The practical sailor’s guide to boat wiring

Pat Manley and Oliver Ballam debunk boat electricals, and explain the wiring and best methods for its installation.

The practical sailor’s guide to boat wiring

All electrical wires used in boats should be soldered (tinned) on their entire length.

This is expensive, so most production boats have ‘automotive-type’ wire, which allows corrosion to spread along the strands of wire under the insulation.

The joints cannot be successfully remade. If the end of a cable is near a damp environment, always use tinned copper wire.

Unless it’s unavoidable, don’t run wires through the bilge where their condition will deteriorate.

Running boat wiring

Push wiring threader is used to thread wires through pipes and conduits.

1. The wire will run through the conduit or hole.
2. Push the threader into place.
3. The threader is used to pull the electrical wire through.

Draw rods can be used to guide wires through difficult to reach spaces. They are rigid, and they can be angled to enter or exit a space. They screw together and can be assembled ‘as you go’.

Many alloy masts come with internal conduits that carry the wires up the mast. These can be extruded into the mast or they can be plastic conduits that are clipped on the side.

If you drop wires through the mast, they will rub 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.

The following are some tips to help you:

1. Install a mouseline in case another cable is needed later. You can run the mouse line through the conduit or other route where you run a cable at the same time that you install the cable. The mouse line can be left in place until you need it again. 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.
   

   Grommets protect insulation. Credit: Fernhurst Books

2. Use grommets on ducting or clips to protect the insulation at bulkheads where wires are run.
   

   Insulation is protected by ducting. Credit: Fernhurst Books

3. The cables are supported by a regular support cable that runs the length of them.
   

   Supported cable. Fernhurst Books

4. Cables should never be strained, as this could result in connections being pulled apart.
5. Corrugated trunking is a great way to support and protect cables. Split trunking, which can be installed around cables in place if they are already there, is available. It also enhances the aesthetics.
   

   Corrugated trunking. Credit: Fernhurst Books
6. If you need to run cables to terminals mounted on a flat surface, use ‘Beta duct’ trunking, which has holes and slots. It is an excellent way to support and organise the cables. They can then exit through the slots and reach their terminals.
   

   Beta duct trunking. Credit: Fernhurst Books
7. Make a wiring diagram of any new work to be stored with the boat’s manual. On the diagram, indicate where there are access points to joints and junctions.

Heavy duty circuits

If the voltage of the supply is reduced, a DC motor will heat up and fail sooner.

For motors that require high current, it is important to take special care in the wiring circuits. This will ensure a stable voltage. Normaly, a maximum voltage drop of 10% can be tolerated.

In order to handle the required current when the operating switch is not able to, a relay must be added into the circuit.

A relay is an electromagnet which can be operated with low current. Its contactors carry the high-current required to run the motor or whatever.

The motor’s high current load is handled by heavy-duty contacts.

By placing the relay correctly, the circuit length for high-currents will be reduced to the minimum.

Let’s see why an electric windlass, for instance, might fail prematurely if its wiring has too much resistance: Say it has a maximum power of 1,000W and should be run at 12V:

• When pulling maximum power, the current would equal 83.33A.
  1,000W ÷ 12V = 83.33A
• Let’s say that the resistance of the cable is such that the voltage drops by 10%.
  2V x 90 = 10.8V
• If the current is to pull maximum load, it would need to be 92.59A.
  1,000W ÷ 10.8V = 92.59A because it would try to maintain power by drawing more current from the battery at the reduced voltage
• If the voltage drops by 20% (to 9.6V), then the current is 104.2A.
  1,000W ÷ 9.6V = 104.17A
• This would result in a current overload of 25%:
  104.17A ÷ 83.33A = 1.25 leading to rapid failure

The voltage at the windlass will be approximately 13.5V if the engine is running.

When the engine has been stopped overnight and the battery is low, it’s easy to reduce the voltage of the windlass to about 10V.

Engine running: 1,000W ÷ 13.5V = 74A

Engine off: 1,000W ÷ 10 V = 100A

Which motor will last the longest?

As you might imagine, the motor of the windlass is a fixed resistor, and by Ohms Law the current decreases as the voltage increases. Motors, however, produce something called back EMF voltage which acts against current flow.

It means that at the speed of the motor, it is operating at its optimal current. It’s at its highest when the motor is stationary.

Back EMF is too detailed to discuss here, but it’s why poorly supplied motors fail early.

Starter motors, sheet winches

They are used with the engine off. The starter motor normally runs for only a short period of time, and has a short cable.

As long as the connections are good and clean, they place only a small demand on an engine-start battery.

The sheet winches can have long cables and battery demands that are significant. Therefore, voltage drops at the motor should be kept to a minimum.

Adhere strictly to the manufacturer’s wiring requirements.

Bow thrusters and Anchor Windlasses

These are usually used while the engine is running in order to reduce voltage drop and prolong the life expectancy of the electric motors.

Bow thrusters especially can’t be powered directly from the engine alternator, as it won’t produce enough current.

There are two schools that believe in the power of these machines.

1. An existing battery – engine or domestic From the battery bank aft, long cables can be very heavy and costly. The cables for a stern thruster may carry 400A, and the circuit length may be 20m. This would require a cross-sectional surface of 200mm2. That’s a diameter of 16mm! Windlass cables will be appreciably smaller as most windlasses draw more like 80-100A (typically 35-70mm²). This is the most common way to install a windlass if there is no bow thruster.
2. Battery separate near demand By placing the battery near the bow thruster or windlass, you can reduce cable length. The battery is charged via a smaller cable that comes from the engine/charger. The charging cable should only be rated at the maximum current for charging and voltage drop. However, the lower cost of a lighter cable may not be worth it if you have to buy an extra battery. This method can have a disadvantage in that it places the battery further forward on the boat. The total weight of the cable and the battery, however, may be similar. This method is not only more complicated, but it’s also the preferred option for bow thrusters.

Wire current rates

A wire must have the capacity to carry the maximum current of the circuit. Each wire has a rating for current. A 5A wire can only carry 5A and no more.

It is important to note that wires bunched together will carry less current as they heat up. If you are wiring sensitive equipment, don’t allow a voltage to drop more than 3 %.

Normaly, this rating is stricter than the current one because it is based on the length (both positive and negatively) of the wires.

The voltage may drop by 10% over the entire length of other wiring. Even 10% requires a cable with a greater diameter than the current carrying capability.

Cables should be chosen to meet the highest demand. Because the current for a 24V circuit is halved, 12V circuits always require a heavier cable. It is for this reason that larger boats with longer runs and heavier loads will use a 24V system.

The size of the wires required to run a particular length of cable

(Length is the sum of the positive and negative wires) – see tables below

When installing new equipment you’ll need to:

1. 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);  An existing bus bar or circuit breaker.
2. Table above shows you the required wire size based on the length of the wire and the current that it must carry.
3. A new positive wire should be run from the source of power to a fuse or circuit breaker.
4. From the new fuse holder, run a new positive cable to the equipment.
5. Install a new negative cable from the negative bus bar.
6. Once you’ve checked your work, fit an appropriately-sized fuse, as recommended by the equipment manufacturer.

Note:

• The fuse or circuit-breaker should be installed as close to the positive source as possible.
• The negative battery cable should be fitted with the shunt if you have an ampmeter.
• Equipment supplied direct from the battery MUST have the negative cable connected via the shunt and NOT taken direct to the battery’s negative terminal.
• You may need a positive or negative bus bar if you plan to add more devices.
• When connection to the battery is necessary, some vessels will have a bus bar already – can you connect to this rather than adding more wires to the battery? Can the battery and the wiring supply handle the extra current?

Continues below…

Essential Boat Electrics, Third Edition (Fernhurst Books, £16.99) is available at fernhurstbooks.com.

Written by Oliver Ballam and the late Pat Manley, it’s a practical guide – with simple language and clear diagrams – to allow owners to tackle electrical jobs on board.

You can find tutorials on everything from how to wire a circuit to understanding switches and relays.

This book covers a variety of topics, including how to choose solar panels, batteries, and connect navigational devices.

Amazon UK – Buy Essential Boat Electrics

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