Is a 200-amp alternator overkill for a modern hot rod, or just what the doctor ordered? Perhaps you need a 400-amp alternator. Let's find out using our small block Chevy-powered 1968 Camaro R&D car. It's equipped with a Powermaster 170-Amp CS130D alternator as part of our Victory HD accessory drive system. The plan is to explain alternator selection using common sense and maybe some basic addition. We'll clarify idle vs peak alternator output.  Let's start with a basic internet search.

How Do I Find Amp Load? (demand)

Your favorite search engine is a great place to start. Research the amp draw of each electrical component. Make a list of all electrical components and systems. It helps to take a walk around your vehicle. In our case, we almost forgot to include wipers, so walking around the vehicle to look for electrical components really helps. Many enthusiasts believe that they need a 200 amp alternator, but never do the math.

Next, have a seat at your computer (or grab your phone) and ask Google, for example, "What is the amp draw of a 1968 Camaro wiper motor?" Repeat this process for each component. Information is readily available these days. You no longer need to dig through technical data. Try to list your vehicle's highest load scenario. For example, a Vintage Air AC with the blower fan on high draws more amps than if the fan is set to low. Here's what Google says is the max load for the electrical components on our R&D Camaro along with the actual tested values.

Google says our Camaro has a max load of 121 amps. That's about 30% below the maximum output rating of 170 amps, meaning the alternator is never operating at its maximum capacity.  That margin of safety will extend the alternator's longevity. Add up your electrical components to the best of your ability, then make sure that your alternator is rated at least 15% higher than your vehicle's load requirements. For our Camaro, we do not need a 200-amp alternator. But that's not the entire story. Keep reading..

Real World Amperage Load Test - Surprising results!

Google research is beneficial, but the results are surprisingly higher than our real-world testing. Say your car or truck is already together. Try this very easy test: Purchase a digital clamp such as a Klein Tools CL390. Make sure you buy a DC-capable digital clamp. Safely loop the digital clamp around your main alternator lead (a helper is recommended) and fire up your engine. CAUTION: Make sure that your digital clamp does not come in contact with moving pulleys or an engine fan. Do not touch your main alternator lead. SAFETY FIRST!

The team at Powermaster have some tips to make sure that the digital clamp test is more accurate. Make sure that your car's alternator is wired correctly with proper wire size and good clean connections on both positive and negative posts. The alternator should be connected in a direct path to the battery without anything taking current from the alternator. Also test voltage at the same time. It should stay above 14 volts (at the alternator, at the battery and at the fuse box). If wiring is not done correctly or the wire is too small, some of or a lot of that amperage can be wasted as heat at the bad connection.

Back to the test. For fun, turn off every electrical accessory that has a switch - such as audio, HVAC, lights, wipers, etc. This will provide a baseline value that includes your ECM, TCM, fuel pump, gauges -- anything that's "ON" all the time. Our test Camaro had a baseline load of approximately 25 amps at idle cold. Note that the electric cooling fan has not yet turned on. We are genuinely surprised that the baseline load is so small. Let's keep going.

Next, we turned on EVERYTHING except the wipers. We cranked the stereo with amp, then cranked the AC and turned on the lights. The result? The digital clamp registered a 45 amp load, without the electric engine cooling fan running. What happens when the when the giant Vintage Air Monster electric cooling fan kicks on? The load actually spikes to 75 amps when it first turns on, then settles in to a 62 amp groove. That's everything running except the wipers.

So even if we were driving around on a hot night with the AC set on MAX and the stereo bumpin', that's only 36% of capacity for our 170 amp Powermaster alternator at speed and 62% of load at idle. Either way, we are running well below Powermaster's recommended 80% load threshold. In theory, this alternator should last longer because it's under-stressed. It's also worth noting that the Vintage Air AC only draws a few amps when the blower fan is set low, so 62 amps becomes 54 amps. Again, we are pleasantly surprised. 

A 200 amp alternator is officially unnecessary for this combination 

There you have it. To be honest, we expected a much higher draw than our 62 amp result. It just seems like a car with this much high tech electrical equipment should demand more amps. So what's the explanation?

In reality, computers and basic electrical components require very little current. EFI, TCM, stereo, and even gauges draw only 5 amps each. An MSD Digital 6AL ignition box draws 1 amp per 1000 rpm. Altogether, these components add up to around 25 amps max.

It's the electro-mechanical components that draw the most current. On this car, the electric engine cooling fan and HVAC blower motor (on high) add load more than all other components combined. That's saying a lot. Furthermore, the current loads for these electrical components spike when they are first turned on. Our 170 amp Powermaster (100 amp at idle) might seem like overkill, but the unused capacity can extend the alternator's lifespan.

Well then.. when do I need a 200 amp alternator?

If you have high load components in your vehicle, you will likely need a 200 amp-rated alternator. In addition to fans (engine cooling and HVAC blower motor), here are some examples of electro-mechanical and electro-thermal devices that require high amp loads. everything listed below either makes things move or makes things hot.

Peak Amps vs Amps at Idle

One last thing. It's VERY important to understand the difference between amps at idle and amps at peak rpm. ..and before we can talk about that, its important to understand that the alternator RPM of Concept One Pulley Systems is THREE TIMES engine rpm. At 6000 rpm engine speed, your alternator is spinning at 18,000 rpm.  Conversely your alternator is spinning at 2550 rpm if your engine idles at 850 rpm. Got it?

Our R&D car has a Powermaster CS130D 170 Amp alternator. It is a standard component in our Victory HD 8-rib system for small block Chevy engines. Whether it's cruising on the highway or racing around at 100mph, the alternator in this car is putting out 170 amps.

At idle however, this very same alternator has an advertised rating of 100 amps.  That's a very important metric. Sitting at a light or stuck in traffic you want to make sure that your vehicle has sufficient charging capacity with all accessories running. PLUS you want to ensure that there is extra capacity otherwise your alternator will be overstressed.

Every Powermaster alternator ships with a "dyno sheet" -- which is pretty awesome. It tells you how much current your alternator puts out at various speeds.  As you can see in the above graph, the curve is not linear. Charging starts at a low rpm and ramps up quickly. In reality, our 170 amp alternator is already outputting closer to 120 amps. 

Sources:

Powermaster Alternators

Klein Tools Clamp Meters