There are many factors to consider when purchasing an alternator.
Things like amperage, voltage, pulley ratio, and wiring all come into play. We’ve covered some of these topics in this earlier post; however, we still receive many questions about alternators from readers. So we teamed up with the tech experts at Summit Racing to get answers for some of the most common questions:
- Why would I need to upgrade my alternator?
- What aftermarket options are available?
- What is a regulator and what’s the difference between internal vs. external?
- How much wiring is involved?
- How do I calculate pulley ratio and why is it important?
Check out the video below for those answers and more. We’ve also included some helpful charts, guides, and formulas below the video to help you figure out your required alternator output, wire gauge size, and overall alternator efficiency.
Amperage of Common Accessories
The following is a list of common electrical accessories and their amperage draw. By adding up your accessories, you can estimate the necessary output from your alternator.
Electrical Load of Common Vehicle Accessories
|Audio Power Amplifiers||10-70|
|CD/Tuner with Amp||7-14|
|CD/Player/Tuner without Amp||2.5-5|
|Electric Cooling Fans||6-15|
|Head Lamp Dimmer||2|
|Head Lamp (Low Beam)||8-10|
|Head Lamp (High Beam)||13-15|
|Lamps, License Plate||1.5-2|
|Lamps, Side Marker||1.3-3|
|Nitrous Oxide Solenoid||5-8|
|Power Windows Defroster||1-30|
|Pumps, Electric Fuel||3-8|
|Voltage Regulators (1 Wire)||0.3-0.5|
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.
Wire Gauge Size
Once you’ve settled on your alternator output, you can use this chart to figure out the ideal charge wire size:
Recommended Cable Gauge and Length for Amp Draw
|Cable Length||Up to 4 ft.||4-7 ft.||7-10 ft.||10-13 ft.||13-16 ft.||16-19 ft.||19-22 ft.||22-28 ft.|
|0-20 Amps||14 AWG||12 AWG||12 AWG||10 AWG||10 AWG||8 AWG||8 AWG||8 AWG|
|20-35 Amps||12 AWG||10 AWG||8 AWG||8 AWG||8 AWG||6 AWG||6 AWG||4 AWG|
|35-50 Amps||10 AWG||8 AWG||8 AWG||6 AWG||6 AWG||4 AWG||4 AWG||4 AWG|
|50-65 Amps||8 AWG||8 AWG||6 AWG||4 AWG||4 AWG||4 AWG||4 AWG||2 AWG|
|65-85 Amps||6 AWG||6 AWG||4 AWG||4 AWG||4 AWG||2 AWG||2 AWG||0 AWG|
|85-105 Amps||6 AWG||6 AWG||4 AWG||2 AWG||2 AWG||2 AWG||2 AWG||0 AWG|
|105-125 Amps||4 AWG||4 AWG||4 AWG||2 AWG||2 AWG||2 AWG||2 AWG||0 AWG|
|125-150 Amps||2 AWG||2 AWG||2 AWG||2 AWG||2 AWG||0 AWG||0 AWG||0 AWG|
You can calculate the amount of horsepower used to operate your alternator with the following formula:
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