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The installation of electrical circuits in boats is covered by both the Boat Safety Scheme (also CE marking requirements) and Codes of Practice. Compliance with one does not guarantee compliance with another. For the practical reason that you will not get your boat licensed without a Safety Certificate the BSS requirements must take precedence.

There will be no attempt to discuss 240 volt supplies because a mistake with these can prove fatal.

Once 240 volts are introduced into boats, especially steel ones, the risk of severe electrical corrosion increases significantly. This risk is again increased when a shore line is in use.

This course is concerned with providing you with "common sense", practical advice that will help you overcome problems, so any advice given is in good faith and does not guarantee compliance with regulation. Your lecturer has yet to have any of his electrical work failed.




The voltage is a measure of how hard electricity is pushing around the circuits. This book assumes 12 volt circuits. The current required by 24 volt circuits is half that of 12 volt ones.

The amperage or current is how much electricity is flowing. This figure is needed to specify fuses, cable sizes, and battery capacity.

Watts or Power or Power enables you to calculate current. A 12v 21watt bulb draws 21w divided by 12v = about 2 amps. A 10w bulb draws 1 amp, a 72w bulb draws 6 amps (half current for 24 volt systems).



A measurement of how many amps a battery can (in theory) supply and for how long it can do it.




I have personally suffered from continuing failures of solid strand electrical cables and therefore endorse the requirement for multi-strand cabling. I have also suffered from difficult to diagnose faults that were eventually traced to crimped terminals.

In the 1970s a regional manager for Ampliversal tried to persuade your lecturer that there were no problems with good quality, crimped terminals. The result was that a piece of cable was prepared and each person attached a similar terminal by his chosen means. The terminals were gripped in large pliers and a tug of war ensued. Almost at once the crimped terminal pulled off, and when the manager started pulling on the cable itself, the cable snapped, leaving the solder joint intact. This took place on the premises of a third party and resulted in no little embarrassment for Ampliversal.

The problem with crimped joints made by normal DIY or cheap professional tools is that they are not airtight. In the marine environment dampness enters the joint and starts corrosion.



The key to a good solder joint is utter cleanliness of the surfaces to be joined and lots of heat from a BIG iron. Soldering may be demonstrated/practised on the course.


Cable sizes

Multi-strand cables are sized thus: 14/0.30mm , 65/0.30mm, etc.

The first number shows how many strands, and the second number shows the diameter of each wire.

These cables originated in automotive use which has lead to an actual current capacity of the cable being quoted e.g. 14/0.30 is quoted as being 8.75 amp cable. This is fine for short runs, but on a boat, with much longer runs, you also must consider the voltage lost as the electricity passes along the cable (both going to the component and back again).

As a rough and easy guide for runs of up to about 2m (4m including the return) just half the strand number of 0.30mm cable and that gives you a safe current capacity, e.g. 28/0.30 gives 14 amps - it actually will handle 17.5 amps, so you will be working on the safe side.

For long runs you need to consider the voltdrop as well. This would be given as a figure per Amp/per metre.

The table below shows cable size, nominal current capacity and volt drop/amp/metre.


  Capacity (AMPS) 

Volt drop/a/m

14/0.30 8.75 0.0189
28/0.30 17.5 0.0091
44/0.30 27.5 0.0062
65/0.30 35 0.0041
84/0.30 42 0.0031
120/0.30 60 0.0022


Take a narrow boat tunnel light, its current draw will be between 2 and 6 amps - say 4 as average. A 50' boat gives a cable run of about 16 metres both ways, that is 32 metres run.

It looks as if 14/0.30 will be fine until you work out the volt drop thus:

4amps X 32metres X 0.0189 = 2.4volts

Thus with a battery charged to 12.5 volt battery the lamp would only receive 10 volts.

It is doubtful if an owner would notice any practical effect on a tunnel lamp, but a horn would sound "odd". Fluorescent interior lights would protest a bit, especially when the battery voltage dropped to about 11.5 volts.

28/0.30 cable gives a volt drop of 1.2 volts, whilst 44/0.30 gives 0.8 volts. Match the volt drop you will accept to the equipment you are running. GLS & halogen bulbs would be fine at 10 volts, the water pump would not.


Calculation Method

Current = amps

Length = out AND back length in metres

Area = cross section area of the conductor


A tunnel light on a 16.5m boat uses a 55-watt bulb – what size conductor is needed?

    55 watts ¸ 12 volts = 4.6 amps
  1. Allowing for the cables to run up and down a bit & to one side of the boat, assume a length of 16m X 2 (16m battery + to lamp + 16m lamp to battery -) = 32 metres
  1. Assume a cable thickness of 28/0.30 which has an area of 2mm2

This means that the lamp would only receive 10.8 volts, so it would be a bit dim – but probably not to any noticeable extent.

44/0.30 (3mm2 ) gives a voltdrop of 0.8 volt

65/0.30 (4.5mm2 ) gives a voltdrop of 0.5 volts

As tunnel lights are only on for a relatively short period, there is no regulation governing them, and 55 watts is far too bright for oncoming boaters anyway, 28/0.30 would do the job.

If it was a yacht’s mast head lamp one would have to take a less cavalier approach to cable sizing because of international regulations regarding the boat’s statutory lights.


Current return path

All marine wiring should be INSULATED RETURN. That is each component has both a live and earth wire connected to it. By and large they are except narrow boats appear to suffer from odd connections to the hull (my boat was purchased with an active radio aerial connected to a window frame). It is not unknown for the NB tunnel light and horn to be wired using the hull as a return path.

Any hull connection can lead to electrical corrosion so on steel hulls this sort of thing needs checking for and rectifying.

Unfortunately the least expensive starting and charging equipment you can get is soured from the automotive industry which uses NEGATIVE earth return (battery negative connected to steel body). This means that a lot of boats are running about with their engines, gearboxes, and shafts at battery negative potential. Experience would indicate that this does not normally lead to problems UNLESS A PARTIAL POSITIVE CONNECTION IS MADE ELSEWHERE.

The partial positive connection is likely to be a terminal hanging in bilge water or screwed to a damp wood plinth. Keep all wires high and dry and there should be few problems.


Cable runs

All cables should be securely clipped and PVC insulated cables should not run against polystyrene thermal insulation (it destroys the cable electrical insulation, although observation shows the cable insulation is just as likely to destroy the polystyrene!). They must be well away from gas or fuel pipes or inside trunking. Cables must not be run through gas tanks.

Good practice dictates that a few spare inches of cable is left so the cable can be cut back during future maintenance/repair.


We will now look at some individual areas, they form no order other than that in which they occurred to the author.



These are not Electrolux gas/electric fridges which have no thermostat when operating on 12 volts so thus flatten the battery. These are the modern 12/24 volt fridges with Danfos compressors.

The latest version of these fridges only draw about 2.4 amps when the compressor is running. Providing the thermostat is set to 2 or 3, and the back of the fridge has plenty of ventilation it only requires about 30 amp hours of battery capacity.

The problem with these fridges is that the compressor starts under load, so to get it turning the control mechanism causes something close to a short circuit across the motor for a second or so. This causes no problems for either the fridge or boat's electrical system, but it does cause a problem with amateur installers.

Because of the very large current flow for that very short period of time on compressor start-up, the volt drop on the cable run becomes critical. The advice is to use supply and return cables of 1mm2 cross sectional area per meter of outward run. This sized cable is used for both the outward and return run. Trying to cut corners and save costs on cables here will ensure that the fridge thinks it has a flat battery and shut down. This is more of a problem when a fully charged battery overcomes the volt-drop, but partial discharge lowers the voltage enough to make the fridge shut down.

Expect to use cables as thick as, or thicker, than your car's battery cables!




The quality of battery master switches is in some doubt, the quoted current capacity can be "optimistic" to say the least. When replacing master switches you are advised to choose the largest you can afford. Try and make sure they are produced by a well recognised manufacturer.

The contacts can oxidise, so if one turns to on, but does not actually turn on, try turning it on and off vigorously a few times, to break the oxide.

If a "key" type switch fails "miles away from anywhere" a small ball of tin foil or paper placed under key often works for a while.


I would always advise separate switches for each battery bank, and fitting them in the positive lead.




Again, the diversity of possible systems makes it impossible to cover everything in detail.

Apart from very simple installations with few "domestic" electrical loads, common sense would dictate that the engine starting and domestic supplies are separated so that the engine can be started to recharge flat domestic batteries. This consideration indicates a two-battery bank system, one set for engine starting and one for domestic use.

There are now two considerations:

How much battery capacity is needed.

What arrangement is to be used to ensure each battery bank is fully charged.


The Energy Audit

Make a list of ALL the electrical items on the boat and their current consumption. If they only quote a wattage you need to calculate the current. For 12 and 24 volt systems divide the quoted wattage by 12 or 24 volts respectively. For anything running from an inverter divide by 10 or 20 respectively. This builds an 80% inverter efficiency into the calculation.

Then decide the time you will be using each item for between the battery charging sessions. I suggest you start with a 24-hour period. Choose the "worst" time of year for your electrical demand. A continuously cruising boat this may well be in January, a holiday boat may use October or March.

Then multiply the amps by the hours used and total all the results as shown below.

I have not done one for a high consumption boat because it gets very scary. For instance running a 600 watt electric drill for half an hour on the inverter will add about 30 Ah – what about that coffee machine!

You now have some idea about how much electricity you need to store, try to store less and you know you will end up with flat batteries.

However this is just a start and batteries are very unlikely to ever deliver what the manufacturer says they will – unless you want a short battery life.

Actual Battery Capacity

Exact figures are hard to come by and in any case an individual may find it more convenient to accept a shorter battery life because they install a less than optimum battery bank.

If you discharge typical domestic batteries to below half-charged (50%) then the more often and the deeper you discharge them the shorter their life.

If you have an alternator more than about 4 years old or one using a split charge diode it is unlikely the alternator will charge the batteries fully. We usually assume you will only charge to 80% of full and this is where an advanced alternator controller might help. It could increase the charge to 95% of fully charged.

Alternators that charge at 14.4 volts and above are likely to get the batteries fully charged – given enough time. It might be wiser to assume the 80% figure.

So 80% - 50% = 30%. We can only use 30% of what is written on the batteries.

Thus the 133 Ah load would require 133/30 x 100 = 376 Ah of battery bank or 4 x 110 Ah batteries.

At this point I should mention that some authorities advocate increasing this figure by 25% to allow for the way battery performance degrades with age.


You now know the size of battery bank you need, but this is only half the story. You also need to ensure you can charge it in the time you are willing to devote to charging.

Alternator Output

A typical charging curve of an alternator is shown below.

Within a few minutes of the alternator starting to charge a partially discharged battery bank it will be delivering its maximum output – the numbers on the plate or in the engine manual.

The output will remain at maximum for a time until the batteries start to get charged.

The output will then gradually fall until a very low "float charge" is reached. It will not drop below this level.

As we can never know how long the float charge will be in use we can never work out an "average" charge to use in any calculations. However if we just consider the shaded section the average will be about half the alternator output. The time represented by the shading will alter depending upon alternator output and battery bank size, but unless you have an ammeter (and ideally a voltmeter) that allows you to monitor the alternator and batteries you could assume that it is around three hours.

Recharging Batteries

Figures quoted for the current required to recharge batteries range from between 10% and 40% more than the discharge. Some of this variation is accounted for by temperature and battery condition but the truth is likely to be somewhere between the two. I tend to assume 30% more.

You will often see it quoted that batteries should not be recharged at more than 10% of their capacity. This may well be true when doing constant current charging, but on the boat we do a form of constant voltage charging. It is true that the higher the charge rate the more you shorten the battery life, but experience shows the 10% figure can now be exceeded. Personally I am happy with a maximum charge rate of 20% of capacity with 30% for a short time.

The Calculations

Using the 133Ah load we would need to put back 130% of that figure so 133 x 1.3 = 172 Ah

I need to put 172 Ah back into the battery during each charging period.

My alternator is rated at 60 amps so it will average about 30 amps over two to three hours.

172 ¸ 30 = 5.7 so I would have to charge for about 6 hours and that will be far too long for most people.

With a 90 amp alternator: 172 ¸ 45 = 3.8 hours

With a 140 amp alternator: 172 ¸ 70 = 2.5 hours


At this point we might decide to have a closer look at cutting consumption!


We had better check that we are not going to damage the batteries by charging at too high a rate.

First we assume straightforward twin alternators.

Taking a 440 amp battery bank (4 x 110 Ah).

With a 140 amp alternator 70 ¸ 440 x 100 = 16%

With a 90 amp alternator 45 ¸ 440 x 100 = 10%

With a 60 amp alternator 30 ¸ 440 x 100 = 7%

This shows that we are not going to damage the batteries with any of the alternators, but the smaller ones are unlikely to recharge the batteries within a reasonable time.


If this were a split charge system the battery bank would be 440 Ah plus the engine battery capacity and the charging times would be increased slightly.

Making Best Use of Twin Alternators

On any system the engine battery will be recharged very quickly so for most of the time the engine alternator is doing absolutely nothing.

The calculations above show just how valuable extra alternator output is. Using a split charge relay to join both battery bank positives together when the alternator is charging easily does this. You can, of course, buy a proprietary system instead!



The alternator charged is controlled by two factors:

Current - - if any charging device produced too much current the charging device will be destroyed by overheating. Dynamos need some form of current control, but the design of the alternator makes it self-regulating for current. This means the alternator cannot destroy itself, even with dead flat batteries, so we do not need to concern ourselves with current control apart from one "oddity".

When an alternator starts to reach its maximum output (current) the self-regulation causes the charging voltage to fall until the voltage produced is no more than that needed to "push" the maximum charge into the battery - self-regulation. The result of this is that on a set of very flat batteries, at first the charging voltage might only be 12 volts instead of the 13.8 to 14.2 normally expected. This is not a fault - it is expected. Just charge the batteries for a while and the charging voltage will rise.

Lucas say that you need less than 10 amps of charging current before you check the charging voltage.

Voltage - - the voltage must be controlled to prevent the battery overheating and being destroyed. The majority of modern alternators are fitted with an internal regulator that limits the charging voltage to between 13.8v & 14.2v (check your own manufacturer's spec, because these values do vary a LITTLE). It is this voltage regulator which is replaced by the alternator controller.

The normal voltage will only give 80% of fully charged. This is because the simple and cheap voltage regulators have to be set so that battery damage can not occur. IF the voltage is any higher than this, after about 15 minutes the battery will start to "boil" its water (gas) and overheat. The controller uses a number of methods of protecting the battery whilst increasing the charging voltage. (Some people say modern alternators with 14.2v charging voltage will fully charge the batteries, but I find this hard to believe because when bench charging I often needed over 15v to get the batteries fully charged).



The majority of automotive alternators (in temperate climates) are what is known as Machine Sensed. This means the charging voltage is measured at the alternator output. This is fine for cars, but on boats with split charging systems, longer cable runs, and master switches there is ample opportunity for a volt drop to develop between the alternator and battery. You will be virtually guaranteed a volt drop of at least 0.7 volts if you use a blocking diode.

Say you have old, dirty & loose connectors and a blocking diode giving a total volt drop of 1 volt.

The alternator is measuring the charging voltage at the alternator output, this is held at 14 volts. You lose a volt between the alternator and battery so you end up with trying to charge your battery with 13 volts. As a fully charged lead acid battery gives 13.2 volts, you can see you will never fully charge your battery.


Alternators for marine use should be Battery Sensed. This means the voltage regulator is measuring the charging voltage at the battery, so in the above case the voltage regulator only sees 13 volts at the battery. This results in the alternator output being raised to 15 volts, so after the 1 volt volt-drop there is still 14 volts to charge the batteries.

You would be well advised to make sure your alternators are battery sensed. This will require you to run another lead from the battery master switch to the alternator. This sensing lead should be connected to the battery bank which is most likely to be the more discharged - the domestic batteries.




There are a number of these on the market, each costs well over £100, each converts the alternator to battery sensing, and each is supposed to fully charge the batteries.

Some allow manual adjustment and setting/re-setting. Your lecturer is very wary of these, there is too much scope to destroy your batteries.

Some years ago Transport Engineer (the magazine of the Institute of Road Transport Engineers) did an in-depth survey of alternator controllers as used by ambulances, lorries with tail lifts, mobile showrooms etc. (All these have similar requirements to boats). The consensus appeared to favour Adverc. This allows no user adjustments. Your lecturer has fitted one to his own boat when he fitted the electric fridge. This has proved to give no battery problems, to date, and definitely improves the state of battery charge.




If you have one alternator and wish to charge two battery banks, you have three choices of system:

Manual - - where there is a switch to connect any combination of battery banks to the alternator. These look impressive, but sod's law states you will have left both engine and domestic batteries connected the one night you have an "all nighter" and flatten BOTH battery banks.


Blocking diode - - two or more diodes are used to allow the alternator to charge both banks, but they prevent one battery bank discharging into the other. Providing they are of sufficient current rating AND have an adequate COOLING air flow over them blocking diodes are long lived and ultra reliable. They usually give at least 0.7volt volt-drop so battery sensing is mandatory.

Using normal alternators, 120/0.30 cable is adequate, but high output alternators would require a larger cable size, especially between alternator and diode.


Split-charging relay - The engine battery is permanently connected to the alternator, and a relay is used to connect the domestic battery bank whenever the alternator is charging. This also is an automatic system, but being a mechanical device, the relay will eventually cause volt drop and fail. Again battery sensing is very important, as is ensuring the relay contacts can easily handle the maximum charging current. In theory a faulty domestic battery, or a very flat one, could burn out the relay, but this is rare in practice.

See below for diagrams and connection details.

Your lecturer has used methods 2 & 3, but is now using a split-charging relay.




There are a number of different battery types available for marine use, ranging from the just about acceptable to down right exorbitant in price. Price and availability alone ensure the most popular form of batteries for small craft are wet, lead acid batteries.

When selecting a battery, remember the requirements for a domestic battery and the starting battery are very different. So different that there are specific batteries available for each job.

A typical inland boat battery set-up will be a pair of 90 or 110 amp hour batteries for domestic use and one for engine starting.

If you choose different types for starting and domestics you will remove the possibility of changing them between banks to overcome failure. Taking this course of action and matching each battery bank type to its duty will extend battery life. You would use "Deep Cycle" batteries for the domestics and a "Starter" duty battery for starting.

Using "Maintenance free" or "Leisure" or "Carbon fibre" batteries allows the same type of batteries to be used for starting or domestic duties, but at the expense of some battery life.




The batteries should be kept clean and dry and their electrolyte should be kept to the required level by the addition of distilled or demineralised water. Their terminals should be kept well coated in petroleum jelly or terminal dressing.

A non-sealed for life battery should be tested with a hydrometer with NUMBERS on it 1.250, 1.150 etc. The hydrometer reading is noted for each cell at least one day after the cell has been topped up and charged. The reading for each cell is noted.

If there is a difference of more than 0.05 between cells the battery is unserviceable. The state of charge of the battery can be deduced from this table:

1.150 Fully discharged

1.200 Half charged

1.280 Fully charged

A fully discharged battery should be put on a bench charger and charged at 1/10th of its capacity until the hydrometer reading stops rising over two consecutive hours. A 110 amp hour battery would be charged at 11 amps maximum.

Alternatively use a MODERN two or three stage charger and wait until the light turns green!


Sealed battery testing

A sealed for life battery is tested by measuring the voltage at its terminals.

This is the method:

Spin the engine on the starter for a second or two OR put all the lights on for 30 seconds.

Switch of all loads.

Measure battery voltage.

The expected readings are:

Less than 12.2 volts        Fully discharged

12.2 - 12.5 volts               Partial discharged

Greater than 12.5 volts     Fully charged.



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Switch or Blocking Diode Diagram

Please note that the diagrams show the sensing lead going to the switched side of the master switches. Ideally they should go to the battery + terminal, but when strictly applied this contravenes the Boat Safety Scheme. Also some controller manufacturers advise connecting to the engine battery in case the domestic batteries are turned off whilst the engine is running - which could destroy the alternator.

The diagrams show a battery sensed alternator with Lucas connections. Many Lucas connectors consists of two large blades and one small one, but machine sensed alternators have the two large ones joined, battery sensed ones have them insulated.

If there is no separate sensing connection, the alternator must be modified so the in-built regulator's sensing connection is brought to outside terminals


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The underside of a split charge relay.

I was appalled when I met someone at the 2001 National who had been sold a split charge relay with no instructions on how to connect it.

Do not buy a relay with ordinary, push on, lucar blades on all connections - they are for caravans and normal marine use will burn the connectors & possible burn the internal contacts.

Site the relay where it is protected from being fallen onto, but with short cable runs. Engine room heat is not an issue with relays.

When wiring one of these I usually connect the main alternator output to the starter motor battery connector with a piece of 120/0.30 cable. This ensures most of the cable back to the engine battery is the main starter cable and therefore minimises volt drop.

The switched sides of the battery master switches are joined, via the nutted terminals on the relay, with another piece of 120/0.30 (minimum) cable. Try and keep the runs as short as possible.

One of the blade terminals is connected to any negative point with a piece of 14/0.30 cable.

The other blade is connected to either the alternator warning lamp connection, or, if the alternator has one, the auxiliary output. If there is no other way of providing current to the relay coil ONLY when the alternator is charging, a rising oil pressure switch may be fitted and used to turn the relay on whenever the engine is running.

To assist in identification of alternator wiring the major features of some makes are detailed below.


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The above information has been obtained from a variety of sources and its accuracy is not above question!



Check drive belt for condition and tension

Check all terminals for cleanliness and tightness

Place voltmeter across one battery and run engine. IF voltage is below 13.8v and rising just keep watching until the voltmeter stops rising.

The reading should be 13.8 to 14.2 volts - check in your manual.

Remember a fully flat battery will pull the alternator voltage down - let it charge before taking a reading.

If less, connect the voltmeter between the alternator output terminal and the battery + terminal, start and rev engine. The meter should read 0 volts, up to about 0.2 is just about OK, anything more and you have volt-drop somewhere along the + line (try loose or dirty connections).

Repeat for - side.

If you have no volt drop and still have less than 13.8 volts across the charging battery seek professional help.




Again, you need to go looking for volt-drops.

Make sure the battery is well charged.

Connect voltmeter across battery - it should read 12 volts or more.

Operate the starter with the stop control pulled. The voltage should not drop below 10 volts. If it does and the battery is well charged, first check the battery terminals for cleanliness and tightness, and if OK suspect short circuit in starter.

Connect voltmeter between battery + and starter +, that is the terminal where the starter solenoid is connected to the starter.

Crank engine - expected reading is less than 0.5 volts. If more look for dirty/loose connections, undersized starter cables, poor contact in master switch and poor contact in solenoid. (The last two can be checked by connecting a volt meter across them and cranking the engine). Maximum reading whilst cranking 0.5 volts - any more and you have found an unacceptable volt-drop.

Repeat for the - side, again the maximum is 0.5 volts.

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