These notes concern 6 v - 12 v - 24 v - 32 v ( low voltage) direct current distribution circuits only. When working with higher voltage circuits, always obtain assistance of a qualified electrical technician. Be aware that sparks from low voltage circuits can cause battery expolsion in a confined space.

Fundamental Electrical Law.    A little bit of essential theory.
Ohm's Law states that the current flowing in a circuit is proportional to the voltage applied and inversely proportional to the circuit resistance.

This may be read as " the current flowing between two points in an electric circuit is directly proportional to the potential difference between these points, and inversely proportional to the resistance of the circuit between these points".

i.e. I = V/R        or         V = I x R        or        R = V/I

where I = current in ampers.    V = voltage or electrical pressure.    R = electrical resistance in ohms.

If any two paramaters are known, the equations can be resolved. e.g. a resistive load drawing 5 amps in a 12 v system has a resistance of 2.4 ohms.

Low voltage electrical power, measured in watts, is the product of voltage and current.

P = V I . Since V = I x R, then also P = I² R . Since I = V/R, then also P = V²/R   e.g. a 15 W navigation light on a 12 v. system would draw a current of 1.25 amps. Operating for 12 hours, it would consume 15 amp hrs., or 15% of a 100 amp hr. battery capacity, requiring an input of approx. 18 amp hrs. to recharge.

In using these equations, it is more appropriate to apply Ohm's Law to component parts of a circuit, and to apply the power equations to the overall circuit or to appliances.               Back to top.


Series and Parallel Circuits
The preceding theory has to be considered in its application to series or parallel circuits. Series connections may be utilized in distribution for connecting batteries to obtain the required supply voltage, e.g. 2 x 6 v = 12 v or 2 x 12 v = 24 v, but invariably, low voltage distribution circuits are in parallel. i.e. all positives are fed from a positive bus, and all negatives return to a negative bus, commonly called a ' two wire system'. Battery terminals are marked + positive, or -negative, and this sequence of connection called polarity, must be maintained throughout the system circuitry. Components connected with reverse polarity may be irreparably damaged.

VTotal = V1 + V2 + V3.
i.e. the sum of the voltage drops across each load = the applied voltage.

RTotal = R1 + R2 + R3. i.e. the total circuit resistance = the sum of all resistances.

The current I is the same value at any point in a series circuit.
VTotal = V1 = V2 = V3 .
i.e. The voltage to any branch of a parallel circuit is the same as the applied voltage.

1/RTotal = 1/ R1 + 1/R2 + 1/R3.
i.e. the reciprocal of the total circuit resistance = the sum of the reciprocals of all resistances.

ITotal = I1 + I2 + I3 . i.e. The total circuit current is the sum of the currents in each branch.

Note that a battery also has internal resistance, adding to the overall circuit resistance. A typical boat installation is therefore be a combination of series and parallel circuits.

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Marine Electrical Cable Nomenclature.
Automotive or AC electrical cable is unsuitable for use in boats. Marine grade electrical wire is tinned (electroplated with a nickel alloy) to resist corrosion and it will outperform and outlast non-tinned wire many times over as it is resistant to wicking.
It is essential to be familiar with the polarity of connections and the color coding of cables. Normally, the positive cable is red, and the negative cable black. On twin core cables, the negative cable may be red with a black stripe. European cables may have different color coding e.g. the positive may be brown and the negative blue. If uncertain, seek advice before connecting. It is good practice to prepare a circuit diagram for your boats electrical installation, with all circuits numbered and matching tags on cables.


Typical Marine Wire Specs.

Amp ratingCoresStrands
No./mm		Nominal Area
sq. mm		Resistance
Ohms/100 m.		Insul. Thickness mmNominal OD mm.
15Single28/0.3020.840.62.95
25Single31/0.4550.350.84.35
15Twin28/.3020.840.64.75 x 7.60
25Twin31/.04550.350.85.95 x 10.30

  • Amp rating is the measure of the cable's current carrying ability, usually at 20° C. Current carrying ability is reduced at higher temperatures and when cables are bundled. Bundled cables within an engine room or compartment can have amperage reduced by nearly 50 per cent.
  • Cores is the number of conductors contained within an electrical cable.
  • Strands is the number of individual strands of wire making up the conductor and their size in millimetres.
  • Nominal area is the cross sectional area of the conductor in square millimetres.
  • Conductor resistance is usually stated as ohms per 100 or 1,000 metres and is an important factor in determining cable size, particularly with long runs. For example: a 5 amp. load is connected to a battery bank with 10 metres of cable. If we're using 15 amp. cable, look at the chart above and we will find that the resistance factor is 0.84 per 100 metres (which is 0.084 per 10 metres). Double this figure to allow for the return circuit, i.e. 20 metres of cable. Now using Ohm's Law: E = I x R, where E (voltage) = I (5 amps) x R (0.084 x 2), or in other words, we have a voltage drop of 0.84 or nearly one volt at the appliance end.
  • Nominal insulation thickness is stated in millimetres. The most common insulating material is PVC, and to meet Australian Standard AS3191 it needs to tolerate temperatures up to 90° C. PVC has good insulating properties and is reasonably moisture resistant. Lloyds specifications require butyl rubber insulation because of its higher insulating properties and moisture resistance.
  • Nominal OD is the overall thickness of the cable in millimetres.
Incorrect cable sizes.
The smaller the cable, the greater the resistance for a given current.
The greater the resistance, the larger the voltage drop. E = IR.
This is of course a major factor in battery charging, particularly from solar panels, and day to day running of a trouble free system. One can liken the problem to trying to run high pressure water through too small a hose. That's the same as a poor join causing higher resistance. All the 'sizes' must match to allow a clean uninterrupted flow through the circuit.       Back to top.

Protecting Your Boat's Electrical System.
Even a medium size boat has many tens of metres of electrical wiring. Obviously, if there is a malfunction, considerable damage can be done, either to the wiring, the appliance or possibly the boat itself. All electrical circuits must be protected, and this is usually achieved by the installation of a ' weak link' in the form of a fuse or circuit breaker, in each electrical circuit.

Fuse Types.
Fuses are cheap and effective; a fusible (weak) link melts at a known amperage. The most common fuse is the 3AG (size 3 automotive glass). it is 6.3 mm (0.25") in diameter and 32 mm (1.25") in length. They are also described by their speed of operation:

Circuit Breaker Types.
Circuit Breakers, although a lot more expensive than fuses, can double up as switches. There are two types:
  • Thermal CB's use a bimetallic strip as the operating mechanism.
    PROS: Because their operation is heat related, they are selfcompensating. This means that the external heating effects of the ambient temperature on an electric cable will similarly influence the bimetallic strip, causing it to trip without delay once the heat threshold within the circuit is reached. Current peaks are also conveniently accommodated because it takes time for the bimetallic strip to heat up.
    CONS: Once tripped, the bimetallic strip must first cool down before it can be reset.

  • Magnetic CB's use a magnetic solenoid and spring to activate the mechanism.
    PROS: Very fast operation and an ability to be reset immediately.
    CONS: Very fast operation can be a hindrance with some appliances (a motor, for example), because the initial startup surge can be many times the normal load current. Nuisance tripping is the result. To help tolerate these temporary overloads, some magnetic CBs have a thermal delay to dampen their sensitivity. Shock and vibration can also cause annoying tripping.
Amperage Rating of CB's.
This is the current carrying capacity, or amperage, on which the tripping or blowing of a fuse or circuit breaker is based. it is the number that is usually printed on the device and is based on a working temperature of 25° C. Fuses operating at higher temperatures need to be de-rated. A fuse or circuit breaker in a boats electrical distribution board is pimarily intended to protect wiring and switches, not appliances or instruments. These should have their own dedicated fuses or CBs.

N.B. If fitting 240 v power outlets to your vessel for shore power, be sure to fit the type with inbuilt residual current protection, or fit a separate residual current device to the circuit. Do not take for granted that such protection will be provided by a marina. Seek advice from a qualified electrician.        Back to top.

Poor Connections.
Low voltage marine electrical wiring systems require particular attention during installation and with maintenance, because the working environment can easily have an adverse effect on the power supply to a range of voltage sensitive equipment.

Poor connections are a major cause of high resistance in circuits and a perfect way to eventually ruin batteries and electrical equipment. There are many electrical connections in boat circuits, and each one represents a resistance and voltage drop. The aim is to have clean, secure connections to minimise this. Glass fuses are one area that can cause resistance, the metal ends eventually get a coating of corrosion quite unseen by the human eye. Increased resistance in the circuit invariably means an increase in heat with the inevitable voltage drop associated with this.
Every now and then it is wise to inspect all such connections and ensure all corrosion is removed either physically by scraping clean to bright metal, renewal with a new connection or cleaning with one of the many products available for the job at an electrical store. Blade type fuses are prefered for marine work as they have a larger and more positive contact area.
Connections to the VHF radio power supply are particularly important, as excessive voltage drop will affect the transmit function, and it pays to minimise the number of connections in this circuit.

Incorrect Cable Joins.
Enter one big villain. Once again, its incredible how many 'temporary' joins become permanent once the sticky tape is around the join. Then, because the join apparently is working well, and the gear hasn't failed, complacency creeps in. A good soldered joint is preferable. Use tinned cable to avoid wicking, especially in bilge areas, solder tails before crimping, and seal line connections with pressure sensitive rubberised tape, or heat shrink tube. Avoid intermediate joins in cables wherever possible. More voltage drops and bad performance in electrical circuits can be attributed to poor joints than any other reason. However, be aware that in some cases where the join is subject to vibration, soldered joins may get brittle and break e.g. on engine harness. In these cases, mechanical connectors such as the Deusch mulitpin type should be used . Support cables adequately with saddles or clamps to minimise movement, and wrap multiple cable runs in spiral loom or similar.     Back to top.

Testing with a Multimeter.
For accurate readings in the low voltage ranges, a digital multimeter is essential. Select the appropriate range before testing.


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Some General Advice.
With the increasing reliance by the boating public on electronic navigation and communication equipment, a little t.l.c. will ensure that equipment continues to work, particularly when it's needed most.

When planning the fishing trip, allow the night before to check your battery, the connections and charge. A generous coverage of petroleum jelly on the connections will solve a lot of electrical problems from developing. If your electronic equipment is mounted in a position which is exposed to salt water ingress, adding petroleum jelly around the plugs will help prevent corrosion and salt build up, which can save you from expensive repairs.

A lot of older GPS units are now getting to the stage where their internal battery is starting to run out of life. Earlier units had batteries with a life from 3 – 5 years, newer units have rechargeable batteries or batteries with a life of around 10 years. You will know when your batteries are flat in your GPS because it will take 15 – 45 minutes to lock on every time you turn it on and any marks or waypoints you've entered will be lost. Your local service technician can replace these batteries in most cases.

Prior to each trip, carry out a radio check with the local VMR. This can be done as you log on for your trip. Remember, if you do get into trouble you may need to contact the VMR and having confidence in your communications will ease any panic that may arise. Be particular about clean connections on your radio power supply circuit. Excessive voltage drop across poor connections can affect the transmit function. Your radio may be ok in Auckland Ck. but not at the reef. Carry spare fuses.

Sounders are useful tools in finding that elusive "feed". Take the time to play with your sounder, read the manual and understand the information displayed on the screen. With some newer units, there is an auto function included. While this may be useful in some cases, it has been found that the best performance will be achieved by disabling this function and setting the sounder for your present conditions.

As with most equipment, a little planning and "tic" goes a long way. Clean your connections, check under the dash for loose, greening or broken cables and use lots of petroleum jelly, and your electrical and electronic equipment should be a useful asset for the life of the vessel.
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