications are needed,just wire diameter?

  3. hi david, thank you for this information!

  4. How can u tell what ratio pulley u have?

    • Get a string and a tape measure, and a calculator.
      Wrap the string around the alternator pulley one time, so the end touches the strings free end, hold or mark that point on the string, then measure from the mark to the end. That’s your alternator pulley (AP) diameter.
      Now do the same measurement for the crank pulley (CP).
      Pulley ratio = CP divided by AP. (PR=CP/AP)

      • the measurement you provided was for circumference(around the pulley) not diameter. Diameter = radius x2. radius is the distance from the center to the edge so diameter is distance from edge to center back to edge again. You can also calculate diameter using your circumference measurement above divided by pi.

  5. I have an amp with a 150 inline fuse for my subwoofer and 2 little amps for my mids and highs one with a 20 amp fuse and one with a 30 amp fuse. all together my stereo is 2000-2600 watts. not cappy overrated brands. will a 240amp 6 phase alternator be enough, I just ordered one and I’m waiting for it now. I also have an optima yellowtop under the hood and one in the trunk. both rated at 48ah. is this enough or what else will I need

    • 2000-2600 watts RMS or max?

      • Rms. This is my situation

        • If it’s a Class-B amp (typical car amp) then it’s about 65% efficient, meaning a bunch of power is turned into heat instead of sound. So your 2600 watt rms amp actually needs 4000 watts to do its max. (2600/0.65=4000)

          Nominal alternator charge voltage is ~13.5vDC.

          4000 watts / 13.5v = 296 amps of current draw.

          So, if you’re driving down the highway (the alternator’s best output), then you’re still discharging your battery. If you’re idling the engine in a parking lot, it’s even worse.

  6. Pingback: Quick Tech: Tips for Choosing and Installing an Alternator - OnAllCylinders

  7. Pingback: Quick Tech: Tips for Choosing and Installing an Alternator | MEP Precision Racing

  8. Maybe in the future some questions could be addressed on Tach installations. Bought a new Auto Meter Sport Comp 3905 and was wondering if all companies now recommend not using solid core ignition wires and they question which ignition system is used. Do these points just come up with this mfg.? Great info on the Alt. most things I read and like this one keep for ref.

  9. Roy Arild Tveit says:

    140 amp alternator and agm battery (Exide EM1000) fried my alternator cable to the starter.
    Recommend to follow the recommended sized charge wire

  10. We’ve been lately installing one of the largest subs setup we’ve ever done and been experiencing a hard time setting multiple amps in our truck and your gauge wire chart just saved me from a lot of headache.

    Thanks for sharing this article.

  11. I have a 2014 Chevy Cruze LT running 2 12 inch sundown subs and 1500 watt amp wired off .5 ohm. As you can imagine my lights flicker when its turned up, and also when I’m driving its like the car doesn’t want to go. Kind of jerks. Do I have to upgrade the alternator or add a second battery?

  12. Hector730 says:

    Get another alternator. Upgrade the chargimg wire and ground wire and you should be good

  13. Very informative.

  14. Hello,

    I have a 2001 toyota highlander. I have installed a sound system consisting of the following…
    1 crescendo BC2000 powering 2 sundown SA 12’s
    1 sundown 50.4 powering 4 memphis prx 6.5’s

    It dims my headlights pretty bad and I was wondering if i’d need to upgrade my alternator or just my battery? Please let me know


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