Rewiring a boat doesn’t have to be a headache.We explain the intricacies of upgrading your boat’s DC electrical system to comply with the latest regulations and cover all the latest technological advancements
Poor connections are the most common cause of electrical failures on board.Make sure all terminals are clean, securely connected, and that adjacent cables are properly secured.Credit: Duncan Kent
Rewiring is a necessity for any yacht that has retained its original wiring for over 20 years, especially if you are keen to avoid endless problems, constant troubleshooting and temporary repairs.
A few decades ago, ship owners generally had minimal requirements for electricity, with shipyards providing only the most basic installation.
Today, however, boat owners seem to want the same level of equipment on board as they enjoy at home, which often requires a rethink of the boat’s entire electrical system, from batteries to equipment, as well as serious upgrades to cable and circuit protection.
When rewiring your boat, choosing the right cable for the job is one of the most important factors, as undersized conductors can overheat under load, creating a dangerous fire risk.
The flexibility of the strands compensates for any motion or vibration typical of ships at sea, and the tinning protects the copper wires from oxidation, which often leads to increased resistance and faulty connections.
Ambient heat also increases the resistance of the cable, so the current-carrying capacity of the cable running through the engine compartment is reduced.
For this reason, they must have greater capacity and be covered with fuel-resistant, flame-retardant insulation.
Cables are specified by their cross-sectional area (CSA), not their thickness or diameter (although the two are related).
A circuit protection device such as a 60A thermal cutout prevents the cable from being loaded beyond its maximum current limit.Credit: Duncan Kent
In most non-critical applications, a 10% voltage drop is considered acceptable, but for basic equipment such as radios and navigation equipment, a 3% voltage drop is desirable.
The temptation is usually to use a smaller, less expensive cable to connect to the bow thruster or windlass along the length of the boat.
However, if the CSA is too small for the desired length, the voltage across the device will drop significantly.
This not only slows down the device, but also increases the current drawn through the cable due to Ohm’s law.
If this current exceeds the rated cable gauge then it is likely to melt and start a fire.
For cables that power many different devices, you’ll need to calculate the maximum current that can flow with all devices fully on, then add a good safety/extension margin of 30%.
To calculate the total current load per cable in amperes (A), divide the power of the device (in watts (W)) by the circuit voltage (V).You also need to estimate the total circuit length as accurately as possible, which will be the sum of the distances from the power source to the device and back.
For the math challenge, there are many websites and apps that offer simple wire size calculators, otherwise see our wire size calculation box (below).
In such a salty environment, it is imperative to ensure that all terminations are clean, securely connected, and adjacent cables are properly secured.
The best way to terminate multiple cables is to use good quality busbar (Blue Seas or similar) and crimp cable terminals.
Before you start wiring, you need to buy good quality wire cutters, strippers, and crimpers.
A decent cutter will make an even square cut so that the wire feeds all the way into the crimp terminal.
Buy a wire stripper that has dies marked for each cable size to ensure you get a cleanly stripped cable without losing any fine strands.
Finally, the ratcheting, double-acting, parallel-jaw crimpers feature dual dies (one side for stress-relieving the outer layer of the cable and the other side for crimping the bare wires), ensuring correct and even application of the crimper Compress the terminal and press the cable firmly into the connector while making sure that all important insulation remains intact.
Note, however, that there are two different “double-jaw” types – one for heat seal crimps and one for simple strain relief insulated crimp terminals.
They are impregnated with adhesive that cures when heated after crimping.sealing joint
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Alternatively, you can apply silicone grease to the entire connector before using heat shrink that overlaps the connector sufficiently (eg, if using a butt connector to connect two cables, at least 25mm on each side).
When sealing, use the heat gun on the lowest setting, as heating too quickly can cause the adhesive to foam and create air pockets in the joint.
Never solder a crimp or terminal on a boat, as it will cure the wire harness, making the joint less flexible and therefore more likely to be sheared by frequent movement or vibration.
What’s more, in an overload situation, the cable can heat up enough that the solder melts and the wire simply falls out of the splice, then it could short out to another terminal or the metal case.
For resistanceless crimp fittings, the terminals must be sized to fit the cable and stud and preferably be electrically compatible with the wire core – i.e. tinned copper terminal (not aluminum) to tinned copper wire.
Always place ring terminals directly on studs, not on washers, this allows moisture and contaminants to enter the joint, causing the joint to overheat due to increased resistance.
If for some reason you really can’t crimp the connector, use a good quality clip-on terminal block (like Wago), housed in a sealed plastic box.
If you absolutely must use plastic, so-called “chocolate block” style terminal blocks, then at least make sure the rods and screws are brass or stainless steel, and apply silicone grease to the blocks, otherwise they will corrode.
Finally, make sure all cables are securely fastened close to the terminals, and insert a drip ring into each cable between the anchor point and the terminal block or device to keep water out of the joint.
For panel wiring, remember to leave enough spare cable on the loom for easy panel removal and handling – you won’t regret it!
Keep the wires as far away from the bilge as possible.If unavoidable, use heat seal crimps or seal any splice or terminal strip in a waterproof case.
After you have designed the wiring layout and selected the cable size, the next step is to determine how best to protect the wiring from short circuits and overloads, and to determine how to open and close the circuit.
One of the most useful improvements that can be made to a yacht’s electrical system is upgrading the switch panel, especially if more electrical items have been added over the years.
While simple toggle switches and cartridge fuses do work to an extent, they often present problems of their own due to corrosion and loosening of their terminals over the years.
Boat owners are increasingly installing more power-hungry equipment, including refrigerators, windlasses, thrusters, inverters, immersion heaters, water generators, and even air conditioners, so it is imperative to ensure that the cables for these high-power devices are completely safe.
An important point to remember when installing a circuit protection device (CPD) in a cable is that its purpose is to prevent the cable from being loaded beyond its maximum recommended current limit.
Drawing too much current through the cable can cause the cable to overheat, melt the insulation, and possibly even cause a fire.
CPDs can take the form of fuses or circuit breakers (CBs), the latter of which many choose for convenience and breaking accuracy.
High-load fuses, such as ANL (35-750A), T-Class (1-800A), and MRBF (30-300A) types, are ideal for high current draw and battery protection, while fast-acting, low-current fuses are better suited for Protects delicate electronics as CB is not available at 5A.