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Amperage tips when wiring your Chevy

All Amped Up! - The Demand for Add-On Electrical Components is Increasing and so is the Load on the Charging System

Jeff Smith Feb 24, 2017 0 Comment(s)
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You just finished installing a dual, 12-inch electric fan package in your Chevelle and life is good. With both fans spinning, it keeps the engine between 180 and 190 degrees no matter how hot it is outside. Everything was right with the world until a couple of days later. You took the Chevelle out to the local cruise night and noticed with the EFI, headlights, and the A/C all running that the voltmeter had plunged to 11.5 volts at idle. That’s when that voice in your head said “Hey, something’s not right here.”

This is an example of the latest battle raging under the hood of your street car. The fight is between all those cool electrical components you’ve been adding over the past few years and your embattled charging system. It’s a losing fight for those older, lower-output alternators so we thought we’d shine a little high-amperage light on this under-appreciated skirmish. We’ll also toss in a few simple techniques that will improve the efficiency of your current charging system. Not everybody needs a 170-amp alternator.

This conflict has been slowly escalating over the past decade with the proliferation of automotive electronics. From the early days of the muscle car movement, alternators hovered for decades around the 40- to 60-amp output level. This was all that was needed in the days when headlights and defroster fans were the biggest amp-hungry items on the menu. Together they might create a 30-amp draw.

Today’s story is far more taxing. A current Pro Touring street car could easily be sporting EFI, twin electric fans, electric fuel pump, an aggressive HVAC system, a stereo, and a litany of other specialty devices like electric water pumps, electric assisted steering, or an electric vacuum pump for power brakes. Individually, these devices would be of little cause for concern. But stack enough of them and the effect is similar to what happens when you start loading for that weekend backpacking adventure—the heavier the load, the less enjoyable the trip.

By now you’ve probably already glanced at the accompanying Amperage Demand Chart—the list is quite extensive and probably incomplete. The point is not that your car would include even a third of these devices. A late-model Cadillac, however, might include a few upscale items like seat heaters and rear window defoggers. A 2016 Silverado 6.2L pickup’s OE alternator is intended to spit out 150 amps and we’ve found an option for a 2017 Chevy 2500 pickup alternator that will crank out 220 amps. High-amperage alternators also put out an enormous amount of heat, which is why there are liquid-cooled alternators already in service. It’s a high-amp/high-temperature world out there.

A wise idea for any street car builder is to take an inventory of the electrical components used and simply add up the ones that would be used simultaneously at idle. In our accompanying example, we chose to package twin SPAL 12-inch fans with a list of other electron-sipping components that would commonly be used on a street car at idle. As you can see, the load could easily reach 90 amps or more. If the alternator could only generate 75 amps at idle, that’s a problem.

The main issue with a high amperage load is that the charging system voltage will fall—often significantly. You may not even notice, but at idle when current demands exceed charging system capacity, the voltage will plummet. Voltage is the electrical pressure used to push the amperage through the wires. A properly designed charging system will be capable of supplying 13.5 to 14.5 volts throughout the entire vehicle even with multiple high-amp devices operating. But as amperage demand increases, this causes heat buildup, which causes resistance, which reduces both the voltage and the amperage.

Another issue is that alternators are rated at their maximum amperage output. This maximum rating is based on a given speed, which is generally much higher than idle or even cruise engine speeds. Older alternators from the ’60s required very high armature speeds to create maximum output. Today’s alternators are more efficient, but still require speed (beyond idle) for maximum output.

As an example, Powermaster tests each of its alternators before shipment and includes a dedicated “dyno” output card with each unit. The Powermaster 140-amp rated CS130 alternator we used for this story tested at 103 amps at idle, 128 amps at cruise, and 156 amps at maximum output at 14.8 volts.

So let’s assume for a moment that we’ve purchased an alternator capable of 105 amps of maximum output and an idle capacity of 70 amps. We’ll also assume the pulley ratios are good so that we have decent alternator speed at idle and our amperage load isn’t excessive. Despite all these positive points, a quick evaluation of the voltmeter in the cockpit reveals a depressingly low 11.5 volts at idle with only a partial electrical load.

Before we bolt on a larger alternator, let’s look at the rest of the charging system. A very common problem with many older street cars is an undersized alternator charge wire between the alternator and the battery. Alternators capable of more than 100 amps should be combined with an oversized charge wire to reduce resistance and minimize heat. This might mean using a 6 AWG wire. These large wires are critical to minimizing resistance and maintaining the proper voltage throughout the system. Luckily, there’s a quick test to evaluate if that wire is properly sized.

Start the engine and engage several systems like the headlights on high beam, the heater blower fan, and cooling system fans to place a load on the alternator. Using a volt-ohm meter (VOM), test the voltage at the alternator’s 12v output post. For this discussion, let’s say that the reading is 14.0 volts. Now connect the VOM to the battery. If voltage at the battery is lower by more than 0.5 volt, then the charge wire or its connections are creating excessive resistance, which prevents the alternator from supplying full power to the car’s electrical system.

This same test can be performed in one step by placing the positive probe from the VOM on the alternator’s output terminal and the VOM’s negative probe on the positive (+) battery terminal. The reading will now indicate the voltage drop between these two connections. In our case, the VOM might read 0.52 volts, which is acceptable although a little on the high side. A lower number of 0.20 – 0.30 indicates lower resistance.

Remember that any electrical circuit requires a return, or ground, side—a portion of the circuit that is completely ignored. To test the ground side voltage drop, place the VOM’s positive probe on the case of the alternator and the negative probe on the negative (-) battery post. If the reading is above 0.50 volt, this indicates excessive resistance. Delco-Remy’s specs claim the total voltage drop of the charging circuit (adding the positive and negative sides together) should not be more than 0.50 volt. In this case, the spec for either side would be roughly 0.25 volt.

The above procedure is called a voltage drop test and is an outstanding way to dynamically evaluate an electrical system. For example, we could also test the delivered voltage to an electric fuel pump. The test compares the voltage at the pump to the voltage at the alternator. If the voltage at the pump is lower by more than 0.5 to 0.75 volt with the engine and pump running, then we know the pump is not running at its most efficient voltage.

For example, if the pump is running at 12.5v when the system voltage at the alternator is 14.3v, this is less than ideal. Of course, the only way to test these components is with the load (the fuel pump) running and the engine running to produce the proper voltage. A simple, key-on but engine not running test isn’t accurate. Companies like Aeromotive, for example, have charts in their catalog that reveal a typical 10 percent loss of pump performance when voltage falls from 13.5 to 12.0 volts. Low supply voltage also means the pump runs hotter, which also reduces its lifespan.

We’ve just scratched the surface of the electrical performance world, but hopefully we’ve offered some interesting situations that might motivate you to upgrade your system or solve some problems you may not even know existed. Deciphering most electrical problems is just a matter of isolating the circuit and running it down. Don’t be intimidated by those annoying charging system problems. Fixing them might just get you amped up!

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1. Use of multiple electrical components like electric cooling fans and fuel pumps places a big demand on the alternator. For our early El Camino we chose this Powermaster CS130 alternator, with a rating of 103 amps at idle and over 150 amps at full output.

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2. High-output alternators need a large charging wire. That skinny 12 gauge wire at the top is too small while the wire connected to the alternator is 10 gauge, which should be considered the minimum. The fat wire is 6 AWG wire and would be the most preferable.

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3. An excellent charging system test compares the alternator’s running, loaded voltage against battery voltage. If the battery reads more than 0.40-0.50 volts less than the alternator output, the alternator charge wire is undersized and should be upgraded. In this test, the battery voltage is 13.8 while our alternator was pumping 14.6, creating a voltage drop of 0.8 volt, which is more than we’d like. The ground side test revealed almost no voltage drop.

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4. Belt tension on serpentine systems is established by the spring-loaded tensioner. But Powermaster insists on bowstring-like tension for V-belts. Put a socket and breaker bar on the alternator pulley nut and turn clockwise. If tension is correct, the belt will attempt to move the engine slightly. Old, glazed V-belts will not be able to transfer the torque necessary to drive a high-amp alternator.

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5. We mounted this large 18-inch Ford two-speed fan to a Holley Frostbite aluminum radiator for a Chevelle project. The fan moves tons of air but demands 35 amps when running at high speed.

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6. This relay package is from a small Texas company called Hollister Road. Owner Dave Chapman assembles this high-quality three-relay system combined with a 50-amp fuse holder for use with the popular Lincoln two-speed fans. One relay controls the low-speed side while the other two share the 35-amp load when running at high speed. This kit also includes temp switches to trigger low- and high-speed operation. Check ’em out at

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7. As horsepower and fuel pressure increases, this places a greater load on the fuel pump, requiring larger pumps with greater demand. This Holley twin pump can feed a 1,600hp forced-induction engine, but it draws as much as 36 amps. With street engines now approaching 1,000 hp, this becomes a realistic concern.

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8. This pair of SPAL fans on our buddy Eric Rosendahl’s big-block El Camino consumes nearly 40 amps of continuous power. That’s a big draw if your alternator’s only capable of 70 amps. If your voltmeter reads less than 13.0 volts at idle with the fans running, you might consider upgrading your charging system.

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9. This is a Bosch 30-amp relay and power terminal we created to feed switched power to multiple underhood electrical components. We recently tested the installation and discovered poor connections to the relay. The terminal indicated 11.7v with the engine running while the alternator was pushing 14.7v. A big part of the problem was a poor ground circuit.

Get Loaded

For this example, we’re driving a small-block, EFI Pro Street car idling at a traffic light. It’s a stifling summer evening and the A/C is on. This is not unusual and yet as you can see, the amperage load is approaching 90 amps. Even with a high-performance alternator with a maximum output of 140 amps, like a Powermaster CS130 alternator we’ll use for this example, it’s output will barely cover the amperage necessary at idle to maintain a minimum 13.5 volts to all the electrical accessories. This assumes a decent alternator speed at idle and a minimal voltage drop among the alternator, the battery, and the rest of the harness.

If you decided to turn up the stereo it’s likely a 100-amp alternator would not be able to maintain system voltage. This isn’t a serious problem on a short-term situation as the voltage would only drop a slight amount. But it’s important to at least be aware of the problem.

Device Load in Amps
Pair of 12-inch SPAL fans 49
A/C compressor clutch 3
A/C blower fan on medium 6
Throttle body EFI system w/O2 5
Electric fuel pump 10
Headlights (low beam) 8
Taillights and instrument lights 4
Total Amp Draw at Idle 85 amps

Amperage Demand Chart
Component Amperage Draw
MSD Atomic EFI 14-18 (30 max)
High impedance injector, ea. (x8) 1.5 (avg.) (x8 = 12)
Low Impedance injectors, ea. (x8) 3 (avg.) (x8 = 24)
MSD-6AL CD 6 at 7,000 rpm
HEI ignition 5 (MSD module – 7.5)
Headlights (Halogen) low beam 42956
Headlights (Halogen) high beam 9-10 (17-19 combo)
Taillights and running lights 42770
Brake lights (1157 bulb) 4 (for 2 bulbs)
Windshield wipers (load dependent) 42776
Emergency flasher (entire system) 12
Horn 15
A/C compressor clutch 2.5-5
Electric A/C pump 32
Defroster / A/C fan (low to high speed) 42776
Single 18-inch Ford MKVIII fan 35
Single SPAL 16” fan 19-45 model specific
Dual SPAL 12” fans 49
Electric fuel pump – (Aeromotive Stealth 340) 43083
Electric water pump – (Meziere 55 GPM) 43051
Electric intercooler pump for supercharger 4-19 model specific
Electric nitrous bottle heater 14-20
Nitrous solenoids 5-30 ea.
Walbro fuel pump 8
Bosch electric inline pump 43052
WBO2 sensor – single (dual x2) 3 (x2 = 6)
Electric steering assist 25-40 or 65-80
500-watt stereo (500 / 14.5 volts = 34.5 amps) 43023
Electric vacuum pump 15
Water injection pump 42770
Electric windows 20-30
Electric choke 1 (6.5)
Remote trans cooler fan 3
Cigarette lighter 42896
Power seats 20

Alternator Charge Wire Sizing

We pulled this information from a larger chart in the Powermaster catalog that recommends sizing based on amperage over a given distance. If the battery is roughly 12 feet away from an alternator in the trunk, it probably won’t be necessary to use a 2 gauge wire because there will be little need to transmit 150 amps just to recharge the battery. However, a 4-gauge wire is probably a good idea. Smaller AWG produces more resistance. In AWG sizing, the larger the wire the smaller the number, so an AWG 6 wire is larger than AWG 10.

AWG Wire Size
Length of Charge Wire (in feet)
Amperage 1-7 7-10 10-12 13-16
105-125 6 4 4 2
125-150 6 4 2 2
150-175 4 4 2 2
175-200 4 2 2 0

Heat Test

According to Powermaster’s J.R. Richmond, the company’s alternator tests are performed at ambient temperature for roughly one minute. However, in the field, it is normal for an alternator to lose a certain amount of output due to heat buildup. Powermaster says this value is usually no more than 20 percent. Since maximum demand will most generally occur during high-heat situations, it would be wise to include an efficiency loss factor when considering a high-output alternator. Generally speaking, very few street cars will need more than a 140-150-amp alternator.

Powermaster Rating on CS130 alternator 140 amp (PN 478021)
Ambient Test Operating Temperature (15% efficiency loss)
Idle 103 amps 87.5 amps
Cruise 128 amps 109 amps
Peak 156 amps 132.5 amps

Watts to HP

All this electrical amperage doesn’t come without a cost. Here’s some simple math.

Amperage x Voltage = Watts
100 amps x 14 volts = 1,400 watts

We won’t get into the math for the conversion from watts to hp but 1,400 watts = 1.87 hp, which does not take into account heat, mechanical, or friction losses. So for an alternator to produce 100 amps at 14 volts would demand a minimum of 2 crankshaft hp. This is why Powermaster insists on a very tight belt tension on V-belt combinations.


Bowling Green, KY 42101




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