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Quick Tech: Tips for Choosing and Installing an Alternator

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

AccessoryAmp Draw
Air Conditioner20-21
Audio Power Amplifiers10-70
Back-Up Lamps3-4
Cigarette Lighter10-12
CD/Tuner with Amp7-14
CD/Player/Tuner without Amp2.5-5
Clock0.3
Dome Light1-2
Electric Cooling Fans6-15
Head Lamp Dimmer2
Head Lamp (Low Beam)8-10
Head Lamp (High Beam)13-15
Heater Defroster6-15
Horn10-20
Ignition1.5-4
Ignition (Racing)8-36
Instrument Panel0.7-1.5
Lamp, Gauges1.5-3.5
Lamps, License Plate1.5-2
Lamps, Parking1.5-2
Lamps, Side Marker1.3-3
Lamps, Tail5-7
Nitrous Oxide Solenoid5-8
Power Windows Defroster1-30
Power Seats25-50
Power Windows20-30
Power Antenna6-10
Pumps, Electric Fuel3-8
Starter Solenoid10-12
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 LengthUp 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 Amps14 AWG12 AWG12 AWG10 AWG10 AWG8 AWG8 AWG8 AWG
20-35 Amps12 AWG10 AWG8 AWG8 AWG8 AWG6 AWG6 AWG4 AWG
35-50 Amps10 AWG8 AWG8 AWG6 AWG6 AWG4 AWG4 AWG4 AWG
50-65 Amps8 AWG8 AWG6 AWG4 AWG4 AWG4 AWG4 AWG2 AWG
65-85 Amps6 AWG6 AWG4 AWG4 AWG4 AWG2 AWG2 AWG0 AWG
85-105 Amps6 AWG6 AWG4 AWG2 AWG2 AWG2 AWG2 AWG0 AWG
105-125 Amps4 AWG4 AWG4 AWG2 AWG2 AWG2 AWG2 AWG0 AWG
125-150 Amps2 AWG2 AWG2 AWG2 AWG2 AWG0 AWG0 AWG0 AWG

Alternator Efficiency

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

 

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2 Comments

  1. Come on guys really? The minimum amp draw by your chart leaving out some items most vehicles DONT have is 150 amps. But the alternator that came with my 99 Z71 was a 105 amp unit.

    You don’t simply add all the loads together and add 50%. One simple example is the headlights. Most vehicles like mine never operate the low beam AND the high beam at the same time. Why on earth would I add all the loads together?

    I certainly don’t drive down the road with the starter solenoid engaged. Or how about the power seats? Could anyone ever operate the electric seats AND the starter at the same time? Let’s not forget the use of LED lighting becoming more and more prevalent which draws significantly less than conventional bulbs.

    Even if a person could operate ALL the loads at one time, it’s only for a moment. The battery then takes the momentary load and then the large load is gone and now you’re back to a normal operating load.

    Incidentally, I switched from a mechanical fan to an electrical fan, quite some years ago and upgraded my alternator from the 105 amp unit to a 150 amp unit to help deal with the up to 30 amps it draws. I have all the normal loads you listed above and have NOT had a single issue with the battery. Yeah your numbers of 6-15 amps for cooling fans is way off.

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