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 Cable Size 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 conductor. 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 either 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 at the 60° Celsius (140°F) 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 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 conductor 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.

If you want to know more, check out the article from Nigel about fuses and circuit breakers on boats.

Calculate the Ampacity of Conductors

The Wiresize Calculators also have an ampacity mode. In this mode, you enter the parameters for a given conductor and you will get the maximum amps that you can safely send through it.

When you choose a fuse or a circuit breaker, its rating should not exceed the ampacity of the conductor (there are some very limited exceptions in the standards which we do not recommend to make use of).

Pro-Tip

If you click or tap on the result of the calculations in the Wire Size Mode, the result and your settings will automatically be transferred into the Ampacity Mode. This makes it easy to determine the right rating for your overcurrent protection device.

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 conductor. The longer and smaller diameter the concuctor the higher it's cumulative resistance. So when we have longer cable runs, we also need larger diameter conductors, or else the resistance and voltage drop 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 Wire Calculators?

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.

Minimum Conductor Sizes

The ABYC requires conductors to be at least 16 AWG except for the following situations:

  1. 18 AWG conductors may be used if included with other conductors in a sheath and do not extend more than 30 in (762 mm) outside the sheath.
  2. 18 AWG conductors may be used as internal wiring on panelboards.
  3. Conductors that are totally inside an equipment enclosure.
  4. Conductors on circuits of less than 60 V having a current flow of less than three amps in communication systems, electronic navigation equipment, and electronic circuits.
  5. Pigtails less than seven inches (178 mm)

The ISO 13297 standard has a similar wording requiring conductors to have a cross-sectional area of at least 1 mm² unless they are in a common sheath or internal wiring of a panelboard, in which case 0.75 mm² can be used.

Thanks for using our calculators. If you have any questions or remarks leave a comment below. And don't forget to check out our boat electrics course!

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.

  • CALCULATOR
    I miss the 96V and 144V. Seadrive.no use this voltages.
    The Result give a miss link to Amazon.de with no sens.

  • Loving this concept. I have Nigels book. Which is great, but the idea of videos is even better.. but, any word when more videos will be available ? Looking forward to it!

    • Hi Justin,
      we are currently wrapping up the final lessons, so the idea is to go live with the course in Mai. If you signed up to our newsletter we will let you know as soon as it’s ready…
      Best
      Jan

  • The wire size calculator mentions that the length need to include the “round trip”. However, my intent is to run a new AC 30amp (120v) shore power line that would be housed in a sort of chase-way that exists near where the deck and the hull meet. Length of the run from the new shore power inlet (including a new breaker I will mount within a foot or so of that inlet) would be about 33 feet. There would be no area that would normally be “wet”. Wire insulation temperature ratings call out a different rating for “wet” vs “dry”. In the “dry” rating I’m well within the range of using 10ga. wire… if AC does NOT include “round trip” for length… thinking that this is true as 30amp shore power cords of 50′, or, so, are typically 10ga wire).

    So, the questions are: 1. With AC (and the calculator set with the AC button selected) do I need “round trip” length?

    2. I am NOT going through any engine area. The only “Wet” would be from some stray condensation and if the wire was ever “submerged”, I’d be having an entirely unrelated water problem…. 😉 Can I use the “Dry” rating?

    Thanks!

    Bill

    • Bill,

      When it comes to AC systems, length typically has very little to do with conductor sizing, unless you have a superyacht (you may be able to get one cheap right now from a Russian oligarch!). Conductor ampacity, for which length is not a factor, almost always takes precedence over voltage drop, for which length is a critical issue. If you are using Boat Cable, which is commonplace in the USA, it will be constructed to comply with the UL 1426 standard which includes various temperature ratings for dry and wet conditions. In the marine world we typically have conductors that are rated BC5W2. The ‘BC’ stands for ‘Boat Cable’. The ‘5’ corresponds to an insulation temperature rating in a dry environment of 105C. The ‘W2’ corresponds to an insulation temperature rating in a wet environment of 75C. If we plug your 10 AWG conductor with a 105C insulation temperature rating outside of an engine room into our ampacity calculator we find the ampacity is 42 amps (regardless of length). This is well above the 30 amps you need. If we change the insulation temperature rating to 75C we find the ampacity is 28A, which is marginally below your 30 amps, but if the conductors are now submerged I think you will have more important things to worry about! So, you are good to go. If we go to the voltage drop calculator to double check things, we need to plug in a voltage drop of 5%, which is the limit we typically use for AC circuits. You will find that a 10 AWG conductor with 105C insulation outside of engine rooms in an AC circuit with 2-3 bundled conductors that is carrying 30 amps is good for up to 193 feet (if you scroll through the circuit length until the conductor size switches to 8 AWG you find this takes place between 193 feet and 194 feet). From a voltage drop perspective, in your 30 amp circuit with 105C insulation and a conductor run of 66 feet we only need a 12 AWG conductor. As I said at the beginning, it is ampacity which almost always rules in AC circuits so length is typically not an issue.

      Nigel

  • Hi,
    I have a doubt about lengh of the cable. For example, if I have 15ft from the battery and device, the total distance have to be 30ft ?
    Best regards.

    • Yes, the length of the cable is both ways. So if your device is 15ft from the battery and both conductors are run on a straight path, you have to enter 30ft in the “Cable Run” field.

      Jan

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