It is remarkably common to see poor electrics in a boat and it’s often the result of the incorrect assumption that 12v DC power isn’t dangerous like 240v mains AC. It’s true that in most cases, you won’t get a massive jolt from 12v DC; it isn’t generally expected to kill a healthy human instantly. However, this is no indication that it’s not dangerous.

Be safe

Poor, unchecked wiring and faulty electrical appliances and connections are the biggest cause of fires onboard.

We can't possibly teach you the ins and outs of electrical safety onboard in a short blog, but we can enlighten you as to the common faults and, specifically, the dangers of getting things wrong.

Fire primarily occurs when one of two scenarios exist:

  • When the power being pushed through a cable is greater than the cable can safely carry
  • When connections work loose and the electricity beings to arc

Boat Electrical dos and don'ts

There are a number of dos and don’ts where 12v DV wiring is concerned; here’s a short list and this is just the basics.

  • Select cable, terminals, switches, relays and connectors that are capable of carrying at least the maximum current that you’ve calculated is required
  • Do not use Screw Terminal blocks for connections – crimp connectors only please
  • Support cables every 18 inches – consider bundling them into a loom.
  • Insulate all possible exposed terminals and have no wire strands visible
  • Ensure that all connections are secure to prevent arcing
  • Do not run 12v DC and 240v AC cables together
  • Do not use solid core cable anywhere on your boat
  • Do not secure cables to gas or fuel pipes
  • Solder connections are not permitted
  • Protect cables from rubbing and chaffing

So what happens when a cable gets too much power though it? Quite simply, it gets hot. If it gets too hot, the insulation will begin to soften and melt, often shrinking to expose the cable strands. If it gets too hot, the cable might ignite and/or ignite materials around it.

A great deal of older boats from the 1960s and 1970s, particularly where cables are hidden in voids and mouldings, are becoming increasingly problematic. Freeman cruisers are well known for wiring problems, particularly in roof voids where the cable insulation slowly degrades and reacts with the foam core material. Because the wiring is totally hidden, replacement is difficult, but often necessary when interior lights or navigation lights fail.

So, what should you check?

The main concerns are cables that carry a lot of power, but all cables are capable of causing fire;

  • Battery connections and cable terminals
  • Power posts and distribution bus-bars for tight connections
  • Starter motor connections
  • Glow Plug connections
  • Winch and windlass connections
  • Fridge & cabin heaters

If any of these terminals are found be loose, tighten them and/or replace the terminals. In the case of heavy battery cables (usually anything from 70amp upwards) a DIY crimping tool is insufficient for the job and a professional crimper must be used. Do not solder any terminal as the resulting solid solder structure becomes prone to breakage through vibration.


Often, people believe that the only cable carrying power is the red, positive one. This is not true and, please if you had this notion, never carry out any electrical on your boat. Both the negative and positive cables carry a current as there needs to be a circuit from the power source (battery), through one or more appliance(s) and then back to the power source. If this circuit is broken, the power will not flow. Therefore, the negative cables are as important as the positive.

Cable sizing

The thickness of a cable generally indicates the level of power it can safely handle. For starter cables, you need a minimum of 25mm cable (170amp in general). That’s a heavy cable and needs specialist tools for crimping on terminal ends. For charging cables, look at the rating of your alternator. Most are at least 30amps and many will be 70amps; you’ll need to select cable that’s capable of carrying these currents.

The most important thing to remember is that all the cable in any given circuit needs to be at least capable of carrying the maximum anticipated current for that circuit and that the safety of the whole circuit is only as good at the thinnest cable within it. So, if you need to extend a 70amp cable, don’t do it with a 20amp one.


The purpose of a fuse is to protect the circuit. That’s all. If you’re wiring in a new appliance, don’t simply connect it to an existing circuit without knowing what the current requirement for that circuit is already and, if the fuse keeps blowing, don’t just insert a bigger one! The idea is that the fuse will burn out before your wiring catches fire; placing some foil or other unsuitable conductor in place of a fuse is asking for trouble.


Honestly, if you're in doubt of your ability to carry out electrical work on your boat to a standard that will keep you and your family, crew and those around you safe, consider employing the services of a professional.

Contact Us for advise