February 6


Wire Size Calculator for Boats

By Jan C. Athenstädt

February 6, 2019

​What's the ​proper size for the wires on my boat? With our wire gauge calculator you can easily calculate the ​right ​cross-section for your wires. Using the appropriate wire gauge is absolutely crucial for a safe electrical system on your boat. If the ​cross-section is too small, there is a serious risk for fire.

​Factors for ​Calculating the Wire Gauge

​The necessary size of your wires depends on several factors:

  • the maximal current going through the wire
  • the length of the wire
  • ​the permissible voltage drop

Our wire gauge calculator takes all these factors into account. It is based on the international ​standard ISO 1​3297-2019 ​Small craft — Electrical systems — Alternating and direct current installations.

​Safety Notice on Fuses

​​You need to make sure that all current carrying wires which are not connected to the craft's earth in your boat​ are equipped with fuses or circuit breakers of the appropriate size. ​If there is already an existing fuse or circuit breaker ​for the cable you are planning to replace, use the amperage of the fuse/circuit breaker instead of the load of the consumer in the calculator.

​​A general rule from ISO 1​3297: do not use wires of a cross-section less than 1mm​² (AWG 1​7 or less) on a boat unless it is inside a switch panel or in multi-core cables. ​Conductors in multicore cables must ​have at least a cross section 0.75 mm² resp. AWG18.  All wires on board need to be stranded wire and it pays off to choose tinned wire to avoid corrosion.

​Wire Size Calculator

​Instructions for the calculator

You want to know the background behind the calculations? Here we go:

​How are the wire sizes calculated?

The wire size calculator takes into account both the maximal current carrying capacity (ampacity) and the maximal ​permissible current drop ​over the total length of the wire. So let's have a look how the calculations work:

Ampacity of Wires on Board

​The ​standard ISO 1​3297 states which type of cable ​may be used on a boat under which circumstances. Since wires tend to heat up easily under heavy loads, we need to make sure that the resulting temperatures do not ​cause a cable fire.

In the text of the ​standard, the choice of the ​cable is ​depending on a variety of factors, such as the temperature of the environment, the temperature rating of the insulation, or whether the wire runs freely or ​bundled.

To simplify things a little bit, we are limiting ourselves to single wires with an insulation temperature rating of ​ 70° Celsius​ (160° Fahrenheit). These are for example cables of type H07V-K, among others.

​The calculator assumes a maximum environmental temperature of ​35° Celsius​ (​95° Fahrenheit) and is based on the following table:

wire size (mm²)


​max. ampacity
































​0 (1/0)



​000 (3/0)



0000 (4/0)








The table assumes that the temperatures on board will usually not exceed the mentioned ​30° Celsius​ (​​86° Fahrenheit). In case the cables are ​used in the engine room, where temperatures of up to ​60° Celsius​ (​140° Fahrenheit) are assumed, the amp​acity​ must be multiplied ​by 0.75. If the corresponding checkmark is set in ​our wire size calculator, the values are adjusted accordingly.

Maximal Voltage drop

The second factor that needs to be taken into account when choosing the wire szie is the voltage drop over the length of the cable. It is caused by the electrical resistance of the copper wire. In this case, if ​you choose too small of a wire size for the maximal allowed voltage drop, there is no immediate danger of a cable fire.

However, it can still be an annoyance if the coolbox shuts down because only 11 of the 12.5 volts of the battery reach the end of the wire. And if the radar or your radio stops working when you need it most, a voltage drop can still lead to some hairy situations.

And, last but not least, you loose a lot of energy when choosing too small ​a wire.

The voltage drop is calculated by Ohm's law: U = R * I.

We know the amperage, what is missing is the specific resistance of ​copper conductors. In the ​Standard ISO 1​3297-2019, this is indicated as 0.0164 Ohm per meter ​at 1 mm² wire ​cross-section. For longer cables, we need to multiply the resistance by the total length of the wire in m (outward and return) and divide it by the wire ​cross-section in mm²:

R = 0.0164 * length (in m) / wire ​cross-section (in mm²)

​The formula for the current drop is thus:

current drop (in V) = 0.0164 Ohm * current (in A) * lenght of wire (in m) / wire ​cross-section (in mm²)

​We hope ​you found this useful. If you have questions, please leave a comment below!

Jan C. Athenstädt

About the author

Jan runs KlabauterKiste, the German online magazine for boatowners and Klabauter-Shop, an online shop for boat supplies. He has worked as deckhand and bosun on various tall ships and has been responsible for rewiring and maintaining the electrical systems of various yachts and research vessels. Currently, he is fixing up an old Laurin32 in Hamburg, preparing the yacht for extensive bluewater cruising. Jan holds a PhD in Computer Science from Konstanz University and a masters from Karlsruhe Institute of Technology.

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