14 Steps To Wiring Your Boat

What you need to know to install or re-wire the electrical system on your boat. A step-by-step practical guide. Covers Planning, Diagrams, Wiring, Batteries, over current protection and more.

I want to thank Ed Sherman for reviewing this page for accuracy. 

A question often asked on boating and boat building forums, and by visitors to my web site, is: “I need a simple wiring diagram for a small outboard boat to wire up the lights and few other things, but no one seems to have one. Is there one, and where can I find it? Are there a set of step-by-step instructions?” 

There are wiring diagrams, websites and forums that tell you how to wire an electrical system for large boats and bigger sailboats. But when it comes to small boats there is a distinct lack of information and diagrams for how to install a simple, safe, and reliable electrical system. 

The following is meant to apply only to small outboard boats under 16 feet with 50 or 60 horsepower or less. It can be applied to slightly larger boats that have a simple 12V DC system using one or two 12V batteries. 

Note 1: I will not deal with the wiring specifically for the outboard motor and controls.  Here is a web site where you can obtain wiring diagrams for most outboard motors.  Most new outboards come with a wiring harness and a manual that has wiring diagrams.  See Master Tech Marine Outboard Wiring Diagrams.

Note 2:  If you are re-wiring a boat with an electrical system installed:  Don't rip out that old system yet!  Use the old system to help make a plan in steps 1 through 7.  Trace out each wire and put that on your diagram.  This will make it far easier to locate wires and equipment.  Wait until you actually start installing wiring in step 12.  Then replace each set of wires with new.  This may take a little more time, but will result in far fewer mistakes and less troubleshooting.

Note 3: Throughout this I will give references to the US Code of Federal Regulations (CFR) requirements that apply to boat manufacturers, and to the American Boat and Yacht Council industry standards. Examples: 33 CFR 183.401, or ABYC E-11. The US Coast Guard Regulations (the CFR) and the ABYC standards are good guidelines to follow for a safe and reliable electrical system. They are used by marine electricians, professional boatbuilders, designers, marine surveyors, and marine repairers. If that’s how the pros do it, so should you. 

Step 1.  Make a Plan.  Decide what you want to install, and where it will go. See Electrical Planning

Step 2. Draw a simple electrical schematic (diagram) that shows each piece of equipment, the fuses, switches, and how all of this will be connected. This is not a diagram of where the equipment is located on the boat. That will come in Step 8. It is simply a diagram of the electrical circuits. Here are two alternative examples. (Click on the diagram to expand.) The first diagram uses a positive buss bar. The second omits the positive buss bar. For clarity I did not use color codes except red (positive) and black (negative).

Do not be concerned if you don’t know electrical symbols. Just make a box or circle and write in what it is, or you can use a picture of the item. As long as you understand what goes where, and how they are connected, it’s Ok. Remember, any 12V DC device must have at least a positive and negative wire connected to it. Put a plus or minus next to the wire or use red for positive and black for negative. On metal boats do not use the hull as a return (negative) path. Connecting your electrical system to a metal hull can result in stray current corrosion.

See also BoatUS diagram:

There are several ways to draw wiring diagrams. The most important thing is that you understand what you are diagraming. It needs to be simple enough and clear enough for you to be able to refer to it in the future and still understand what each item is, what the wiring is and how each item of equipment is connected to the electrical system. That way, in the future if you want to add or subtract equipment you can do so by referring to your diagram and determining where and how the new item fits into the system.

Step 3.  Batteries: Decide where you will put the battery.  Later we will decide the capacity and type of battery but for now we only need to decide where to put it.

The battery is the source of power for starting, instrumentation, and lighting.  There may be a second battery on some boats for running a trolling motor or other equipment. 

Batteries should not be too close to anything that can cause an accidental short. There should be 12 inches of space all around them. Batteries must not be directly under or over fuel lines or under other electrical equipment such as a charger or inverter. If they are, there must be a floor or panel separating them. ABYC E-10.7.5 and 10.7.6 Storage Batteries

Batteries need to be in a space that is ventilated to the atmosphere. 33 CFR 183.420(e) This applies to all batteries, not just lead/acid batteries.

Batteries must not move, so they have to be fastened down.  33 CFR 183.420(a)

There should be a tray under a battery for spilled electrolyte, or it should be in a battery box, and fastened down so it won’t move under any conditions. (ABYC E-10.7.2) The Coast Guard does not require a tray or a battery box but ABYC does require some means to contain spills. If it is strapped down in a tray, spilled acid won’t damage the boat and the battery won’t move. The terminals need to be covered with a boot or some other device that protects them from accidental contact with metal tools. But, if the battery is in a box the terminals are protected against accidental contact with tools, spills are contained, and it won’t move.

A Single Battery Tray

Two Battery Trays

Note the large red and black wires in both pictures. Those are the battery cables.  The red block on the end of the red wire is the boot that covers and protects the Positive battery terminal. Click on the photos for full size.

The battery should be close to the engine.  Since starting current is so high, and the wires to the starter are not fused, you want to keep the wires as short as is practical. 

The battery should be a combo starting/deep cycle battery, usually sold as a marine battery. An auto battery would do for starting and lights. But, for running a radio, and other electronics while anchored or fishing, a battery with a little deep cycle capacity is needed so the battery doesn’t go flat and leave you stranded when you try to restart the engine.

How big a battery (capacity, not physical size) do you need? That depends on the amount of load on the battery. I will show how to determine that in Step 12.

There is one non-electrical consideration; weight. Lead acid batteries can weigh up to 50 lb. Think about how the weight of the battery will affect weight distribution on your boat, especially if it is on the same side as the helm and controls. You may have to move it to balance the boat side to side. If you have a very low transom, how will the weight of the battery affect the water line at the transom?

Step 4. Battery Switch: Some people think that a battery switch is not necessary on a small boat. I think a battery switch is necessary to turn everything off when you are not using the boat.

Where the battery is located determines where the battery switch goes. It should be close to the battery but easily accessible to be switched off in an emergency. ABYC E-11.6.2.

A good brand is Perko but there are others. Avoid any battery switch that is not UL Marine Listed. There are cheap ones on the market that are not UL listed and can get hot and melt.

A battery switch must be ignition protected. (33 CFR 183.410)

Ignition protection means that it will not ignite gas fumes if they are present. This is extremely important if you have a gasoline fuel tank in the same compartment as the battery.

Use only ignition protected electrical components. You don't want anything in there that will set fuel vapors off. Batteries are not considered a source of ignition because there are no moving parts, but if you make accidental contact with metal tools it can create an arc. So, the terminals must be protected, and battery switches and other electrical equipment in this compartment must be ignition protected.

Buy a switch that has a provision for two batteries because you may want to add a battery in the future. The switch will have three positions. OFF, 1, 2, and BOTH. The 1 position connects the one battery and allows charging of that battery when the engine is running (if your outboard is large enough to have an alternator). The 2 position connects and charges the second battery, if there is one, and the BOTH position puts the two batteries in parallel doubling the battery capacity and charging both at the same time. You won’t need the BOTH and 2 positions now, but this gives you the option to add a second battery.

Step 5.  Fuses: Next, install a fuse block close to the battery switch. Fuses must be within seven inches of the source of power (33 CFR 183.455) but you can go up to forty inches if the wire is sheathed. Standard wire loom is fine as a sheath. Be aware, the fuse is there to protect the wire, not the equipment. If you overload wiring it gets hot, melts and starts a fire. We will determine the size of the fuse later. See Step 12. Buy a fuse block with two fuse holders. That way you have a spare if the fuse blows. This is generally a good idea. When installing fuse blocks get ones with more fuse holders than you think you need. You will need them eventually. One or two extra fuse holders is good.

Step 6. Equipment Location: Determine where each piece of equipment will be.

Think about where you want things to go. Depth finders need to be where they are easy to see, but not blocking your vision when operating the boat. Radios should be where they can be easily reached, and for VHF, reach the mike. The back of the console or surface you are mounting them on needs to be easily accessible for access to the wiring.  

Step 7.  Locate the fuses, buss bars and switch panels. 

Decide where to put fuse boxes, buss bars, switch panels, etc. Each of these must be close to the equipment they power, and easily accessible to be worked on. They cannot be hidden behind equipment or inaccessible panels. This may sound obvious, but I have seen some very bad installations. Also, they should be protected from spray or rain.

Most electrical and electronic equipment comes with pigtails. Pigtails are wires coming out of the equipment and may only be a few inches to several feet long. Sometimes they have a connector attached to the ends of the wire. When determining where stuff goes consider the length of the pigtails, because you don’t want a rat’s nest of wires hanging loose.

Switch boxes: A box or panel where switches can be mounted to control stuff. On a small outboard boat this is usually the dash or the console. 

Fuse block: A panel with fuse sockets on it. It can be open or covered.

Buss bar:  A block with studs for connecting wires.

A Battery Switch

FuseBlock

Buss Bar

Typical Buss Bar: This buss bar is for the negative wires. The large wire on the left is the battery negative.

There are some devices that are connected directly to the source of power and do not go through fuse blocks and switches. They need to always have power. One is the bilge pump. Bilge pumps may have a float switch that automatically turns the pump on when water in the bilge gets to a preset height. This won’t work if the pump is not wired directly to the battery. It is not good practice to wire it directly to the battery though. Wire it to the power input side of the battery switch. It is good to install a switch at the helm that turns the pump on manually.

If your boat has an anchor light, you may also want to wire the switch for the light directly to the power input side of the battery switch. That way you can turn on the anchor light when the battery switch is off.

This is a typical submersible automatic bilge pump. It has a built-in float switch that activates it when the water gets to a preset level. Note the black connector on the right with two black and two red wires. One set are the positive and negative wires to power it, the other two go to the manual switch.

Step 8. Make a diagram of the boat showing where the wiring, equipment and fuse blocks will be located.

Make a rough drawing of the boat looking down from the top. This is called a general arrangement and shows how the boat is laid out. Using your electrical schematic, put in where the equipment, fuse boxes, buss bars, switch boxes and wiring are going to go. Check this against the actual boat to make sure you aren’t missing something.

Wiring cannot go through pieces of equipment, pipes, tubes, and other solid objects. They can go through walls and bulkheads and panels. Wiring must be easily accessible for installation, trouble shooting and replacement. It must be fastened down at least every 18 inches (ABYC 11.15.4.1.9) so it isn’t or chafing on something. Where wiring goes through a bulkhead, wall or panel, it must have a grommet or padding to protect the wire. 33 CFR 183.445(a)

Your diagram may look something like this; (Click on image to expand)

Step 9. Wiring: Figure out how much wire you need, what size wire you need, and what color it should be. Wire standards.

What about the wires from the battery Switch to the starter? The wire needs to be a very heavy gauge, at least a 4 AWG on small outboard boats, because starters draw a lot of current. Both the positive and negative wires should be the same size. If the outboard has the wires for the starter already installed, the wires from the battery to the switch should be the same size as those wires. The engine manufacturer has determined the amount of amperage the starter draws and correctly sized the wires for the load.

The positive wire (red) goes from the battery to the input side of the battery switch. The negative (black) wire goes to a buss bar. One post on the buss is for the wire from battery to the engine block (ground). Another wire goes from the buss up forward to the dash. The others are for other equipment. There should be as many terminal posts as you need plus a few extra.

Color Codes: The positive wire should be red. Negative can be black, or yellow, or black with a yellow stripe. Throughout the boat negative wires should be black or yellow or a combination. AT the dash or console, all positive wires from the fuse block to the instruments and the equipment, should be color coded using the standard color codes for marine wiring. Direct Current Color Codes: From ABYC E-11.15.2.3 Table 11 and Table 12.

Direct Current Color Codes: From ABYC E-11.15.2.3 Table 11 and Table 12.

 Color codes tell you what the wire is for. But label the wire on both ends. A simple piece of tape with a name written on it will do. They do not need to be fancy labels, but if you prefer, you can buy labels at electrical suppliers or hardware stores.

Wire must be marine wire. (33 CFR Sec. 183.435) Do not use auto wire. It is not made to the same standards as marine. Most marine wire is labeled UL 1426. It must be copper stranded wire. It does not have to be tinned, although tinned wire will last longer. On a small boat it is not necessary. Do not scrimp on wire though! Cheap wire could mean the difference between a reliable system and one that you constantly have trouble with. Buy good quality wire. I have seen 100 ft spools of Ancor 16 AWG Tinned Marine Wire for sale on-line for as little as $24.00 USD.  

What size wire? American Wire Gauge (AWG) is in reverse order. The larger the number, the thinner the wire. The thickest wires are 00 or 0 AWG. The smallest gauge allowed on boats for a single wire is 16 AWG, or 18 AWG if it’s in a bundle or sheath (33CFR 183.425), but this may be way too thin for the equipment or the length of the wire run. The only exception to this is wire inside electronic devices or part of the electronic controls on the engine. 33 CFR 183.425(g)

The thicker a wire is, the less resistance it has. The longer a wire is the more resistance it has, and so there is a larger voltage drop. You want to minimize the resistance and the voltage drop. So you first need to figure out the wire size based on how many amps are being used, and then by how long the wire is. Use the tables in Appendix A, at the end of this page, to determine the correct size. Don't just guess at wire size and buy larger diameter wire such as 14 or 12 AWG. See Wire Size:

For the purpose of determining wire size, the fuse block the wire is coming from is considered the source of power. For the wires running from the battery to the starter, or to the under-dash fuse block, the battery is the source of power. In the two examples below the fuse block under the dash or console is the source of power.

Here is an example: 

A Hummingbird Model 345C depth sounder draws 380ma (milliamps from the specifications). The installation includes a 6 foot power cable of 18 AWG wire. This may be fine for connecting it to a fuse block near the dash. But we need to size the cable running from the battery to the dash. It is going to be at least 10-12 feet long on a 16 foot boat. Double that length for the negative return wire. 

Use table 3 in The Appendix for voltage drop. Most boat manufactures use wire rated for 105C (degrees Celsius - the temperature rating of the insulation on the wire). Looking at the table under the column for 105C we see amperages starting at 20 amps, 25 amps, 30 amps, and so on. Following the row for 20 amps to the left column we find 18 AWG.

From the table on voltage drop an 18 AWG wire 20-24 feet long (30 feet in the table) with a 15-ampere load will have less than a 10% voltage drop. But it can only be 18 if it’s in a sheath or bundle. So go up one size to 16 AWG.

Another Example:

Suppose I have three electronics running off a fuse block in the dash or console. Each piece of equipment requires 1 amp at 12 volts to run. The total amperage for the three items is 3 amps. From the fuse block in the dash or console to each item of equipment, there is a positive wire from the fuse to the equipment, and a negative wire running back to the buss. Using 1 ampere, we determine the size the wire should be, by using table 1 and 3 in Appendix A. For instance, if the positive wire is two feet long then the total length of positive and negative wires is 4 feet. Looking at the Table 1, the line for 18 AWG wire at 105C allows up to 20 amps.

So, we could use 18 AWG. Look at Table 3. We see that an 18 AWG wire, 10 feet long, will have less than a 10% voltage drop for up to 5 amperes. Again, we could use 18 AWG but since 18 AWG wire has to be in a bundle or a sheath we add a level of safety by using 16 AWG.

This is done using the tables developed by the US Coast Guard and ABYC. You don’t have to know any formulas to figure it out. The first table determines the wire size based on the load in amps and the second table the size depending on length and voltage drop. You use the larger wire if there is a difference.

See the table in Appendix A at the bottom of this page. or ELECTRICAL TABLE: 33 CFR 183.42: ALLOWABLE AMPERAGE OF CONDUCTORS FOR UNDER 50 VOLTS  or: ELECTRICAL SYSTEMS VOLTAGE DROP

Step 10.  Wiring tools. Wire connections (terminals).  See Connectors:

Tools: Use good quality tools, especially good quality crimpers and wire strippers. Cheap crimpers make bad crimps. Bad crimps make bad connections. Poor wire strippers nick the metal conductor which may cause the wire to break or have a high resistance.   See My Page on Practical:

Wire terminals must be used. Connections should never be a bare wire wrapped around a stud or post. This is bad practice, and can easily come loose or result in a high resistance connection. High resistance equals heat, which results in fire. Never use wire nuts to connect wires on a boat! They are prone to vibration and corrosion. ABYC E-11.15.3.7 Twist-on connectors (i.e., wire nuts) shall not be used.

Use crimp type ring or captive spade terminals. Captive spade terminals have a tang on the ends. This prevents them from being pulled off or slipping off the stud or post. Connections must resist being pulled off. In the ABYC wire standard there is a table listing how much of a pull they must withstand depending on the size of the wire. A 16 AWG wire must withstand a ten lb. pull. A 4 AWG wire must withstand a 70lb pull.

You can solder connections if you like but crimp them first. ABYC standards do not prohibit soldering, but they do not allow soldering to be the sole source of support for the connection. (ABYC E-11.5.3.8) This is because solder creates a hard spot in the wire which is not as flexible as the wire itself and not as resistant to flexing and vibration. So, if you solder you must also crimp. Crimp first, then solder.

Seal wire connections with a good waterproof sealant, usually marketed as dielectric grease. There is no requirement to do this, but it prevents water from getting in the connection and wicking up the inside of the wire insulation or corroding the connector.

My method.  I do not solder.  First I slide a short length of heat shrink tubing onto the wire. https://en.wikipedia.org/wiki/Heat-shrink_tubing  How long it is depends on the wire and connector size.  Usually if the tubing extends about 1/2 inch (1 centimeter) beyond the end of the connector, that is enough.  Then I use dielectric grease. See Wikipedia on Dielectric grease. Dielectric grease is non-conductive grease, usually silicone that is also waterproof and can be used to seal connectors. Before crimping the wire in the connector, I squirt a little dielectric grease into the connector where the wire goes. I then insert the wire and crimp it. Then I slide the tubing down over the connector and shrink it with a heat gun or hair drier so it seals itself around the wire and connector. The combination of grease and heat shrink tubing should keep the water out. 

Heat Shrink Tubing And Connectors, AAA protection, How to install and repair. http://youtu.be/jCRsx38WRw8

How to get a good crimp: Marine How to: Wire terminations:  https://marinehowto.com/marine-wire-termination/

Step 11.  Fuses.  How big should your fuses be? 

Fuses are rated by amperage and protect the wire from overheating and fire. Fuses must be rated at the same or less rating of the wire. If you have a wire that is rated at 15 amps you need a 15 amp fuse. Each circuit is rated for a certain amperage, such as 15 amps or 20 amps, and more equipment is not added to the circuit if it would cause it to draw more current than the fuse is rated for.

This can become an issue on little boats too if you have more equipment, or something like a powerful stereo system that draws a lot of amperage. Then it should have its own circuit and its own fuse for the circuit. 

The question is how many fuses in the block?  That depends on how much stuff you are running.  I would have a fuse for the lights, one for the instrumentation, and one for any electronic devices, plus a spare.  That is four.  But for expansion maybe a six or 8 fuse block would be better. Again, in the future you won’t have to buy a new block. See Overcurrent Protection:

Step 12.  Installing equipment.  

Start with the battery, the battery switch, and the main fuse block. 

Selecting a Battery: Batteries are rated by voltage and capacity. We are using a 12V battery. There are two ratings, CCA and MCA See Batteries at:

CCA Means Cold Cranking Amps. MCA means Marine Cranking Amps. These are measures of how many amps the battery can deliver for 30 seconds and maintain the voltage at 12V. Basically the higher the CCA rating the longer the battery will maintain its voltage. Batteries are also rated by amp-hours. 1 amp for 1 hour is 1 amp-hr. Generally the rating is based on how many amps the battery will discharge for 20 hours until the charge drops to 10.5 volts. The higher the amp hour rating, the longer the battery will power your equipment. Also, batteries are rated for Reserve Capacity which is how many minutes it will deliver the same voltage at 80 degrees. An average marine battery should have a Reserve Capacity of 60 to 90 minutes. Anything less is not adequate.

Double the result to determine what the rating of the battery should be. For this case, 200.

Another consideration is the battery group size. Batteries come in different physical sizes. A Group 24 battery is 10 ¼ inches by 6 13/16 inches by 8 7/8 inches. A Group 27 battery is 12 1/6 inches by 6 13/16 by 8 7/8 inches. The physical size is determined mainly by how much space you have for the battery and its weight. A bigger battery weighs more. A large group size does not necessarily mean it will last longer. That is determined by the battery ratings for amp hours and reserve capacity. The most commonly used size on small boats is Group 27. 

Install the battery box if you are using one, or a tray, then the battery. Now that you have installed a battery you can begin installing equipment. Install lights and electronic equipment. You want everything in place before you begin wiring. Put in switch panels and fuse blocks.

From Step 5. We need to determine the size of the main fuse at the battery. The continuous loads add up to 10.5 amps. The fuse in a DC circuit should be about 150% of the load so a 15 amp would be appropriate. (ABYC E-11.10.1.5.)

The fuses for each circuit of our example should be at least 3 amps except for the VHF radio because on transmit it draws 6 amps. So, use a 10 amp fuse for the radio circuit. Check the manufacturer's installation instructions for recommended fuse sizes for each piece of equipment. Remember, this fuse is to protect the wire to the equipment, not the equipment. Some equipment may have built in or in-line fuses for that purpose.

Step 13. Installing Wire: 

Begin installing wire, starting at the battery and working outward to each fuse block and buss bar, and then on to each piece of equipment. Remember to follow the color codes and label the wires on both ends. If you decide to make any variations from your diagrams make sure you change the diagram for future reference.

Step 14. Turn on the power. Test by turning on each item, one at a time, to see if it works. Troubleshoot as you go. If there is a problem, fix it before you proceed. Once everything has been tested individually, turn on everything, one at a time, until everything is on. If a fuse blows or something doesn’t work the last item you turned on is where the problem lies. Turn everything off, fix it and then try again from the beginning.

An  Excellent Article:  Avoiding Boat Electrical Mistakes by Ed Sherman;  Boat US Magazine https://www.boatus.com/expert-advice/expert-advice-archive/2016/august/avoiding-boat-electrical-mistakes

An excellent article by Owen Youngblood on Wiring Your Boat, from the Metal Boat Quarterly

How to Wire A Boat from New Wire Marine https://newwiremarine.com/how-to/wiring-a-boat/

The USCG Boat Builders Handbook for Electrical Systems is available on-line at https://safeafloat.com/wp-content/uploads/2021/04/I-Electrical-Systems-Final-4-14.pdf

Contact ABYC for a copy of E-11, AC and DC Electrical Systems on Boats. There is a fee. See: https://abycinc.org

Appendix A:  Allowable Amperage and Voltage Drop Tables

Note: This is the table that is in the Federal Regulations. The Federal Regulation now uses the ABYC table. It is published in 33 CFR Subpart I sec 183.425. ABYC Standard E-11 has five separate tables based on how many conductors are in a wire bundle.

The table for voltage drop is below. This is only for 12V DC. Contact ABYC for a copy of E-11, AC and DC Electrical Systems on Boats. There is a fee. See: https://abycinc.org

This is the table to determine wire size due to voltage drop based on the length of the wire. This table is for 12 volts only. The top row is the length of the wire in feet. The first column below Total Amps, is the amount of maximum amperage. The number in the row to the right of the total Amps column, is the size of the wire for a 10% or less voltage drop. Example: 25 feet of wire (top row) at 15 amps (first column) the wire would be 14 AWG.

Navigation Lights: I added this section because many people asked for it.

Wiring Navigation Lights for boats with combination red/green bow lights and an anchor/sternlight on a pole. I have been asked many times if there is a standard wiring diagram for hooking up the lights on a small outboard or inboard boat. There are some variations on this but here is how I did it on my boat.

The below diagram is for small boats with a red/green combo light on the bow, and a single sternlight that can also be used as an anchor light. Usually these have a single switch with 3 positions; Off, 1. anchor light, 2. combo bow light, sternlight/anchor light, and instrument lights. The diagram shows a Cole-Hersee switch that is in common use, but there are other manufacturers that also make switches for this, such as BEM and Blue Seas. They all serve the same function. In this diagram the lights are wired directly to the battery. However, some people prefer to wire it through the battery switch so the battery is not discharged if the lights are accidentally left on. It is just a matter of switching the power wire from B on the lights switch, to the number one position on the battery switch.

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