Boat Wiring Harness: All You Need To Know Before Buying

Boats do most of their work in the water. So, it’s crucial to protect your wiring from the obvious pervading threat of moisture. Also, it’s essential to keep your wires organized and safe from jumbling with others by the boat’s vibrations as it moves. Thus, a custom boat wiring harness is a critical investment in the electrical functionality of your boat. Today, we will discuss some information to help you customize the boat wiring harness.

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Chapter 1: Understand Boat Wiring Harness

A custom boat wiring harness is critical to the organization and safety of your boat’s electrical system, and that’s because of its character.

  1. Boat Wiring Harness Construction

A large number of wires can be packed into a single custom boat wiring harness. These wires have a coating to protect them and are put together with a band, sleeve, tape, or lacing, so the correct custom boat wiring harness will safeguard them in a structured fashion. Also, custom wiring harnesses take your boat’s particular wiring challenges into account. So, Your boat will function at its pinnacle of safety and electrical efficiency. 

2. Boat Wiring Harness Functions

Without a boat wiring harness, your boat would leave a mess of wires unprotected from the dangers of water and vibration. One false move and one of your boat’s vital electrical functions could compromise. A custom boat wiring harness will eliminate that worry and help your boat run at its peak capacity for much longer.

3. Boat Wiring Harness Color Code

Because your boat’s wiring is so complicated and should be organized efficiently, your custom boat wiring harness should be color-coded for protection and ease—the harness groups different colored wires about their functionality and power. For example, light blue cables are usually used for the boat’s oil pressure reading and gauge.

If there’s a problem with an aspect of your electrical system, you can easily find the wires which cause the trouble. 

Supply boat support oil and gas industry

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Chapter 2: How Boat Wiring Harness Improves Your Boat

As soon as you start to realize just how many wires it takes to make your boat run, a boat wiring harness becomes essential. If you don’t purchase a stock boat with boat wiring harnesses already in place, it’s a sign that you need a custom boat wiring harness. Without proper protection and organization, your boat’s functionality and overall condition will be in massive jeopardy.

Close up the rear side of boat transom four-stroke outboard motor

Chapter 3: Signs You May Need Custom Boat Wiring Harness

When you hire a company to build your custom boat wiring harness, they’ll work first to organize your wire types. They will use an easy-to-understand color-coded system to ensure each set of wires for each particular need is organized. They’ll group these wires accordingly for ease of replacement when an inevitable power failure happens down the road.

The right company of custom boat wiring harnesses will safeguard your wires in saddles that are moisture-resistant. Furthermore, they’ll build custom boat wiring harnesses that ensure that the vibrations of your boat don’t disconnect them or ruin them. Again, the perfect boat wiring harness will vastly decrease the potential for electrical fires onboard.

The proper way to build a custom boat wiring harness is with the safety of your boat’s passengers in mind. The right company of custom boat wiring harnesses will work to make harnesses that ensure no one trips over wires. 

Chapter 4: How To Build A Wiring Harness For A Boat

Boat electrical wiring is a whole different ball game from wiring a home or even a car. The wiring must withstand excessive moisture, unusual vibration, high temperatures, and salty water. If you do not wire the boat’s electrical circuit to mitigate against these factors, there is a risk of equipment failure and, in the worst case, boat fires. 

That is why hiring a boat is not truly complete if it does not include boat wiring harnesses. Let’s take a closer look at how to wire a boat.

1. The Battery as the Electrical Source

A boat will have one or more batteries charged by the engine’s alternator or the auxiliary battery charger. The batteries are the source of electrical energy and can generate hundreds or thousands of amps, more power than an entire house needs. For this reason, take all appropriate circuit protection precautions.

Boats would usually have two types of batteries–a starting battery and a deep cycle battery. A starting battery has a high current rush capacity, while the deep cycle battery can perform a deep discharge without causing any harm.

The two most common battery setups are the single-engine (one starting and one house battery) and the twin-engine (two starting and one house battery).

2. Marine-Grade Main Battery Switch

Your boat wiring system should usually have a marine-grade main battery switch. For a 1-2-BOTH type switch, both battery positives run through it, allowing you to choose the battery you would like to output. Though this is similar to the A-B switch, the 1-2-BOTH will enable you to parallel the two batteries. With that, you can turn off everything in a single action. 

You could use both settings when running your engine and looking to change both batteries from the alternator. You may also use the two sets to parallel your batteries to start the engine after depleting your battery during an emergency.

Remember to turn the battery switch to the house circuit when the engine is off, so you are only tapping into the deep cycle house battery.

Note that your best bet is the double pole ON-OFF-COMBINE battery switch for a two-battery, single-engine boat wiring system. It allows the start battery to stay isolated except during emergency conditions.

3. Bypass Main Battery Switch for Bilge Pump Float Switch

It is pretty standard boat wiring practice for electricians to bypass the main battery switch for one reason–the bilge pump float switch. The pump will kick in with that setup if the boat starts to fill with water, even though the battery switch may be off. The rationale here is you would rather have a dead battery than a sinking boat.

Make sure you circuit-protect the fuse via an inline fuse. Pay attention to the bilge pump’s negative return wiring.

4. Get the Source to the Boat’s Helm

Next, source power from your house battery to the switch panel. Use a marine-grade primary wire to run a positive from the battery switch and a negative from the battery negatives (ganged together) to the central switch panel. 

It can be a reasonably long wiring run. Bear in mind that the further away your battery is from the switch panel, the greater the voltage drop your wiring has to contend. You can minimize the voltage drop by using a large cable. 

Also, the two conductors are typically thick as they have to carry your combined electrical loads. For small boats, 12 AWG wires would work, 10 AWG is normal in larger vessels, and 8 AWG is overkill except for 30 feet or longer boats.

Note that if your switch panel does not come with circuit breakers, you will need a fuse block right before the board and then conductors from each board to each fuse. Many boat electricians recommend ordering circuit breakers with your switch panel to make installing and maintaining the board easier.

Whether you are using circuit breakers or fuses, it should install these overcurrent protection devices as close as is practically possible to the circuit’s power source. 

The principal house battery positive conductor feeds directly to the switch panel. On the other hand, the negative goes to the negative busbar, where you eventually attach the load negatives.

5. Install Terminal Block

It will be easier for you to install the boat wiring harnesses with pre-installed heat shrink labels and ring terminals when you wire the switch panel, and it fits onto a terminal block.

Crimp a #8 ring terminal on your positive load wiring running around the boat to various loads. Each switch output has its gang on the terminal block. With labels already in place, it simplifies the process of troubleshooting or adding items later on.

6. Load Wiring to Busbar and Terminal Block

From this point onward, the wiring is relatively straightforward. Connect your boat wiring infrastructure to the busbar and terminal block. Negatives go to the busbar while the positives to your terminal block. 

You would mainly use #8 ring terminals for termination. Ensure you install the positives on the right gang associated with the corresponding switch for the load. You can install the negatives on any busbar screw since the goal is to get them to the appropriate negative post of the battery.

There is nothing quite like the marine environment to push electrical circuits to their limits. Getting the wiring process right and using boat wiring harnesses is key to a boat electrical circuit’s quality and resilience. 


The above will be very helpful for you to know enough information before customizing the wiring harness. Of course, if you contact a company with rich experiences, such as  CLOOM Tech,  it will help you consider all the factors and provide professional services to ensure your sailing smoothly.

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