Marine Wire Size Calculation

How to Calculate the Correct Marine Wire Size

Ahh...this is what you've wanted to find. We get this question all the time, "What size wire do I use for..."

There are some variables that need to be filled in first in order to begin figuring out the correct wire size. First of all we will deal with DC (Direct Current) which is most common on boats, coming from your batteries to whatever equipment you are trying to figure out a correct wire size for. There are some limiting factors to consider with electricity and they are the voltage and current. 

Often in the marine industry we are dealing with low voltage 12 Volt circuits. A big issue is voltage drop which occurs as the voltage at the end of a circuit is less than what was sent through the circuit to start with. This occurs as the electricity travels...the further it has to travel, the more voltage will be lost.  

In the boating world, the ABYC (American Boat and Yacht Council) has set forward some guidance on the acceptable voltage drop limits on boats.  The ABYC has two levels
of acceptable voltage drop. 

3 % Voltage Drop- Applies to conductors used for panel-board or switchboard main feeders as well as conductors used on bilge blowers, electronic equipment, navigation lights and any circuit considered critical to safety and operation of the boat. These circuits should be sized to keep voltage drop to a minimum and not to exceed 3% voltage drop

10% Voltage Drop- Conductors used for lighting other than navigation and other circuits which are not critical should be sized so the voltage drop does not exceed 10%

 Okay, that's one half of the issue, the other half is the current (amps) that a wire is rated to carry. The amount of current that a cable can carry is related to the size of the gauge conductor, the jacket temperature rating and the ambient temperature. So the larger the conductor, the more current it can carry. The higher the temperature on the jacket rating also increases the amount of current that can be carried. The ambient temperature which is the surrounding air temperature is considered as well so that if the wire were installed in a cool area it could safely carry more amperage whereby in a high heat area (such as the engine compartment) an adjustment has to be made to lower the amount of current the wire should carry. 

Yadda yadda...let's get to wire sizing...again, we are working with 12 Volt Circuits for these examples.

What you'll want to do first is figure the distance of the wire run from power source, to device and back to power source. In other words you'll need the round trip distance of the wire run since the DC circuit is comprised of the Positive and Negative runs making a complete circuit. 

Second you will need to know the current draw (amps) of the circuit. Usually a device will have a nameplate or manufacturer data to alert you to the voltage rating and the current draw of the device. If you don't have that information you can often find the manufacturer's contact information online and call their support department and get the information.

As our goal is to make your life easier please use the VOLTAGE DROP CALCULATOR (opens in a new window) and plug in the variables with regard to the voltage, the amps, the one way distance from the power source to the device and then choose a wire size you think might be correct and click the CALCULATE button.

The calculator will return information with regard to the voltage drop percentage and other end circuit voltage information. If your voltage drop is too high then go back to the wire selection and choose a larger wire gauge and recalculate until you find the correct wire size for the voltage drop.

USCG Diagram of Wire AWG Sizes 

Double check that the wire size is rated to handle the amps for the circuit by using the table on the right side of the calculator which displays the rated ampacity of the the cable sizes in the engine area or outside the engine area. Cables in the engine compartment are rated lower due to higher ambient heat given off from the engine.

If you have any specific calculations you think you need help with, feel free to post a comment below and we'll do our best to help you out.