With the BOATHOWTO Wire Size Calculators, you can determine the proper size for the conductors on your boat based on ISO and ABYC standards.
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?
- 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.
- 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.
- 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.
- 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.
- 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!
- 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.
- 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).
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:
- The maximum temperature rating of the conductor's insulation,
- The ambient temperature (whether part of the conductor runs through an engine compartment or not), and
- The number of current carrying conductors that are bundled together.
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:
- 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.
- 18 AWG conductors may be used as internal wiring on panelboards.
- Conductors that are totally inside an equipment enclosure.
- 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.
- 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!