Operating a classic car takes 3 elements: fuel, oxygen, and spark with the spark coming from the ignition system driven by the classic car’s electrical system. The primary component of the electrical system is the alternator or generator. The selection process of the right alternator for your classic car is course of action to avoid potential damages to the electrical system.
Most factory alternators are rated at 65 to 100 amps and can handle your vehicle’s 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.
Amperage requirements several factors must be taken into consideration when choosing an alternator that is right for a classic car or Hot Rod. It is best to calculate the amp draw of the accessories and items installed, and then the size of the alternator to the loads.
The fact is, not having enough amps to properly feed your vehicle’s electrical accessories can cause the death of 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 will 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 can produce. If your vehicle’s electrical load exceeds the amperage—or maximum capacity—of your alternator, you are asking for trouble.
Most factory alternators are rated at 65 to 100 amps and can handle 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 are looking for hard evidence that you need to upgrade your alternator, 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
The selection process of 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 do not 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 have 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.
Final Steps
Lastly a few other steps to take before you start shopping for your new alternator:
- Determine the available space and proper mounting location for your alternator
- Determine the style alternator you want to install
- Determine what kind of finish you want (brushed, chrome, painted)
- Determine how you want to wire your new alternator (Factory, 1 wire, etc.)
The selection process for your classic car alternator takes some time and patience to calculate the correct load for your car’s electrical system. This time is well spent to avoid possible damage to your classic car.
