February 13


10 Commandments for Electrical Systems on Yachts

By Michael Herrmann

February 13, 2019

Is the electrical system on your boat safe and reliable? Check out these ten rules for a proper electrical installation on board. And don't forget to sign up to get free practical information on how to (re-)wire your boat - the right way!

A note from the author: The 10 Commandments listed in this article are not my personal opinion or armchair knowledge. They are taken from established standards and guidelines which must be observed when installing AC or DC systems on board of „small craft“ which are defined e.g. in the Recreational Craft Directive 2013/53 („RCD“) or ISO 13297-2019 „Small craft — Electrical systems — Alternating and direct current installations“ formerly known as ISO 10133 for DC and 13297 for AC systems.

Ensuring ​Safe Installation for Boat Electrics

These rules ​enable you as a yacht owner to check if the electrical installation corresponds to the technical state of the art. If not, you should consider to modify or extend parts of the system – or the system itself – to guarantee the electrical safety on board. If – on this basis of these „commandments“ – deficiencies come to light – e.g. missing insulation of conductors –, stay tuned for more than 1,800 pages comprehensive and detailed advice coming soon on YachtKnowHow!

The Commandments



​Thou shalt not connect any conductor to the positive terminal of a battery or panel board or other sources of power without a fuse or circuit breaker suited to the cross section of the conductor.

Especially quite thin conductors added later to the system are often connected without a fuse or circuit breaker. In case of a short circuit the complete battery current can flow through this conductor and set the cable on fire within seconds.

The only exception: The supply of starter motors for engines. These conductors may not be fused or protected by a circuit breaker. Instead they must be routed in such a way that they are mechanically protected, e.g. in cable trunks or conduits. Special rules apply for „fully insulated two-wire DC systems“, i.e. systems with no connection to the craft‘s earth. (See commandment V)



​Thou shalt choose the conductor‘s cross sections so that the rated temperature of the insulation – generally 70° or 80° Celsius (158° or 176° Fahrenheit) – is not exceeded.

These cross sections are specified in two tables in ISO 13297. The values given in these tables may vary from those used in house systems. Use our wire size calculator to find the right size of cable according to the ISO standards.


​Voltage Drop

Thou shalt choose the conductor cross section so that the voltage drop does not exceed 10 percent for standard loads and 3 percent for loads with special requirements, e.g. navigation lights or bilge pumps.

Our wire size calculator helps you choose a cable size that keeps the voltage drop within the limits over the length of the cable.


​Non-Polarized AC Systems

Thou shalt protect both current carrying conductors, i.e. neutral and phase conductor, by a two-pole circuit breaker in non-polarized AC systems without isolation transformer. This way, both conductors are separated from the system simultaneously in case of a short-circuit. .

In non-polarized systems the positions of neutral and phase conductors are not defined. Both are current carrying and must consequently both be switched off.


​Fully Insulated DC-Systems

Thou shalt protect both – positive and negative – conductors by a two-pole circuit breaker in fully insulated DC systems. (Systems which are not connected to the craft‘s earth.)

​According to the new ISO 13297-2019 this applies also to the conductors connected to the negative battery terminals. Fuses may not be used.



​Thou shalt not use soldering!

The origin of this requirement is probably lost in history. It is as least older than 40 years and again and again discussed in various online forums. Wether you agree or not: It is good practice (as used for decades by professional electricians) to use solderless connectors. Or, in other words: You can spot amateur work by finding soldered cable connections.


​Ground AC-Systems

​Thou shalt ground/earth AC systems on your boat! On all boats!! Especially on those with metal hulls!!!

Anything else is potentially lethal. Yes, even if there is an RCD fitted!



​Thou shalt fit at least one RCD in your AC system!

This is – electrically speaking – the best life insurance since the invention of protective earthing/grounding. In case of more than one source of power – e.g. land, generator and/or inverter – each current source must be ​provided with an RCD.



Thou shalt generally ​switch ​batteries in parallel only ​if they are of the same type (liquid acid, AGM or Gel), of equal capacity (Ah), and if possible  of the same age. Make sure to only charge your batteries with constant current - constant voltage chargers.

​...and not discharge more than 60 percent DoD (Depth of discharge) – that keeps batteries young and strong!


Wire ​Connections

Thou shalt make all connections using crimp connectors and good quality crimping tools.

​Luster screw terminals and connectors without end sleeves are considered arson.

Do your boat electrics adhere to these commendmenst? Congratulations!

​You're not sure? We ​support you in setting up a safe and reliable electrical system on your boat. Enter your E-Mail below to get free ​expert advice on boat electrics and mechanics!

Michael Herrmann

About the author

Michael is a leading expert on boat electrics and mechanics. He set up and ran two businesses in boatbuilding and marine engines. In the last decades he has published hundreds of articles on the topics in the German technical magazine for boatowners, "Palstek", and other publications. In addition, he has written several books on yacht engineering and electrics and runs the German website Yachtinside.de.

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