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Resistance Overcurrent Protection Power: Amps - Watts  
Wire: Conductors General Rules References  

"Basic Electricity - Page 5 - Simple Circuits

Basic electricity for boat builders, boat repairers and owners. What you need to know about the electrical systems on your boat. Resistance, Power and Over Current Protection. Wiring rules.


Now that you have seen what simple circuits look like, you need to know some other things before you can start wiring up your boat.  Resistance is one of those things. All electrical devices have a certain amount of resistance. (Except superconductors but you will never have any occasion to deal with superconductors.) The amount of resistance in a wire is dependent on the diameter of the wire, and the length. The smaller a wire, the greater it's resistance. The larger the wire the less it's resistance. The amount of resistance determines how much of a loss occurs in the wire when the electrons flow through, or what is called voltage drop. Because of this, wires are sized so that there is as small a voltage drop as possible. See Current and Electricity

Resistance is directly related to the amount of volts and amperage in a circuit: The formula is Resistance (Ohms) = Volts / Amps. (volts divided by amps) Why is this important?  It becomes especially important when there are multiple loads on a circuit, and whether or not the loads are in parallel or series.  This determines the amount of voltage drop, that is how much the voltage reduces as it goes down the line.

On most small boats each piece of equipment usually has its own dedicated circuit, such as the bilge pump, cranking motor,  and accessories such as a radio. However, Navigation lights are usually on one circuit so using a parallel circuit becomes important.  However, as boats get bigger and have more equipment and appliances this becomes a critical issue for DC circuits. 

Most electrical devices such as motors, electronic equipment, and lights, are designed to operate within a very specific range of voltages. If the voltage starts at the battery as 12.5 volts, but loses 1 volt by the time it reaches the device, then it is already down to 11.5 volts. Most devices simply won't operate properly when the voltage is too low.  Electric motors get hot, and will actually burn out if the voltage is too low.  So, the circuits are designed to prevent this.

Series Circuits: In a series circuit each item on the line has a certain amount of resistance. As you add items the resistances add up.  Amperage stays the same in a series circuit but voltage varies with the resistance. The greater the resistance the larger the voltage drop. (that is voltage decreases as you go down the line.) This is why on DC circuits there is usually only one item on a series circuit.

Parallel Circuits: In parallel circuits the voltage stays the same in both legs, the positive feed and the negative return.  Amperage varies across each branch of the circuit but the voltage stays the same. So each load sees the full battery voltage. The resistances do not add up, in fact the total resistance in the circuit decreases depending on how many items are on the circuit.  Because the voltage stays the same each item has high enough voltage to operate correctly.  If an item fails, the others keep working.

When designing the electrical system on your boat keep this in mind.  Instrument panels, and lighting are always on parallel circuits. Cabin lighting and navigation lights are on parallel circuits. 

Critical items like blowers, bilge pumps, the cranking motor, are on series circuits, and are usually the only item on that circuit.

See Wikipedia: Series Parallel Circuits

Wire: (Conductors)

Wire comes in many sizes, and can be single strand, multiple strand, multi strand tinned, and there are other types. Wires for boats should comply with standards published by NEC (The National Electrical Code), UL (Underwriters Laboratories) the US Coast Guard, ABYC, ISO, and Canadian Standards. Specifically you should use wire that is listed as Boat Cable, UL 1426.

See Wiring General 


Wire on boats is required by law to be stranded copper wire. Title 33 CFR 183.425 requires it.

§183.425   Conductors: General.

(a) Each conductor must be insulated, stranded copper.

For  Canada the requirements are in TP 1332 Part 8,  and for the EU look in ISO 10333 Section 7.1

The ABYC Standard is E-11,  AC and DC Electrical Systems on Boats

Single strand, called solid core wire, should never be used on boats. Romex or typical three conductor house wire must not be used.  One of the biggest causes of damage to wire on a boat is shock and vibration. Boats pound and slam and get tossed around a lot.  The engine also transmits a lot of vibration to the boat. Vibration causes wire to fatigue. Eventually it breaks and fails.  The fewer strands in a wire, the quicker this happens. Properly sized and mounted, stranded wire, will last many, many, years under these conditions but solid core single strand wire will fail.

Tinned stranded copper wire is not required but may be used. Tinned is considered better, but is more expensive.  Many marine electricians insist on tinned wire because it lasts much longer in the marine environment, is resistant to corrosion, and wicking of moisture in the wire.  See ABYC E-11.14 System Wiring. 

Wire is rated for the temperature of its environment. On boats, much of the wiring is in the engine room which easily gets to 100 degrees Fahrenheit (38 C) or more. So the insulation has to be capable of withstanding these temperatures over the life of the wire.

Wire is also rated for resistance to chemicals, oils, and other liquids.

See the US Coast Guard regulations for wire on recreational boats in 33 CFR Sec. 183.430. and Sec. 183.435

Wire comes in different diameters, called gages (or gauge if you prefer).  The larger the gage, the smaller the wire. On a boat the smallest gage you are allowed to use is 18 gage but only if bundled with other wire or in a sheath. Otherwise the smallest wire permitted is 16 gage. There are exceptions to this in the regulations and in ABYC standards. The largest gages are 000, 00, 0, 1 and so on. Also rated as 3o, 2o, 1o, or 3 ought, 2 ought, 1 ought. This is really big wire.  If you look at the cable coming off the battery in your car it is probably 1 ought or 2 ought cable. Here is link to Wikipedia that explains American Wire Gauge.

In metric countries wire is sized by it's cross sectional area, in mm squared. The bigger the millimeters the bigger the wire.

Video on Youtube on Battery Cables:

So how do you pick the wire?  Well, most of the work has been done for you. If the wire is boat cable, then UL has already tested and determined that the wire meets the standards for marine wire.  NEC and ABYC (American Boat and Yacht Council) and the US Coast Guard have prepared tables that allow you to pick the size of the wire right off the table. Look at the table:  Just look, don't try to use it yet because before you do, you need to know a few more things.

Power: Amperages And Watts

As you may have seen in the table, it speaks about allowed amperages.  Back to basics. The amount of electrons flowing through a wire is measured in Amperage.  The force pushing the electrons is Volts. But here is a new concept, Power. The amount of power in the circuit is called Watts. Watts are equal to the Amps times the Volts.

W = A X V.

The equipment you are hooking up is probably rated in the amount of Watts needed to operate it.  We know the voltage is 12, so to find the amps needed to operate it, we simply divide the watts by the volts.

W/V = A

By the way, this is about as  difficult as the math gets. If you can add, subtract, multiply and divide, you can do the math. If you have a simple calculator you are all set.

So suppose you have a depth sounder that uses 300 Watts.  300 Watts divided by 12 volts is 25 amps.  So this device needs 25 amps to operate. This is the amount of load on this circuit. If nothing else is on this circuit then that is the total load.

On a very small boat with just a few devices, each circuit may have only one load on the circuit, as in the simple circuit diagrams. But on larger boats such as an inboard with a cabin, galley, head, and berths, there may be several, or many devices on a circuit.  As an example let's use something simple like lights.

If your lights are all on one circuit, then you need to know how many amps each light draws and add them up to get the total amps of the circuit.  Suppose you have 10, 12V lights on the circuit, and each light draws one amp.  Then the total load for the circuit is 10 amps.

Now you need to go back to the table. Look at the table. At the top it says below Allowable Amperage of Conductors,  Temperature Rating.  You need to decide what temperature rating you wish to use. Most boat builders use 105 C (221 F).  Look at the column that says 105 C. Going down from 105 C are two columns, one labeled inside the engine room, the other outside the engine room.  Since these are cabin lights they are outside the engine room. The very first row in the column says 20. Look to the far left of the row and in the far left column is the wire gage in English units and metric. For 20 amps it is 18 gage (0.8 metric). However, 18 ga. is only allowed if it is used in wire bundles that are in a sheath. So, since this is going to be a single wire circuit, we must use the next larger size, which is 16 ga. 16 ga. is capable of handling 25 amps which is much more than our 10 amps.  We could actually have twice as many lights on the circuit, and some could be in the engine room. See Electrical System-Wire Size

If you have a circuit that has many devices on it that draw lots of amps then you would have to go to much larger sizes.

Over Current Protection

Before I continue I want to make one thing perfectly clear. Over current protection is there to protect the wires, not the equipment. It is a common misunderstanding by laymen that fuses and circuit breakers protect the electrical appliance.

Since we have determined the circuit load and the wire size, the next thing to choose is the size of the fuse or circuit breaker.  Which to use?  That is up to you. On small boats much under 20 feet with only DC circuits, fuses are usually used but this is not a requirement. You can use circuit breakers.  On larger boats circuit breakers are commonly used for DC, but occasionally you also see fuses.

 Fuses should be sized to carry 150% of the load on a DC circuit.  This is to prevent the fuse from blowing every time you turn something on. When an electric device starts up there is a momentary surge of current that is higher than the normal circuit current, especially with devices that have electric motors. So if you don't want the fuse to blow when you turn on the switch, it has to be a little larger.

Circuit breakers used in DC circuits are usually single pole breakers that are only in the positive side of the circuit. 

Switches and Circuit Breaker Poles

What does Single Pole mean? Single pole is term used for both switches and circuit breakers.  It means that when you flip the switch it activates or breaks  only one side of the circuit.  Double Pole means both sides of the circuit.   On DC circuits double pole switches can also be used to activate more than one circuit at a time. 

 Shown below is a fuse block. This uses ATO or ATC fuses. Connected to it is a small double fuse holder that takes SFE or AGC type fuses.

_MG_8862.jpg (298281 bytes)

ATC Type use

Typical ATC Type Fuse.

There is a difference between ATO and ATC fuses.   ATO  fuses have a hole in the bottom and are open to the atmosphere. If the fuse is in an engine compartment or other space where fumes may accumulate, and the fuse blew, this could cause an explosion.  ATC fuses are sealed.  No holes.   O stands for OPEN. C stands for CLOSED.  Use ATC fuses in engine compartments or fuel tank compartments or anywhere gasoline fumes or hydrogen gas from the battery could accumulate. See Wikipedia on Auto Fuses:

Our example circuit is 10 amps.  So the fuse for this circuit would be 15 amps.

10 times 1.5 (150%) = 15.

Typically 15 to 20 amps is about all you want on a single circuit. If you have some item, such as a windlass, a pump, or other device that draws 15 or more amps, then put it on a single circuit with it's own fuse.

The one item that is normally not fused is the starter (cranking) motor. The power surge is so large, and the amperage so high, that the fuse would blow. Since the cranking motor is only used momentarily to start the engine, it is normally not fused.  However, in recent years fuses have become available for this use, but they are not required.

For a nice neat installation you should use a fuse block, like that in the picture above, rather than in-line fuses or separate fuses.  This way, all of your fuses are in one place, they can be labeled so you know what circuit they protect, and what size they are.

On AC circuits double pole circuit breakers are always used because they break both the hot (black) and neutral (white) wire at the same time. I will cover AC Circuits later. See Basic Electricity AC Circuits

The Federal Regulations, ABYC, ISO and other standards organizations require that circuit breakers be manually reset, trip free, circuit breakers. What does manually reset, trip free mean?  It means you cannot hold the breaker in the on position. If the situation causing the breaker to trip still exists, it must be able to trip regardless of whether you are holding it in the on position, or taped it in the on position, or in any other way to prevent it from tripping. 

Additionally, if breakers are used in an area where explosive or flammable fumes may collect (such as the engine room, battery compartment, or fuel tank compartment) they must be ignition protected. 

General rules:

Support Wiring: Wiring should never be just strung though the boat. It should be laid out neatly, securely supported, or fastened to the boat at 18 in. (46 cm) intervals or less.  If many wires are going to the same location, say from the distribution bus to the instrument panel, they should be bundled together.

Avoid water and heat:  Wire should be kept out of the bilge (except for bilge pumps) and away from hot stuff such as engine manifolds or exhausts.

Protect wire:  If wire passes through a hole in a bulkhead, or into a panel box, it should be protected against chafing, or wear, by a rubber grommet, duct seal, or some other means to prevent damage.

Label Wires: Wires should be labeled at each end to identify their purpose.  Just a piece of tape with a name on it will do.  There are labels made just for wire, but they are not necessary. But if you want a professional looking job, labels are best.

See also Basic Electrical Practices:

Here are the titles of some good reference books:

Boatowner's Mechanical and Electrical Manual;   Nigel Calder
Powerboater's Guide to Electrical Systems, Second Edition;  Ed Sherman
Boatowner's Illustrated Handbook of Wiring; Charlie Wing
Sailboat Electrics Simplified;  Don Casey
Your Boat's Electrical System: Manual of Electrical and Electronic Projects; Conrad Miller, Elbert S. Maloney
The Twelve-Volt Bible for Boats; Miner K Brotherton

Some excellent web sites for boat electrical questions:

12V Basics for Boaters By Ed Sherman

Getting Grounded By Ed Sherman

Marine Electrical Checklist

© 2007 All rights reserved. revised 02/23/2022

Basic Electricity
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American Boat And Yacht Council  Boat Design Net  Wooden Boat Foundation

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