What Gauge Wire from Battery to Starter? | Choosing the Right Wire Gauge

The wire connecting the battery to the starting motor is crucial in supplying high currents for starting a vehicle. Selecting the right gauge wire ensures effective power supply, and voltage drop, and avoids electrical issues.

In this article, we’ll look at the aspects to consider when deciding on the proper gauge wire to run from the battery to the starter, so you can make an informed selection for best performance.

Different Wire Gauges for Battery-stater Connection

Wire gauge measures the thickness or diameter of a wire using the American Wire Gauge (AWG). AWG provides information about a wire’s diameter in a specific number, with a higher number indicating a thinner wire. A wire with a low AWG number carries more electricity. The diagram below illustrates how the wire gauge works.

Figure 01

The Starter and its Ampacity

The battery and starter are two essential components for starting a car. The starter uses battery power to crank the engine and turn on the ignition. However, its ampacity fluctuates according to a number of circumstances. We’ll go through these aspects so you may have a better understanding of what goes into choosing a wire gauge.

Figure 02

Factors to Consider When Choosing the Wire Gauge

Now, various deciding variables must be examined in order to determine the proper wire size. 

Ampacity

The current draw of the starter motor during the beginning process is the most important aspect in choosing the optimum wire gauge. Starter motors may demand a substantial amount of electricity, ranging from 100 to 400 amps depending on the size and engine type of the vehicle. 

Use the following table below to figure out what the ampacity will likely be, or check the label on the starter for precise information.

Engine or Vehicle Type	Typical Starter Draw (Ampacity)
Large trucks and other more powerful engine	125-150 amps
V6	175 amps
V8	225 amps
SUV	200-300 amps
Large trucks and another more powerful engine	Up to 1500 amps or more (max ~ 2000 amps)

A starter’s current draw varies depending on whether it’s spinning freely or cranking an engine. Tests on a workbench may draw 80-90 amps, while starting an engine may draw 250-350 amps. Factors like battery and starter state, terminal corrosion, ambient temperature, and engine oil thickness affect the actual amps flowing through the cable.

Length of the Cable

To determine the wire length, we will measure the distance between the battery and the starting motor. Longer wire runs have more resistance and a greater voltage loss. As a result, at longer distances, selecting the proper wire gauge becomes increasingly critical. To prevent voltage loss, use a bigger gauge wire for longer lengths.

The voltage drop when using AWG wires and a 12V battery should be less than 3%. The maximum voltage loss should therefore be,

12V × 3% = 0.36V

Cross-Sectional Area of the Wire

The gauge of a wire is its cross-sectional area, which determines its current capacity. A larger wire with a lower gauge number can handle more current, while a thinner wire with a higher gauge number can handle less current. To ensure proper starter system power and voltage drop, use the thickest gauge wire.

Figure 03

Voltage Drop

The acceptable voltage drop in automotive electrical systems is typically kept within 0.5 to 1 volt. Anything beyond this range can cause problems with the starter’s operation and potentially lead to starting difficulties.

Exact Gauge Wire to Use for Connecting Battery with Starter

Knowing exactly what gauge wire we need from the battery to the starter involves some calculations to establish the wire’s cross-sectional area.

The formula for determining this is,

Where:

• A is the cross-sectional area in square meters;
• I is the maximum current through the wire in amps;
• ρ is the conductor’s resistivity;
• L is the wire’s length in meters;
• and D is the maximum allowable voltage drop in volts.

Convert this to AWG (American Wire Gauge) to get the equivalent. I made things simple for you by including a chart at the top, but if you want to do the math, use these values:

• I: The current draw depends on the engine size, starter motor’s condition, oil viscosity, and temperature. A typical 4-cylinder engine draws 125-150 amps.
• ρ: A typical PVC-coated copper cable used in motor vehicles has a resistivity of about 1.7 x 10^-8 Ωm.
• L: This is the cable’s length from the battery to the starter in meters.
• D: The typical voltage drop for a 12V starter circuit (for maximum load at 20°C) is 0.5 volts. [Denton, 2007, p. 84]

Using the above formula, we will obtain a square-meter metric value when calculating the cross-sectional area. Convert this to the area in square millimeters:

area in meters squared x 10⁶ (or 1,000,000) = area in millimeters squared

Then, refer to the table below to find the equivalent in the AWG system (within the range of 12 to 2/0).

Frequently Asked Questions

What Happens If the Battery Cable Size Is Too Small?

Using battery cables that are too small for the electrical demands can lead to problems such as voltage drop, overheating, reduced efficiency, component damage, safety risks including fire hazards, voltage instability, and frequent breakdowns.

What Happens If the Battery Cable Size Is Too Big?

When the battery cable size is too long, it will increase your costs and weight. The longer the cable will be, the more you need to pay. Also, longer cables increase their weight which you might not want.

What is the difference between 2 gauge and 2/0 gauge wire?

2 AWG can carry around 94 amps, whereas 2/0 AWG can handle approximately 190 amps. To be true, the length of the wire, as well as the percentage voltage loss, must be considered.

To Conclude

Selecting the right wire gauge from the battery to the starter is crucial for reliable starting performance. Factors like starter motor current draw, wire length, and voltage drop help make informed decisions. Opt for a slightly larger gauge for efficient power delivery and minimize voltage drop. This ensures a confident start and prevents potential issues in the future.