The most common questions are for battery cable sizes relating to either the motor, engine, trolling motor or inverter and each has it's own set of variables we need to find in order to make the correct calculation. In this article we will cover engines (we will tackle trolling motors and inverters in separate articles and put links to them here)
The issue is the cloudiness surrounding calculating battery cables for starting an engine as an engine will momentarily have a large inrush current to get the motor moving and as the motor begins to spin the current draw lowers considerably through the remaining cranking cycle.
The recommendation for battery capacity is based on engine displacement and there are different variations over the years but I've grouped all the research together and have established the following protocols. Cold cranking capacity of a battery should be at least 1 amp per cubic inch of displacement for more efficient engine sizes such as 8 cylinder. For 6 cylinder engines 1.5 amps per cubic inch of displacement and 4 cylinder engines are 2 amps per cubic inch of displacement. These ratings are for the battery capacity, the ability to deliver current to start the motor but it's logical to use them as a guideline in determining the battery cable size to allow for that current to flow with minimal voltage drop.
With regard to voltage drop I've found that under 5% is a solid target for the voltage drop to ensure easy starting and definitely under 10% if you're wanting to squeak by with the minimum. The lower the voltage drop the better so keep that in mind.
Steps to calculate battery cable size for starting your engine:
1) Check literature or contact manufacturer to see if they will specify the size of cable you should use. While this seems obvious, most people begin the search for knowledge when it would be a whole lot easier to just ask the manufacturer what size cable to use..it's their product, they should know!
2) Find the displacement in cubic inches for your engine. If you have the engine size in liters then you can convert it by 1 liter = 61.02 cubic inches so that a 5 liter (5L) engine is 5 x 61.02 = 305.1 Cubic Inches.
3) Using the 8 cylinder = 1 amp , 6 cylinder = 1.5 amp or 4 cylinder = 2 amp suggestion multiply the cubic inches of displacement by the correct amps per cubic inch rating for your engine size. The 305.1 Cubic Inch engine from the previous example is an 8 cylinder so we would take 305.1 x 1 amp = 305.1 amps.
4) To keep it simple, use our VOLTAGE DROP CALCULATOR (opens in a new window) and begin to plug in the variables.
- You will select Copper Wire, then in the voltage drop down select your voltage...most of you will be 12 VDC (12 volts DC).
- Then enter the distance in feet from the battery to the motor. This assumes the positive and negative cables are the same length, if you have a setup with a longer run of one and a shorter run of the other then take the total round trip distance and divide it in half and enter that number. For example, you have battery to switch of 6 feet and switch to engine of 8 feet and then the negative is from engine to battery at 7 feet you would take 6 feet + 8 feet + 7 feet = 21 feet round trip and divide it by two to get a 10.5 ft one way distance.
- Then enter the load in amps we calculated in step 3 above (the engine starting current draw) and type it in the box.
- Finally select a cable size from the drop down list. I suggest starting with the cable rated to handle that amount of amps continuous from the table to the right of the voltage drop calculator. You can use the outside amperage rating as it will give you a good starting size so you see 2/0 AWG is rated to handle 330 amps continuous so we will select 2/0 AWG from the drop down list on the wire size.
- Now hit the "Click to Calculate" button and see the results.
- Look at the Per Cent Voltage Drop and with the variables from this example of 305.1 amps, 10.5 feet, 12 Volts and 2/0 AWG we get 4.28% voltage drop which is a good number. It's under the 5% target and should work fine. If the voltage drop was too high you would go up to the wire gauge size and choose the next larger wire size and recalculate until you get what you feel is an acceptable voltage drop percentage.