Wire Size Calculators for Boats

With the BOATHOWTO Wire Size Calculators, you can determine the proper size for the conductors on your boat based on ISO and ABYC standards.

If you are based in the United States, the ABYC Wire Size Calculator gives you the wire size you need according to the American Wire Gauge (AWG) system.
The calculator is based on the ABYC (American Boat and Yacht Council) E-11 standard.

If you are based in Europe or any other "metric" country, the ISO Wire Size Calculator gives you the required cross sectional area of the conductor in square millimeters.
It is based on the brand new ISO 13297 standard from the International Organization for Standardization.

Disclaimer: Even though we have thoroughly checked them, we cannot be held liable for any errors in the calculations. Make sure to cross check the results with the ABYC or ISO tables.

How to use the Calculators?

  1. First of all we need to set the length of the total cable run from the battery to the consumer and back, or, in some cases, the point of connection of the circuit (for example, a circuit breaker in the main panelboard). This cable run has to include the length of the positive and the negative wire! We can choose the unit for the length between meters or feet. For the ISO-Version we have preset this to meters, for the ABYC version to feet.
  2. Now we set the maximum total load of the consumer(s) that are connected to the cable. We can choose either Amps or Watts as the unit. This should be written on the back of the devices or found in their data sheets.
  3. Now we set the system voltage, which on boats is typically ether 12 or 24 Volts in a DC system and 120 or 230 Volts in an AC-system.
  4. The maximum voltage drop is preset to 3%, which is required for safety relevant devices. For less important loads you can also set it to 10%, but we recommend to start with the 3% setting and only change it if the required wire sizes become excessive. If you want to, you can also set a custom value.
  5. The next setting is the temperature rating of the conductor's insulation. In the US, this should be printed on the conductor's insulation. In other parts of the world, this is unfortunately not always required. In this case, we recommend that you leave the setting to the 60° Celsius setting to be on the safe side. Note, however, that cables with a 60° Celsius rating on the insulation are not allowed to be installed in engine compartments!
  6. Now we select the number of current-carrying conductors that are bundled together, for example in the same cable trunk or wiring harness and whether or not the conductor is installed in an engine compartment. Note that you have to check the engine compartment box even if only a small section of the cable run leads through the engine room. Note also that the AC grounding conductor (green or green and yellow) and any bonding conductors (the same color) are not normally current-carrying and so do not count in the bundling calculation.
  7. In the ABYC version we have a box to check if the conductor is used in an AC-circuit because with the ABYC the derating factors for bundled AC conductors vary slightly from DC conductors.

Requirements for Overcurrent Protection

You need to make sure that all positive wires in a DC system, and all 'hot' wires in an AC system, are protected with fuses or circuit breakers that have an appropriate amp rating.

If there is already an existing fuse or circuit breaker for the cable you are planning to replace, you can enter the amperage of the fuse/circuit breaker in the calculator instead of the amp rating of the consumer.

How do the wire size calculators work?

The wire size calculators calculate the required size of the conductor based on two factors: the maximum ampacity and the voltage drop.

The maximum ampacity

...is the maximum amount of current that a conductor can carry safely without overheating and melting down it's insulation. The greater the diameter of the conductor, the more amps can flow through it without it reaching a critical temperature. But there are three other factors that come into play:

  1. The maximum temperature rating of the conductor's insulation,
  2. The ambient temperature (whether part of the conductor runs through an engine compartment or not), and 
  3. The number of current carrying conductors that are bundled together.

Our calculators use the ABYC and ISO Ampacity Tables as the basis for these calculations.

The voltage drop 

...is the voltage that gets lost on the passage from the battery to the consumer and back to the battery as a result of resistance in the cable. The longer and smaller diameter the cable the higher it's cumulative resistance. So when we have longer cable runs, we also need larger diameter cables, or else the resistance goes too high.

Our wire size calculators use a formula that is based on Ohm's law in order to calculate a diameter for the conductor that keeps the voltage drop below the value you set in the calculator.

According to both ISO and ABYC standards, the voltage drop should be kept below 10% in all cases and below 3% for sensitive or safety relevant consumers such as navigation lights, bilge pumps and main feeder cables for switch panels.

Why are there two different versions?

The European ISO and the American ABYC standards differ slightly in some of the details when it comes to calculating a conductor's ampacity. However, since the laws of physics are the same on both sides of the Atlantic (or wherever else you want to use the calculators), you can choose whichever better fits your situation.

In practice this means: If you are in a place where you have access to cables that are labeled with a gauge according to the AWG system, use the ABYC version. And if you are in a metric country where the cross-sectional area of the conductors is given in square millimeters (mm²), use the ISO version.

Thanks for using our calculators. If you have any questions or remarks leave a comment below. And don't forget to sign up for our newsletter. For up-to-date information on boat electrics and mechanics straight from the experts.

About the author 

Jan C. Athenstädt

Jan runs KlabauterKiste, the German online magazine for boatowners and Klabauter-Shop, an online shop for boat electrics. He holds a PhD in computer science and loves to teach people new skills. He is responsible for running the BoatHowTo website and will guide you through most of the courses. Jan lives aboard his Laurin 32 ahora and is currently sailing the Med.

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