How To Choose a Hot Rod High-Amp Alternator

High-Amp Alternators
4 Things To Know Before Choosing a High-Amp Alternator

Generally, OEM alternators are rated between 65 to 100 amps and are capable of handling the factory power demands from systems like headlights, gauges, A/C, etc. Original equipment alternators also typically come with a 10 to 15 percent reserve to handle additional accessories.

Do You Need a High-Amp Alternator

There are several obvious signs that your current alternator is undersized. For example, dim headlights, sluggish starting, or an alternator that simply wears out too quickly.

You can also check your electrical load using an ammeter or estimate your vehicle’s electrical load by checking the accessory fuses. The amp ratings, although slightly higher than the highest draw of each component, will give you a good estimate of your load.

What Amperage Alternator Do You Need?

Determining the alternator amperage required is the current draw, plus any future accessories you plan to add.

Each accessory will have drastically different draw: an electric fuel pump can draw from 3 to 8 amps while a high-power audio amplifier can pull as much as 70 amps or more.

Lighting System4-6 Amps
Halogen Bulbs≤ 10 Amps per bulb
Power Windows15-20 Amps *
Dash System2-4 Amps
Heat/Air Conditioning19-22 Amps
Electric Cooling Fans30-50 Amps
Audio System4-10 Amps
Audio Amplifiers15-250+ Amps
HEI Ignition10-12 Amps
Electric Water Pump18-25 Amps
Electric Fuel Pump8-10 Amps
Air Suspension Compressor20-50 Amps *
* Intermittent draw.

As you calculate the total of your accessories draw, keep in mind that you can never really have too much amperage — amperage is basically the amount of electrical current your alternator can supply.


What Accessories Do You Need?

Although a high-amp performance alternator really doesn’t require much in the way of modifications, we do recommend that you replace both the ground straps and charge wire.

Keep in mind that 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. Here is a chart with recommended cable length and gauge for your amp draw.

Alternator Output (Amps)Wire Length
 4 ft or less4-7 ft7-10 ft10-13 ft13-16 ft
20 or less1412121010

Alternator Pulley Ratio

Pulley ratio is a comparison between the crankshaft pulley diameter and alternator pulley diameter.


For example, a 6-inch crank pulley with 2-inch alternator pulley will yield a 3 to 1 pulley ratio.

Follow this common rule of thumb:

  • Street Use = 3 to 1 ratio or slightly higher
  • Drag Racing = 1.75 to 1 ratio
  • Circle Track Racing = 1 to 1 ratio

4 Things To Know Before Choosing a High-Amp Alternator

Powermaster Performance put together an easy to understand PDF entitled How to Choose a Hot Rod Alternator that reiterates each of the points covered in this article.