Stop using ‘automotive wire’ on your boat: The critical wiring mistake that makes remaking joints impossible

Learn why automotive-type wire will fail you in damp conditions and how proper tinned marine cable can prevent corrosion and save your systems

Electrical wire used on boats should be tinned (covered in solder) along its whole length. However, this is an expensive process, so most production boats have ‘automotive-type’ wire, which allows corrosion to spread along the strands of wire under the insulation.

This makes it impossible to remake joints successfully. Where one end of a wire is located in a damp atmosphere, tinned wire should always be used. Unless it’s unavoidable, don’t run wires through the bilge where their condition will deteriorate.

Running wiring

Use a push wiring threader to run wires through conduits and pipes. Insert one end into the conduit or gap through which you need to run the wire, and push the threader through. Then use the threader to pull the electric wire all the way back through.

If you need to run wires straight through inaccessible spaces then draw rods are better because they are rigid and you can angle them to come out of, or go into, a space. They screw together and can be assembled ‘as you go’.

Most alloy masts have internal conduits, provided to carry wiring running up the mast. Sometimes these are extruded as part of the mast and sometimes they are plastic conduits clipped or riveted onto the side of the mast.

Wires dropped through the main section of the mast will slap and chafe. A push threader can be used to run mouse lines and cables up the mast’s conduit. If possible, it’s usually easier to pull one existing cable back with a mouse line attached, and then attach the new cable and the old cable together to the mouse line.

Expert tips

Run a mouse line in case you need to run another cable later. The mouse line can be run through the conduit (or other routes where a cable is run) at the same time as you install a cable.

The mouse line is then left in place for the next time you need it. The mouse line should be twice as long as its route, so that you don’t pull it all the way through when you use it.

Use grommets or clips on ducting to protect the insulation where you run wires through bulkheads and so on.

Support cable runs regularly along their length.

Cable should not come under strain as this can cause connections to be pulled apart.

A very neat and secure way to protect and support cables is to run them in corrugated trunking. If cables are already in place, split trunking is available that can be placed around the cables in situ.

This also improves the aesthetics of the wiring run.

If you need to run cables to terminals mounted on a flat surface, use ‘Beta duct’ trunking, which has holes and slots. This is a very good way to organise and support the cables and they can exit through the slots to their relevant terminals or equipment.

Make a wiring diagram of any new work to be stored with the boat’s manual. If there are joints/junctions access points, try to indicate on the diagram where these points are.

Heavy-duty circuits

A DC electric motor will overheat and suffer early failure if the supply voltage is reduced. Motors with high current requirements need special care with their wiring circuits to maintain an adequate voltage. Normally, a voltage drop of 10% is the maximum allowed.

The operating switch will not handle the required current, so a relay is introduced into the circuit. A relay consists of an electromagnet that can be operated by the low current; its contactors then carry the high current required to operate the motor, or whatever. This keeps the switching current low, but heavy-duty contacts handle the high current load of the motor. Proper siting of the relay will keep the length of the high-current circuit to a minimum.

Electric windlass

Let’s see why an electric windlass, for instance, might fail prematurely if its wiring has too much resistance: If, for instance, it has a maximum power of 1,000W and should be run at 12V, then the current would be 83.33A at maximum pulling power (1,000W ÷ 12V = 83.33A).

If the resistance of the wire is such that there is a 10% voltage drop (12V x 90% = 10.8V), the current would now need to be 92.59A if required to pull its maximum load (1,000W ÷ 10.8V = 92.59A) because it would try to maintain power by drawing more current from the battery at the reduced voltage. With a 20% voltage drop (to 9.6V), the current would be 104.2A (1,000W ÷ 9.6V = 104.17A). This would give a 25% current overload (104.17A ÷ 83.33A = 1.25) leading to rapid failure.

For the same reason, if you had the engine running, the voltage at the windlass would be about 13.5V. With the engine stopped and the battery down from overnight use, the windlass voltage could easily be reduced to 10V or so (engine running: 1,000W ÷ 13.5V = 74A; engine off: 1,000W ÷ 10V = 100A).

You might think that the windlass motor is a fixed resistance so by Ohms Law, as the voltage decreases, so does the current. However, motors produce something called ‘back EMF’ voltage, which acts against the current flow. This means that the operating current is optimal at rated speed. It’s at its highest when the motor is stationary. Back EMF is too involved to discuss here, but it’s why poorly supplied motors fail early.

Starter motors and sheet winches

These are operated with the engine stopped. Starter motors normally run for a very short time and have a relatively short run of very heavy cable.

Provided connections are clean and well made, they make only small demands on a dedicated engine start battery. Sheet winches may have relatively long cable runs and the demands on the battery system can be significant, so that voltage drop at the motor needs to be minimised. Adhere strictly to the manufacturer’s wiring requirements.

Bow thrusters and anchor windlasses

To keep voltage drop to a minimum, these are normally used with the engine running to prolong the life of their electric motors and to minimise battery drain. Bow thrusters especially can’t be powered directly from the engine alternator, as it won’t produce enough current. There are two schools of thought on powering these machines: An existing battery – engine or domestic.

Long, very heavy and expensive cables run from the aft battery bank. For a bow thruster these cables may carry 400A and may have a circuit length of 20m, in which case a cross-sectional area of 200mm² may be needed for cables. That’s a diameter of 16mm!

Windlass cables will be appreciably smaller as most windlasses draw more like 80-100A (typically 35-70mm²). If no bow thruster is installed this is the most common method for a windlass.

A separate battery close to the demand. The heavy cable length is kept to a minimum by placing a dedicated battery close to the windlass or bow thruster. This is charged by a smaller cable from the engine/charger. The charging cable needs to be rated to carry the maximum charging current and associated voltage drop only.

The lower cost of the lighter cable, however, may be outweighed by the cost of the extra battery. The disadvantage of this method is the weight of this battery forward in the boat, although the total weight of cable and battery may be similar for both methods. It also complicates the charging system significantly, but this is the favoured option for bow thrusters.

Wire current rates

A wire must be capable of carrying the maximum current in the circuit. All wires have a current rating. A 5A wire must carry no more than 5A, and so on. Note that wires bundled together can carry less current because they will heat up.

Wiring for sensitive equipment should not allow more than a 3% voltage drop. Normally this is more restrictive than the current rating because it depends on the length of the wires (both positive and negative). Other wiring may be allowed a 10% voltage drop along its total length.

Even 10% will usually require a larger diameter of cable than the current carrying capacity. The cable should be specified to suit the largest requirement. 12V circuits will always require heavier cable than 24V circuits, because with 24V the current is halved for the same power appliance. This is why larger boats (with longer runs and heavier loads) will run a 24V system.

Wire sizes for a given length of cable run

When installing new equipment you will need to decide where you will need to power the equipment from: the battery (if it needs to be on all the time); the battery isolator (best if it is turned off by the battery isolator); or from an existing bus bar or circuit breaker. Check the wire size required, according to its length and the current it has to carry.

Run a new positive wire from the power source to a fuse holder or circuit breaker. Then run a new positive wire from the new fuse holder to the new equipment. Run a new negative wire from the existing negative bus bar or a new one as appropriate. Once you’ve checked your work, fit an appropriately-sized fuse, as recommended by the manufacturer.

How to troubleshoot onboard cabling problems

If you have power at the boat’s switch panel but it’s not reaching a particular device, ‘buzz’ the cable through with a multimeter, writes Duncan Kent. Check that the terminal at the switch panel is clean and securely connected, before following the cable along the boat. If there’s power at one end and not at the other then there’s a good chance there’s a fuse, circuit breaker or dodgy junction somewhere along the line.

It’s not unusual to come across cabling that is too small for the job (often caused by owners adding extra items to the circuit without upgrading the cable size). There are plenty of tables online showing the correct cable cross-section size for a known circuit distance, load current and voltage drop.

If you come across a blown or burned-out fuse/breaker along the line then check the maximum load against the cable’s carrying capacity. If it has become too small for the increased load, either remove some of the load or upgrade the cable. Make sure you replace both the positive and negative wires.

The most likely causes of a poor connection are cheap, unsuitable terminal blocks (such as DIY store ‘chocolate block’ strips with steel blocks and screws). These hate moisture and will quickly corrode unless slathered with grease and sealed in a watertight box.

If you find these during your troubleshooting, replace them with decent marine-grade blocks. While you’re at it, cut back, trim and re-terminate the wires with crimped ferrules or ring terminals. Do not use solder as this creates a hard spot, which can cause the wire to break. Another common cause of failure is using cheap, cloned circuit breakers, which have been found to either trip out well below their stated capacity or slowly melt under continuous load.

Meters and monitors

The most useful tool for troubleshooting electrical systems is a multimeter. Simple ones are cheap and adequate for basic tasks, but spend a little more to get one with a DC clamp ammeter that will enable you to measure current flow through a cable without disconnecting any wires. It also helps a great deal if you have a battery monitor fitted, which will indicate at a glance (some even remotely on a smartphone) what current is going in and out of the battery bank and its approximate State of Charge (SoC).

Terminal connectors

The best method of terminating a wire is with a crimped terminal or ferrule. There are many different types depending on its purpose but try to find those that have a tinned copper barrel and a self-sealing heat shrink cover that you heat up after crimping to melt the sealant and shrink-wrap the joint.

These give the optimum security in both physical strength and moisture resistance. If you’re doing a lot of rewiring buy a good quality, ratchet-style crimper. Note that terminals and ferrules come with different colour covers depicting their recommended wire size.

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