banner6a.jpg (17058 bytes)

Printer Version
6.5 in (17.1 cm) wide


I want to thank Ed Sherman of ABYC and Boat Wiring - Ed's Tips for reviewing this page for accuracy. 

The following is meant to apply only to small outboard boats under 16 feet, 50 horsepower or less. 

A question often asked on boating and boat building forums, and of me 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? Is there a set of step by step instructions?”

The simple truth is there isn’t one.  This is because no two boats are alike.  But some generalizations can be made.

Note: 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 with 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.

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

Step 2.  Draw a simple schematic (diagram) that shows each piece of equipment, the fuses, switches, and how all of this will be connected.   Here are two alternative examples.

Wiring Diagram
ALt Circuit

Don’t be concerned if you don’t know electrical symbols. Just make a box or circle and write in what it is. As long as you understand what goes where and how they are connected it’s Ok. Remember, any 12V DC device has to 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 path.  None of the wiring should be electrically connected to the hull.

See also BoatUS Diagram:

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.

Batteries need to be in a space that is ventilated to the atmosphere.

  Batteries must not move, so they have to be fastened down.

 There should be a tray under a battery for spilled electrolyte, or it should be in a battery box, and the box fastened down so it won’t move under any conditions. If the battery is in a box the terminals are protected against accidental contact with tools. If it is not in a box the terminals need to be covered with a boot or some other device that protects them from contact. 

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. 

Battery Tray 
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.
Two Battery Trays   

The battery should be a combo starting/deep cycle battery, usually sold as a “marine battery”.  An ordinary auto battery would do for starting and lights, but for running a radio and other electronics something with a little more of a deep cycle capacity will be needed so the battery doesn’t go flat while you’re fishing and listening to the radio, and leave you stranded when you try to restart the engine.

How big a battery (capacity, not size) do you need?  That depends on the amount of load on the battery so I’ll figure that out in Step 12.

There is one non-electrical consideration; weight.  Batteries weigh a lot.  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.  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.

The battery switch is necessary to turn everything off when you are not using the boat.  A good brand is Perko but there are others. See Marine Battery Switches  Avoid any battery switch that is not UL Marine Listed. There are cheap ones on the market that will get hot and melt.  Also a battery switch must be ignition protected. 

Ignition protection means that it will not ignite gas fumes if they are present. This is extremely important if you have a fuel tank in the same compartment.  Most small outboard boats have a motor well forward of the transom and the space under the motor well is where the battery, bilge pump, battery switch and fuses, and most importantly the fuel tank are placed.  It may be a portable tank.  If it is, it vents into the compartment.  If it is a permanent tank it will have an overboard vent, but if there is a leak, it will leak fumes into the compartment. You don't want anything in there that will set this off.  So use ignition protected components. Submersible bilge pumps are usually ignition protected.  If you use circuit breakers instead of a fuse they must be marine ignition protected circuit breakers.  However, batteries are not considered a source of ignition because there are no moving parts to cause a spark, but if you make accidental contact with metal tools it can create an arc. That is why the terminals must be protected, and why battery switches in this compartment must be ignition protected.   See Ignition Protection Protection

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 an OFF, 1, 2, and BOTH position.  The 1 position uses the one battery and allows charging of that battery while the engine is running.  The 2 position uses and charges the second battery, if there is one, and the BOTH position puts the two batteries in parallel doubling the capacity and charging both at the same time.  You won’t need the BOTH and 2 positions right now,  but if you decide to add a second battery you won’t have to buy a new switch.

Step 5.  Next we need to install a fuse block close to the battery switch. The requirement is within seven inches but if you can’t do that you can go up to forty inches if the wire is sheathed. Standard wire loom is fine.  The thing to remember here is,  the fuse is there to protect the wire! Not the equipment.  If you overload wiring it gets hot, melts and starts a fire.  This fuse is in the main power feed to the instruments and all the electrical equipment so it will most likely be 15 amps. But we will determine that later.  See Step 12.  Buy a fuse block with two fuse holders. That way you have a spare if the fuse blows.

Step 6.  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 the operator can easily reach the controls, and reach the mike if this is a marine VHF radio. The back of the console or place you are mounting them needs to be easily accessible for mounting the device and for access to the wires.  

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

Use the locations of each piece to 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 where they are 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 or may be several feet long. Sometimes they have a connector attached to the ends of the wire. You need to take this into account when determining where stuff goes 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. 

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

Buss bar:  A block with studs for connecting wires.

Battery Switch 
Fuse Block  Buss Bar 

There are some devices that are connected directly to power and do not go through fuse blocks and switches.  In other words, they always have power to them. One is the bilge pump.  Bilge pumps usually have a float switch that automatically turns the pump on when water in the bilge gets to a preset height. This won’t happen if the pump is not wired directly to the source of power. It is not good practice to wire it directly to the battery though. Wire it to the power input side of the battery switch. Most automatic bilge pumps have an inline fuse or a built in fuse. It is also a good idea to install a switch at the helm that allows you to turn the pump on manually.

Bilge Pump  This is a typical submersible automatic bilge pump. It has a built in float switch that activates it.  Note the black connector that plugs into the pump, 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 at the helm.  

Step 8.  Make a drawing showing where the wiring will be.

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

Remember, wiring cannot go through pieces of equipment, pipes, tubes and other solid objects that would be damaged. 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 so it isn’t swinging in the wind or chafing on something. Where wiring goes through a bulkhead, wall or panel, it must have a grommet or padding to protect the wire from damage.

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

Boat Wiring Diagram

Step 9.  Figure out what size wire you need and how much.

What about the wires from the engine to the battery?  Because starters draw a lot of current, the wire needs to be a very heavy gauge, at least a 4ga on small outboard boats.  This includes the positive and negative side. Both wires should be the same guage. If the engine has the power wires already installed, the wires from the battery to the switch should not be smaller than those wires. The engine manufacturer has already determined the amount of amperage the starter draws and correctly sized the wires for the load.

The positive wire (red) goes to the input side of the battery switch. The negative (black) wire should go to a buss bar.  A buss bar is simply a block with a lot of posts on it.  One is for the wire from battery to the engine.  The others are for the equipment. There should be as many posts as in the fuse block. 

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.  But from the fuse block to the equipment all positive wires should be color coded using the standard color codes for marine wiring. 

Direct Current Color Codes: From ABYC E-11 Table XIV and Table XV.

Green DC Grounding Conductor Bonding Wires (insulated)
Yellow or Black DC Negative Conductor Negative Mains
Red DC Positive Positive Mains
Yellow/Red Starting Circuit Starting Switch to Solenoid
Brown/Yellow Bilge Blowers Fuse or Switch to Blower
Dark Gray Navigation Lights Circuit Breakers or Switch To Lights
Dark Gray Tachometer Tachometer Gauges and Senders
Brown Generator Armature Generator Armature to Regulator
Brown Alternator Charge Light Generator Terminal or Alternator Aux Terminal to Regulator
Brown Pumps Circuit Breakers or Switch to Pumps
Orange Accessory Feed Amp Mtr to Alt or Gen Output & Acc Circuit Breaker & Switches
Orange Common Feed Distribution Panels To Accessory Switches
Purple Ignition Ignition Switch to Coil & Electrical Instrument
Purple Instrument Feed Distribution Panel To Electrical Instruments
Purple Main Power Feed Positive Mains (particularly un-fused)
Dark Blue Cabin and Instruments Circuit Breakers or Switch to Lights
Light Blue Oil Pressure Oil Pressure Gauges & Senders
Tan Water Temp Water Temp To Sender To Gauge
Pink Fuel Gauge Fuel Gauge Sender to Gauge
Green/stripe Tilt Down/Trim in Tilt and Trim Circuits
Blue/Stripe Tilt Up/Trim Out Tilt and Trim Circuits

This way you know what the wire is for just by looking at it. But for additional help in later troubleshooting, label the wire on both ends.  A simple piece of tape with a name written on it will do. They don’t have to be fancy labels although you can buy labels at electrical supply or hardware stores.

Wire must be marine wire.  You should not use auto wire. It is not made to the same standards as marine. Low voltage conductors must comply with SAE standard J1127 and J1128 and the insulation temperature rating of SAE J378b or UL standard 1426.   Most marine wire is labeled UL 1426.  It does not have to be tinned wire,  although tinned is better and will last longer, but on a small project is not necessary. Do not scrimp on wire! Cheap wire could mean the difference between a reliable system and one that you constantly have to troubleshoot. Buy good quality wire. I have seen 100 ft spools of 16ga Tinned Marine Wire for sale on-line for as little as $24.00 USD.

What gauge wire (the diameter)?  Wire gauge is in reverse order. The larger the gauge number, the thinner the wire.  The thickest wires are 00 or 0 gauge. The smallest gauge allowed on boats for a single wire is 16 gauge, or 18 gauge in a bundle or sheath, but this may be way too thin for the equipment or the length of the wire run.  The thicker a wire is the less resistance it has.  The longer a wire is the more resistance it has, and so the more the voltage drops from one end to the other. 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. But don't just guess at wire size and buy larger diameter wire such as 14ga or 12 ga. Use the tables to determine the correct size. See Wire Size: Sizes

Here is an example: 

A Hummingbird Model 345C depth sounder draws 380ma (from the specifications). Their installation includes a 6 foot power cable of 18ga wire. This may be fine for connecting it to a fuse block near the dash, but the cable running from the battery to the dash is going to be at least 10-12 feet long. You need to double that for the return wire.  So I would go up a size to 16ga for the run from the battery to the dash. A 16 ga wire 15 feet long and 15 feet back, with a 15 amp load will have less than a 10% voltage drop. Since this wire may be powering other equipment from the in dash fuse block it would be good to make it larger than the wires from the fuse block to the equipment. Confusing?  I’ll try to make it simpler. 

Suppose I have three pieces of electronics running off a fuse block. Each has its own fuse and power cable from the block to the equipment. A single wire runs from the battery fuse block to the block in the dash. And a wire runs back from the negative buss to the battery. Adding the two gives the total length of the wire. Each piece of equipment requires 1 amp at 12 volts to run. So the total amperage is 3 amps. Therefore the wire from the battery to the fuse block and back has to support three amps without any significant voltage drop, or without getting hot and causing a fire. So you size that wire for the total load (amps) for that circuit. 

This is done using 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 load in amps and the second the size depending on length and voltage drop. You use the larger if there is a difference. See Appendix A at the bottom of this page. 

For instance 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 18ga. But it can only be 18 if it’s in a sheath or bundle. So go up one size to 16ga. 

See the table in Appendix A. or and

Step 10.  Wire connectors. See Connectors:

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.  

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 have to withstand being pulled off. There is in the wire standard a table listing how much of a pull they have to withstand depending on the size of the wire. A 16 ga wire must withstand a ten lb pull.  A 4ga wire must withstand a 70 lb 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. 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. You can tin the wire before crimping if you want.  

Never use wire nuts to connect wires on a boat! They are too prone to vibration and corrosion.

Seal wire connections with a good waterproof sealant.  There is no requirement to do this, but this 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.  How long it is depends on the wire and connector size.  Usually if the tubing extends about 1/2 inch (centimeter) beyond the end of the connector, that is enough.  Then I use 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.  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 combo of grease and tubing should keep the water out.  

A good page on wiring and connections  on the Friendship Sloop Society web site explains it well. Yes he is talking about much bigger boats but the basics apply to all boats. Also see Heat Shrink Tubing And Connectors, AAA protection, How to install and repair.

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

Fuses are rated by amperage and are there to protect the wire from overheating and starting a fire. Fuses must be rated at the same or less rating of the wire. So if you have a wire that is rated at 15 amps you need a 15 amp fuse. This is why on bigger boats you will see many separate circuits for different systems on the boat. 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: 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:

CCA Means Cold Cranking Amps.  MCA means Marine Cranking Amps. This is a measure 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 they 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.

There are three types of batteries commonly used on boats, Wet Cell, AGM and Gel,  but for now I’ll stick with the standard wet-cell battery.  They are relatively inexpensive, can be purchased anywhere, and for a small boat, more than adequate.  A battery with a CCA or MCA rating of 200-300 should do.  Install the battery box if you are using one, or a tray, then the battery. See table below on how to calculate load. Battery Capacity should be at least twice the load.  See Also Electrical Planning

continuous Loads Intermittent loads Total
Item amps hours Amp Hours    item amps hours Amp Hours
Nav Lts 1 8 8    Horn 1 0.25 0.25
Bilge Pump 2 8 16
Radio 1 8 8    Radio Tx 6 0.5 3
Depth Sndr 0.5 8 4
Engine 2 16 32
Instruments 1 8 8
GPS 1 8 8
Bait Well 2 8 16
Totals 10.5 100 7 3.25 100.3

Then 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. 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.

The fuses for each circuit should be 3 amp except for the 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.  Begin installing wire, starting at the battery location 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 each item, one at a time to see if everything works. Troubleshoot as you go. 2011 All Rights Reserved

Appendix A:  Allowable Amperage and Voltage Drop Tables

Note: This is the table that is in the Federal Regulations.  ABYC Standard E-11 has five separate tables based on how many conductors are in a wire bundle.

Temperature Rating of Conductor Insulation
Conductor Size English (metric) 60 C (149 F) 75 C (167 F) 80 C (176 F) 90 C (194 F) 105 C (221 F) 125 C (257 F) 200 C (392 F)
Outside Engine space Inside Engine Space Outside Engine space Inside Engine Space Outside Engine space Inside Engine Space Outside Engine space Inside Engine Space Outside Engine space Inside Engine Space Outside Engine space Inside Engine Space Outside or Inside Engine Space
18 (0.8) 10 5.8 10 7.5 15 11.7 20 16.4 20 17.0 25 22.3 25
16 (1) 15 8.7 15 11.3 20 15.6 25 20.5 25 21.3 30 25.7 35
14 (2) 20 11.6 20 15.0 25 19.5 30 24.6 35 29.8 40 35.6 45
12 (3) 25 14.5 25 18.8 35 27.3 40 32.8 45 38.3 50 44.5 55
10 (5) 40 23.2 40 30.0 50 39.0 55 45.1 60 51.0 70 62.3 70
8 (8) 55 31.9 65 48.8 70 54.6 70 57.4 80 68.0 90 80.1 100
6 (13) 80 46.4 95 71.3 100 78.0 100 82.0 120 102 125 111 135
4 (19) 105 60.9 125 93.8 130 101 135 110 160 136 170 151 180
2 (32) 140 81.2 170 127 175 138 180 147 210 178 225 200 240
1 (40) 165 95.7 195 146 210 163 210 172 245 208 265 235 280
0 (50) 195 113 230 172 245 191 245 200 285 242 305 271 325
00 (62) 225 130 265 198 285 222 285 233 330 280 355 316 370
000 (81) 260 150 310 232 330 257 330 270 385 327 410 384 430
0000 (103) 300 174 380 270 385 300 385 315 445 378 475 422 510

Notes for the above table

Temperature Rating of Conductor Insulation


60oC (140oF)

75oC (167oF)

80oC (176oF)

90oC (194oF)

105oC (221oF)

125oC (257oF) 200oC (392oF)
1. See the following table: Temperature Rating of conductor


0.75 0.78 0.82 0.85 0.89 1.00
2. See the following Table: Number of current carrying conductors             Correction Factor
3             0.70
4 to 6             0.60
7 to 24             0.50
25 and above             0.40

The table below is from ABYC. There are tables for 24 volts and 32 volts, and for AC systems in the ABYC Electrical Standard E-11.  Contact ABYC for a copy of E-11, AC and DC Electrical Systems on Boats.

Table to determine wire size due to voltage drop based on the length of the wire. For 12 volts only.

Length of conductor from power source to the device and back to the power source

Feet 10 15 20 25 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170
Total Amps 12 volt  -  10% drop wire sizes gauge
5 18 18 18 18 18 16 16 14 14 14 12 12 12 12 12 10 10 10 10
10 18 18 16 16 14 14 12 12 10 10 10 10 8 8 8 8 8 8 6
15 18 16 14 14 12 12 10 10 8 8 8 8 8 6 6 6 6 6 6
20 16 14 14 12 12 10 10 8 8 8 6 6 6 6 6 4 4 4 4
25 16 14 12 12 10 10 8 8 8 6 6 6 6 4 4 4 4 4 2
30 14 12 12 10 10 8 8 6 6 6 6 4 4 4 4 2 2 2 2
40 14 12 10 10 8 8 6 6 6 4 4 4 2 2 2 2 2 2 2
50 12 10 10 8 8 6 6 4 4 4 2 2 2 2 2 1 1 1 1
60 12 10 8 8 6 6 4 4 2 2 2 2 2 1 1 1 0 0 0
70 10 8 8 6 6 6 4 2 2 2 2 1 1 1 0 0 0 2/0 2/0
80 10 8 8 6 6 4 4 2 2 2 1 1 0 0 0 2/0 2/0 2/0 2/0
90 10 8 6 6 6 4 2 2 2 1 1 0 0 0 2/0 2/0 2/0 3/0 3/0
100 10 8 6 6 4 4 2 2 1 1 0 0 0 2/0 2/0 2/0 2/0 3/0 3/0 2011 All Rights Reserved. Revised 12/05/2015