Every automotive wiring application is different, and because of this every wiring solution is unique. There are a number of factors to consider when choosing an appropriate wire size for use in your application.
While current draw is very important there are other factors to consider:
 Wire Length
 Ambient Temperature
 Freetoair or Bundled
 Wire Construction
 Cost
You don’t want a harness that catches on fire due to not having adequate wire for the application, but you also don’t want to run 10AWG wire through the whole car, not only would it be incredibly cumbersome and heavy, it would be an expensive endeavour.

Spec 44 Wire – Hook up Wire – M81044/12$1.35 – $12.36
The below table is a general guide for automotive use (under 3m lengths), but as per the above factors, other considerations should be made before deciding on a wire size. This table was created for use with Spec 44 wire and may not be appropriate for other types of wire available.
Wire Size  Use Case  Current Rating (When Bundled)  Wire Size (mm²)  Diameter(mm) Including Insulation 
24AWG  Sensor wiring  4A  0.35  1.0 
22AWG  Sensor wiring, small injectors, coil triggers, relay triggers  6A  0.74  1.2 
20AWG  Sensor wiring, injectors, engine actuators (VVT solenoids etc), coil triggers, and lights (30w)  8A  0.8  1.4 
18AWG  Small fans, pumps, coil pack power, motors (wiper/window), and lights (55w)  11A  1.3  1.6 
16AWG  Fans, transfer/vacuum pumps, and lights (100w)  16A  1.7  1.8 
14AWG  Medium duty fuel pumps, and small radiator fans  17A  2.6  2.2 
12AWG  Heavy duty fuel pumps, and large radiator fans  21A  4.2  2.7 
10AWG  Power distribution, and electric power steering pumps  28A  6.6  3.9 
8AWG (1000V)  Power distribution  55A  15.2  5.2 
6AWG (1000V)  Power distribution  70A  23.9  6.4 
4AWG (1000V)  Power distribution  95A  38.0  7.8 
This table isn’t the beall and endall, but it can help you get started. If you want to be more exact you can calculate the current requirements of what is being powered. Ohms Law can be used to determine what our current requirements are, if we again look at the injectors we can use the equation
Amps = Volts/Ohms
If we are going to use a set of Injector Dynamics ID1700x injectors we can look up the impedance which is 8.5ohms. While running our car should be providing around 14 volts. Therefore our current is equal to
Amps = 14/8.5
Amps = 1.65
Current (I) = Voltage (V) / Resistance (R)
This means our peak current draw from the injectors will be 1.65A, using the table above, we can easily run each injector on a 20AWG wire.
We can also calculate what size wire we need for items which draw a larger current, like the ignition coils. As an example Haltech IGN1A coils can draw up to 15A of peak current. Using this peak current will cause us to overspec our wiring, but can help us to find a ballpark figure. Below is a table showing the effective current draw at 100% duty cycle at different RPMs.
RPM  Current (A)  Charge Time (ms)  Engine Cycle Time (ms)  Effective Current Draw at 100% Duty Cycle 
2000  15  4.5  60  1.125A 
4000  15  4.5  30  2.25A 
6000  15  4.5  20  3.375A 
8000  15  4.5  15  4.5A 
10,000  15  4.5  12  5.625A 
To calculate the above the formula is:
Effective current draw = (current * charge time) / engine cycle time
Referring to the top table, again we would run the ignition coils on a 20 AWG wire. If we needed a power wire to supply all coils we can calculate that as:
Total Amperage = Effective current draw * Number of coils
If we had a 4 cylinder engine revving to 8,000rpm, then our total maximum current for all 4 coils would be 18A.
At first glance this seems like we would need to run 12 AWG wire, but there are a few factors which can be taken into consideration. All 4 coils won’t be firing at once, and our calculations are based off the peak current draw rather than the average. On top of this, our rating table is for wire up to 3m in length, and a short run from the ECU to the coils will be less than this. With these combined factors, a 16AWG wire will be sufficient for this application.
As you can see, calculating wire size can be complicated, from finding the draw of various components to finding how long your wire lengths will be, there are a lot of factors to consider. As always PT Motorsport is here to help, and we are happy to answer any questions you might have about your project.
We also have a number of other useful calculators available: