How Tos / Tech / Tech Articles

How to Choose an Alternator


This Powermaster 140 amp alternator (PWM-8-57140) comes with a unique one wire regulator to make installation easy. The only thing required to hook it up is a charge wire from the battery terminal on the alternator to the positive terminal on the battery-your stock wiring harnesses can just be secured out of the way. It fits a wide variety of Ford trucks and Jeeps. Powermaster also has 200 amp alternators for late model 4.6L V8 applications.

A high-amp alternator's output can be negated by a charging wire or cable that is too small to handle it. The more amperage an alternator produces, the bigger the cable required. A 140 amp alternator, for example, requires a cable from 2-gauge to 00 gauge depending on the length (refer to the Charging Cable Gauge chart in the story). This Taylor Diamondback cable features a multistrand copper conductor with a polyethylene insulator, braided alloy cover, and solid brass terminals.

Any alternator can deliver "feedback"-feeding the electrical system power even when the system is shut off. That's what causes the infamous engine run-on when you switch off the ignition. Painless Wiring makes this Alternator Shutdown Kit (PRF-50105) for high amp alternators. It has a 250 amp solenoid that prevents the alternator from feeding voltage back to the electrical system. It's recommended for racing and should be used with a master battery disconnect switch.

Even a big honkin' alternator needs some help when it comes to handling power-draining audio/video systems, digital ignition systems, neon lighting, and more. That's where a battery with a large reserve capacity comes in. This Optima Group 31 battery has a 155 minute reserve capacity, plus 1,125 cranking and 900 cold-cranking amps of power. You could probably light a small town with the thing!

Good thing: Kickin’ audio system with megawatt amps, big subs, zillions of speakers, crossovers, and enough wire to hook up your average city.

Bad thing: Melted, smoking, fried vehicle electrical system ’cause you didn’t have a big enough alternator.

The fact is, not having enough amps to properly feed your vehicle’s electrical accessories can DOA a typical OEM electrical system. While it does have some reserve power for small accessories, the extra amperage draw created by a high-power audio system (or race electronics, or lighting, etc.) can cause a stock electrical system to literally melt from the inside out.

Luckily, a simple alternator upgrade can prevent such a catastrophe. We’ll guide you through the alternator selection process, how to decide whether you need an upgrade, how to find the proper size alternator, and tips for getting the most out of your new alternator.

Do You Need a High-Amp Alternator?

Deciding if you need a more-powerful alternator is easy once you understand exactly what kind of power, or amperage, you need.

Amperage is defined as the maximum capacity or maximum volume of electricity an alternator is capable of producing. If your vehicle’s electrical load exceeds the amperage—or maximum capacity—of your alternator, you’re asking for trouble.

Most factory alternators are rated at 65 to 100 amps and are capable of handling a vehicle’s basic accessories—headlights, gauges, fuel pumps, transmission, A/C, etc. While many alternators have a 10-percent to 15-percent power reserve to handle additional accessories, this is often insufficient capacity to power high end audio systems or other high-amperage items.

For example, a typical 500-watt stereo system draws upwards of 60 amps when cranked. A stock vehicle’s electrical accessories draw an additional 60 amps total. To run 120 amps’ worth of goodies with an 80-amp alternator, it will have to run at 100-percent capacity—and draw reserve power from the battery—with no cool-down time. The result is predictable—drastically reduced alternator life.

If you’re looking for hard evidence that you need to upgrade your alternator, take a look at your voltmeter. When you are drawing reserve power from your battery, the voltmeter will read below 12.7 VDC. If your voltmeter spends a lot of time below that figure, you are surpassing the maximum capacity of your alternator.

Choosing the Right Alternator for Your Vehicle

Selecting the right alternator comes down to figuring out your vehicle’s total electrical load. The most accurate way to determine electrical load is with an ammeter. With the engine off and the battery charged, connect an ammeter in series with the battery’s ground terminal. Switch each electrical component on and off, noting their amperage draws. Add up the total ammeter readings. Your alternator output should be 50 percent greater than that figure.

If you don’t have an ammeter, you can estimate electrical load by checking the accessory fuses. The amp ratings will be slightly higher than the highest draw of each component , but the sum of all fuse ratings will give you a general idea of the vehicle’s electrical load.

This chart shows the amperage draw of common electrical accessories:


Amp Draw of Some Common Accessories

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

Once you’ve determined the electrical load of your vehicle, there are a couple of things to keep in mind as you are choosing your alternator. First, you can never have too much amperage. Again, amperage is defined as the total electrical capacity of your alternator, and it is impossible to have too much electrical capacity.

A good-quality, high-amp alternator can also help you gain horsepower. While most alternators are only about 75-percent efficient (some power is lost in the form of heat and wind resistance from the cooling fins), a higher amp alternator will recover lost horsepower by allowing your electrical system to run at maximum voltage.

Getting the Most From Your Alternator

Here are some ways you can get optimum performance from that new alternator:

  1. Use the proper gauge charge wire. The charge wire is the wire that carries power from the alternator to the battery and the electrical system. Too small a wire will restrict the flow of electricity. Use the chart in Paragraph 16 to select the right charge wire.
  2. Make sure the alternator belt and tensioner (if equipped) are in excellent shape. High-amp alternators usually have a smaller pulley than stock to overdrive the system by 16 percent. The slightly heavier load will cause added stress to your belt, so it needs to be in good shape.
  3. If you are strapped for space, many alternators can be run in reverse (pulley side toward the driver). The alternator will still charge properly, but cooling efficiency will be reduced and life span of the alternator may be shortened.

Balance of Power

Like most things, an alternator is a compromise. It strike s a balance between drawing power from the engine (via the drive belt) and delivering back that power by helping the ignition system perform at its peak. By using the proper size alternator, you can tip the balance of power in your favor and create a horsepower gain.

By using the following formula, you can determine the amount of power it takes to operate an alternator (where 745.7 equals one horsepower and a 25-percent alternator efficiency loss is assumed):

Amps x Volts = Watts
Watts / 745.7 = Electrical Horsepower Produced
Electrical HP X 25% (.25) Efficiency Loss = Horsepower Lost
Electrical HP Produced + HP Lost = Total Horsepower Used

Let’s apply the formula to an alternator that produces 57 amps at 14.9 volts:

57 x 14.9 = 849.3 Watts
849.3/745.3 = 1.14 Electrical Horsepower Produced
1.14 x .25 = .285 Horsepower Lost
1.14 + .285 = 1.425 Horsepower Used

As the formula shows, this alternator doesn’t take much horsepower to operate. And by supplying the proper voltage to your electrical/ignition system so it operates at peak efficiency, the alternator can actually help your engine produce more power—more power than it takes to operate the alternator itself.

Recommended Charging Cable Gauge Size


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




  1. Nev Baker says:

    Under heading of Balance of Power” he 6th line shows “Amps x Volts + Watts”.
    I believe this should read “Amps x Volts = Watts”

  2. Excellent information ! I just learned some very important things I never thought about ! Thank You for putting this together, I can move forward with confidence. Thank You ! Steve.

  3. Pingback: Alternator help - Page 2 - Vintage Mustang Forums

  4. I am installing an alternator in a 11/2 litre riley. Im not a sparky,but the info, gives me an idear of the alternator I need thanks

  5. My 120 amp alternators instructions call for a 10 gauge wire, you state using a 4 gauge wire, I planned on using an 8 gauge, now who do I believe……Ron

    • Aaron Geister says:

      Ronald Although the 10AWG wire can push alot of amperage for a few seconds I would instead use a thicker gauge because if you pull to many amps for long periods it is possible that wire could burn up. Bassically get the wire that is suited for how many amps you are drawing. Perferable a little bigger. The bigger the wire, the more amps that cant flow saftley. The smaller the wire, the more it will TRY to pull the amps through it and it will burn up/Get super hot if it is past its rating/duty cycle .

  6. Chris Park says:

    If you add up all the minimums, you get about 188amps. To the best of my knowledge, cars generally don’t come with this kind of amperage (about 2.2KW). Obviously not all of these things are going to be on all the time.
    This article is also making a large claim for itself, free energy:
    ” the alternator can actually help your engine produce more power—more power than it takes to operate the alternator itself.”
    This is absolutely impossible. The best possible interpretation is that if you have a failing alternator, replacing it would improve performance. But by definition, doing work takes power and adding an additional load to an engine is not going to make it produce more power, it will lower the power available to the rest of the system.

    • Yes; agree it will reduce horsepower available: as much as 1.14 hp. Negligable, imho.

    • It’s not free power. Lets say an alt takes 1.14hp to run. When you upgrade a better alt, it allows your ignition system to run more efficiently letting your engine convert the energy more efficiently. Say the upgrade allows you to produce 7.8 more hp, you still lose the power draw of the alt, but you are left with a 6.66hp net gain. The potential energy that is stored in the gas is the same in both cases. The better atl just lets the system convert chemical energy to useful mechanical work better.

  7. I never realized using the proper gauge wire can help you get the most from your alternator. My wife’s car has been having some electrical issues. We’ll definitely have to take a look under the hood. Thanks for the info!

  8. AnalogDan Wilson says:

    WOW . Some of the questions are more confusing than the attempted answers. I guess the willingness of readers to get involved is a good thing.
    But I’m afraid this train wreck was caused by way too much smoke…not from hot smoking wires. Automotive electrical wiring can be difficult sometimes.

  9. my gm 10 si alternator with internal regulator keeps shorting out i had a 95 amp the store only had a 63 amp i have replaced it twice all my wires are new 12v to batt 8g #2 to 12v 10g and #1 toi ign 12g. so do i need to go back to a bigger alternator or are there 65amp remans all junk??
    thank you

  10. My car keeps dying i have halo headlights and led tailights and an aftermarket radio with speakers but my battery keeps dying idk if i need a high output alternator

  11. Pingback: max charger amps with stock alternator? - Page 2 - Ford Transit USA Forum

  12. If I go from a 125amp alternator to a 160amp alternator is there going to be a noticeable difference? Or should I just go with a 250amp alternator?

    • OnAllCylinders says:

      It really depends on the electrical load your vehicle, Jordan. You can’t have too much amperage, so it really depends on if you plan on adding more electrical components or greater demand to your vehicle.

  13. I am planning to install an additional AC on the roof in my new 250 Transit. The manual says that the current drawing Amp is 30. Will the heavy duty alternator 250 amp be enough to power it?

  14. Pingback: Alt Text: 5 Things You Should Know Before Using a High-Output Alternator - OnAllCylinders

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