How To Choose A Battery Starter Cable

Author: JRAutofix  Date Posted:27 May 2021 

Choose your Battery cable wisely and avoid any potential damage. High quality battery cables will help ensure you have power and avoid battery drain—but only if they are properly sized, installed and maintained.

Battery cable selectionThe battery cables are an important piece of any electrical system, transferring power from the battery to the electrical accessories.

Replacing them or adding more accessories mean you will have to choose the right cable to ensure  that you that your installation remain safe and efficient.

A battery cable must have quality terminals and have protection like heat shrink and cover to avoid external contact.

But more important is the gauge and lenght. With the lenght you get a voltage drop.

Longer the cable is more resistance you will have to compensate the drop we must choose a thicker cable.

Voltage drop can be calculated using Ohm’s Law: Voltage Drop = Current in Amperes x Resistance in Ohms.

The resistance can usually be found n Manufacturer spec as per below.



So now the Gauge the most common term for it are: B&S (as per table above) and AWG.

B&S stand for Brown & Sharp Wire Gauge and it is identical to AWG. So a 6 B&S cable is identical to a 6 AWG cable, for example.


AWG (American Wire Gauge) and B&S (Brown & Sharp) are both American Standard Wire Gauge Standards. 

The resistance of a wire is based on its cross-sectional area over a distance information expressed in mm2.

The area in mm2 of the copper conductors can be determined by:

As an example with a 3B&S cable.

Count the total number of strands. (329) 

Measure the diameter of one of the strands. (0.32)
In this case we have 14 strands @ 0.32mm each. Knowing this we now do the following calculations:

Square (multiply by itself) the diameter of each strand (0.32 × 0.32 = 0.1024)
Multiply this result  x (Pi) 3.14  (0.1024 x 3.14 = 0.321536)
Divide this result by 4 (0.321536/4 = 0.080384)
Multiply this result by the number of strands (0.080384 x 329)
Therefore the approximate area for 3 B&S auto cable is: 26.45


To start working out the size of cable you will need is what will be the total amperage that will be required to power your equipment.

If you know the wattage of the components to be wired, the amperage can be calculated by dividing the wattage by 12volts. For example, if the wattage of the component is 2800watts divided by 12volts =233amps. 

Now you need to consider the lenght of the cable for the voltage drop.

To calculate voltage drop in a copper wire, use the formula below: 



Volts= Length x Current x 0.017


Volts= Voltage drop.
Length= Total Length of wire in metres (including any earth return wire).
Current= Current (amps) through wire.
Area= Cross sectional area of copper in square millimetres.



This formula only apply to copper at 25°C, voltage drop increase, at approx 0.4% per °C.

0.017 This figure only applies to copper.


A trailer has 10M of 4 square mm wire so, how much voltage drop at 20A?


10 x 20 x 0.017= 3.4. Divide this by 4 (cross section area of wire): 3.4/4= 0.85V.


In this example, the drop is 0.85V. This would mean that if there was 12V at the front of the trailer, there would only be 11.15V at the back.


This is when the wire temp is 25ºC, if the wire temp was 35ºC there would be a 0.88V drop, meaning only 11.12V at the back of the trailer.

Inverters: When buying cable for inverters divide the wattage by 10volts. 

Volt drop: The longer the cable the more resistance, the more Volt Drop will occur. To reduce the Volt Drop, use a larger capacity cable. 

As a general rule for Battery/Starter cable it's preferable to select a bigger cable gauge if the volt drop is bigger than 0.24 Volt. 

Regarding the amperage required by your equipment as a general rule it's safe to use a cable tolerating 20% more Amp.


For 233 Amp the cable should be at least (233X20/100)+233=279.6 Amp

The next cable available in the Table above for this range is: 00B&S 292 Amp.


All these formula might sound complex but it is worth knowing as you can verify the Amperage rating advertised n the product. The only useful information is the cross-sectional area of the cable and from there you can verify the rating of the cable (Amp and Size) and avoid a scam.


Other specs to consider when choosing quality cable:

  • The manufacturing process called "OXYGEN FREE Plain Copper Wire" which is the highest purity       (A-grade) copper.

   Oxygen free copper is produced by melting and casting the copper wire in reducing atmosphere. 

   The reduction of oxygen from the melt can be done either by carbon or vacuum. Both methods result     in a low oxygen ontent.

   Benefits are: Oxygen-Free copper cables runs cooler than other conductors more durable and long-                           lasting, and is far less likely to corrode.

  • Insulation, the most common found in Battery/Starter cable is V90 PVC which mean that the cable      is rated to an operation temperature of 90°C. 

  PVC is the material which is resistant to heat, water, acid, and alkali, as well as being flame-retardant.


If you want to have a clearer picture how your cable are performing you can perform a Voltage test drop.

To check the starter circuit (Battery cable) for excessive resistance, you need to measure the voltage at the battery first to know what the battery voltage is (for example: 12.5V) using a digital voltmeter.

Then measure the voltage at the battery cable connections between the battery post and the connection is there is a different voltage then the connection need to be cleaned. Reckeck the voltage after cleaning should be zero.

Third step measure the voltage at the battery cable connections and starter to give you the resistance of the cable and the real voltage drop of the cable. 

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