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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 to be as small a voltage drop as possible. See Energy Story

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.  Motors get hot, and will actually burn out if the voltage is too low.  So, the circuits are made to prevent this from happening.

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. Stranded copper Wire is required.   See Don Casey at BoatUS

Tinned stranded copper wire is not required. Tinned is considered better, but is more expensive.  Many marine electricians insist on tinned wire because it lasts much longer in the marine environment.

Single strand, called solid core wire, should never be used on boats. 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.

Also, 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 requirements for wire in 33 CFR Sec. 183.430. and Sec. 183.435

Wire comes in 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. 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 milimeters 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 the 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.  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.

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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.

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 motor. The power surge is so large, and the amperage so high, that the fuse would blow. Since the starter 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, 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

General rules:

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.

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

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.

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 of these.
Boatowner's Mechanical and Electrical Manual   Nigel Calder
Powerboater's Guide to Electrical Systems, Second Edition by 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:
Ed's (Sherman) Boat Tips
Easy AC/DC Boat Wiring Tips
Marine Electrical Checklist
Getting Grounded By Ed Sherman
Electrical On Board

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