What Size Battery Cable Do I Need for My Engine / Motor ?

"What size battery cable do I need for ..." is by far the most common question we get on a daily basis and we always do out best to help customers figure out the correct size.

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.


Crimping vs Soldering Marine Cable and Wire Connectors

So this is one of the most debated topics in the marine wire world where there are some valid arguments on both sides but I will present some fact and opinions on why I prefer to use crimped connections only. 
When it comes to ABYC here's what they have to say on the matter: Solder shall not be the sole means of mechanical connection in any circuit. If soldered, the connection shall be so located or supported as to minimize flexing of the conductor where the solder changes the flexible conductor into a solid conductor.

EXCEPTION: Battery lugs with a solder contact length of not less than 1.5 times the diameter of theconductor. 

 So it appears that ABYC will allow for an "only solder" connection 

An important factor to consider is the requirement of stranded cable for use on boats. The same stranded wire requirement is present in many motive (moving) applications where machinery or vehicles sustain vibration which will cause a solid wire to fatigue much faster and induce breakage. The stranded wiring allows vibration to be absorbed over the length of the wire where a solid conductor will focus the vibration at the point of least mobility which by default is the connector since it is secured and immobile. 
The importance is the call for a mechanical means of securing the wire to the connector which is accomplished via crimping. With the proper crimping tool using the correct crimping force you'll have a connection that well exceeds any recommended pull out requirements and still allows the stranded wire to do it's job of absorbing the vibrations. 

Soldering has with it some inherent drawbacks which are non-disputable. I'm not looking at a benefits comparison but strictly a drawback comparison. 

Wicking - Drawback #1
With soldering the melted solder will flow into and between the strands traveling in both directions from the point of soldering. This effect is known as "wicking" and the solder is "wicked" up into the strands of the wire much as a traditional wick will soak up a liquid.  The issue arises that the solder creates a solid mass from the stranded wires just outside the connector and creates a new focal point for vibration which can cause the solder to crack inside or crate fatigue on the individual strands facilitating their premature breakage.

No Mechanical Bond - Drawback #2

If a connection is solely soldered there'd be no mechanical connection, nothing to physically hold the connection in place other than the solder itself functioning as a metal "glue" which was not solder's intent. Under a short circuit condition sufficient heat could be focused at the solder connection to cause the solder to re-flow and the wire and connector could separate. This will not occur in a mechanically crimped connection which relies on the connector and wire which have substantially higher melting points than the solder.

Solder then Crimp will Crush the Solder - Drawback #3

There are those that will attempt to solder and then crimp for "doouble protection" but this is a bad idea on multiple fronts. The first being that the crimp may not form properly with the added mass from the solder in the connector being crimped. The crimp force could crack the solder causing a high electrical resistance point.

Crimp then Solder also Functionally Unnecessary - Drawback #4

There are others who will crimp the wire then remove some of the insulation from just beyond the connector and attempt to backfill with solder to "seal out" moisture and air from entering the stranding there. The issue is that the solder will not flow past the crimp so you'd in essence be putting a mass of solder into the stranded wire just outside the connection which again creates a solid mass as the focal point of vibration and would cause premature strand breakage or other fatigue related problems.
Yet others will drill a hole into the closed end of the connector and fill the small air void with solder. Well, the connection was already closed and sealed to start with and drilling a hole and then filling it with solder accomplishes no more than was already there before this is attempted.

Increased Resistance - Drawback #5

Lead is not as good a conductor as copper. The resistance of copper is 13 times less than lead so why introduce it into the circuit if not necessary? The crimp will be a copper connector and a copper wire and the crimp pressure with seal out air and moisture creating a low resistance connection

In all fairness let's take a look at possible drawbacks to a crimped connection:

Crimp Not Done Properly - Only Drawback

Simple. If you don't crimp properly using low quality tools or the wrong size connector and wire combination then you might have a problem. I fully believe this is where any popularity arose for solder since at a glance it would be easier to use solder as a glue to hold a connection and know it there over the "risk" of making a poor crimp connection such as those who "just hit it with a hammer". That's not really apples to apples, use good tools and follow manufacturer recommendations and you won't have a problem.

When a crimp is properly formed it will last indefinitely. Add to the crimp a piece of adhesive lined heat shrink tubing to cover and seal the joint where the wire meets the connector and you have a totally sealed connection. 

So in review, just looking at the potential disadvantages I have yet to hear an argument that would move soldering ahead of crimping...heck, I haven't heard an argument than even makes them equal choices. Crimping is the way to go.