Electrical Systems for Boats

Posted on Tuesday 26 June 2012

If you want to upgrade the electrical system on your boat or add one to a boat that really doesn’t have one you will need a few things. Before I mention what you’ll need, I would point out that it is often far less expensive and faster to rip out the entire previous system and replace it than it is to try and do an extensive upgrade if you consider the time dealing with the legacy wiring may cost. This is especially true on older boats where the electrical system may have been cobbled together by previous owners and may have a lot of things wrong with it.

I have seen ROMEX solid untinned household wire used on boats, I’ve seen lamp cord and speaker wire used for higher amperage circuits that have nearly caused fires. I have seen wire nuts used on wires in a bilge. These are really things you should avoid if at all possible. They are unsafe, unreliable and can actually be dangerous.

The things you will need to replace or upgrade the electrical system on a boat are:

First, you need common sense.

If you don’t have good common sense, then I would highly recommend you pay someone to do the work instead. While the voltages are fairly low, make no mistake, the power involved is dangerous and can kill you if you make a mistake. Common sense can not be taught. Common sense can not be bought. Either you have it or you don’t.

Most of all boat work—in fact most of all specialized work—is 95% common sense mixed with 5% esoteric or specialized knowledge. This is true of working on diesel engines, electrical wiring, plumbing, electronics, fiberglass repair or almost anything else you might need to know owning a sailboat. Without the foundation of common sense it is almost impossible to do anything safely. The esoteric knowledge can be gained through classes, watching someone do it, learning from books/videos/the internet.

Second, you need to get a basic foundation of knowledge about 12 VDC electrical systems on sailboats.

Get yourself a good book on marine 12 VDC electrical systems, like Charlie Wing’s Boatowner’s Illustrated Electrical Handbook, Miner Brotherton’s The 12 Volt Bible for Boats or Nigel Calder’s Boatowner’s Mechanical and Electrical Manual.  

Another good way to get a handle on 12 VDC basics would be to take a marine or automotive electrical system course at the local technical/vocational school.  The electrical system on a car is quite similar to the 12 VDC system on most boats.

One commonly asked about system is air conditioning.  Running any sort of A/C on a boat requires either an engine-driven system, which is expensive, or a generator to drive a 110 VAC A/C system—less expensive, but more maintenance and costs, or using shorepower to run it—which is pretty easily done if you keep the boat in a slip with a shorepower connection.

I’d point out that air conditioning isn’t usually required when at anchor or on a mooring, as the boat will swing head to wind and opening the hatches should give fairly good ventilation.

A basic 12 VDC electrical system will require:

1) Batteries—preferably at least two, one for a starting bank and one for a house bank. 

Please note: in the rest of this article, I will refer to the house battery and starting battery, but the battery may technically be a battery bank consisting of several batteries in serial or serial/parallel combination—this is especially true of the house bank, which is generally much larger than the starting bank on most cruising boats.

2) A main DC panel, preferably using circuit breakers rather than fuses

3) A main DC battery switch—I prefer the BlueSea Dual Circuit Plus series of switches, since they isolate the starting and house loads.

4) A main fuse for each battery bank—like the BlueSea MegaFuse.  This is to protect the wiring and boat in case the batteries have a problem.  This fuse should be as close to the batteries as possible.

5) A main fuse—that goes between the main DC panel and the battery switch to protect the panel.

6) A ground bus bar—which will be the common termination point for all of the electrical circuit ground wiring.  This is connected to the battery bank negative posts.

7) A positive bus bar—which will be the common termination point for some of the 12 VDC wiring.

This is connected to the house battery bank at the main 12 VDC battery switch. This would be used for things that may need to be “hot” all the time, like the memory circuit for a stereo, the maintenance bilge pump, the fume alarm, etc. Not all boats will require one. This should be have a cover or be protected from accidental contact in some way.

8) Battery cables—I recommend getting them from GenuineDealz.com on the internet.  They will crimp the lugs you need on the cables if you ask them to for a very reasonable fee. I highly recommend that you use Yellow for the 12 VDC ground—the reason for this is to differentiate the 12 VDC ground wiring from the 110 VAC hot wiring if you ever add a shorepower or inverter setup to the boat.

9) Marine-grade, tinned, stranded Wire—You can buy wire from many different sources.  I like the Berkshire marine wire, as it is better than the Ancor IMHO and far less expensive.  Genuinedealz also sells pretty good marine wire.  Pacer makes good marine grade wire as well. Again, use Yellow for the 12 VDC ground.

10) Terminal blocks—you may find you may need some terminal blocks.

Terminal blocks can be very useful where you have to make a series of connections on a regular basis. Some places they are used are to connect the hull wiring harness to the main electrical panel. This allows you to disconnect the main panel and remove it if necessary, but leaves the wiring in a fairly neat and organized fashion.

Other places they might come in handy include at the junction where the mast exits the cabin or at the base of the mast step, or where external equipment is connected that may be removed on a regular basis—like the compass light wiring or for instrument display heads.

I would also highly recommend getting the following to make the 12 VDC system more foolproof and user-friendly.

1) A battery combining device—like the BlueSea ACR, or the Xantrex Echo Charge or the Balmar DuoCharge.  Having one of these will allow you to charge both battery banks without user intervention.  All charging sources should be wired to the house battery bank if possible. The starting battery bank will be charged via the ACR/EchoCharge/DuoCharge unit.

2) A good battery monitor—like a Victron BMV 601, which will let you see how much energy you are using, and what the state-of-charge of your house battery bank at any time.

If you keep the boat at a dock with shorepower, you will also want:

1) A 30-amp shorepower inlet socket. I prefer the newer style SmartPlug connector, as I believe it is a more intelligent and reliable design.

2) A main shorepower panel that has a double breaker for the incoming 110 VAC shorepower line. Ideally, this would be an ELCI type breaker panel, such as the Blue Sea 1502 panel. 

The ABYC requirement for a double main breaker on the shorepower panel is to ensure that the 110 VAC system is not live when the main breaker is in the off position.  If you used a single breaker and the shorepower outlet had reversed polarity, the 110 VAC system wiring would still be live, even if the main circuit breaker was off, since the voltage would be coming in on uninterrupted neutral line on a system with a single main 110 VAC breaker.

3) A 110 VAC-powered, three-stage, intelligent battery charger—Iota makes a good unit at a reasonable price.

4) A GFCI-type 110 VAC outlet or GFI device for each circuit you have coming off the AC Shorepower panel. You can also use ELCI/GFCI type breakers on the panel instead—which is actually the preferred option, since the entire circuit is protected this way.

If you keep the boat on a mooring, or plan on cruising longer term, you will probably want:

1) One or more solar panels—to recharge the batteries when not connected to shorepower without running the engine.

2) A charge controller for the solar panels. Ideally, you would want an MPPT-type charge controller to maximize the useful energy gained from the solar panels. An MPPT-type controller can allow you to capture as much as 25% more useable energy from the solar panels.

You should probably read the article on Solar Power on Boats I wrote on on this blog previously, as I will not be going into the details of setting up solar power in this article.

Tools you will need/want:

1) A good digital multimeter—I prefer an auto-ranging unit, since they’re easier to use.

2) A good crimper for heat-shrink insulated crimp terminals, like the Fastenal 4 nest heat shrink terminal crimper.

3) An ultra-fine tip Sharpie marker

4) 3M White Electrical Tape

5) A good wire stripper, like the Ancor 702030.

6) A small butane-powered torch for shrinking heat shrink insulation.

7) Assorted adhesive-lined heat-shrink terminals for various ring terminals, butt splices and blade/spade connectors.

8) An electrician’s fish tape—either fiberglass or steel, and a 25′ one is probably the longest you’d need for most small sailboats. Some good 1/8″ cord to use for pulling wires is also a good idea.

9) A good pair of diagonal cutters

10) Basic tools like wrenches, pliers, screwdrivers—these should already be aboard as part of your boating tool kit.

You should read Maine Sail’s excellent article on terminating electrical wire connections.  It goes into far more detail than I probably would, and is an excellent tutorial on how to properly terminate marine electrical wiring.

As for equipment for your boat:


I’d go LED with the lighting.  It will be more reliable and lower draw than going with either CF, Incandescent or Halogen lighting.

Interior Lights—I’d recommend getting SensiBulb LED-based interior cabin lights.  They are the best in terms of color, area of coverage and durability.  You can either buy the fixtures and retrofit them, or buy the fixtures with the Sensibulbs pre-installed. The Sensibulbs will fit many different model fixtures, ranging from reading lamps to dome-type area lighting.

Navigation Lights—I’d highly recommend getting USCG-certified LED-based navigation lights, like the AquaSignal Series 32 navigation lights. These will be lower-maintenance, lower energy use, and higher reliability than traditional incandescent navigation lights.


Stereo—Get a good car stereo, unless you need one that is water-resistant.  If you have to mount it near the companionway, get a marine unit instead.

GPS Chartplotter—I prefer the Garmin units.  While they don’t integrate as neatly with instruments from other vendors, they are very reliable, have probably the best user interface on the market, and good charts.  The higher end units will integrate with the Garmin 18HD radome, which is quite good for small sailboats.

VHF—I would highly recommend getting a DSC capable unit that has two receivers.  Some of the Class D units sold in the USA do not, but are called DSC-capable receivers because of a loophole in the specifications.  I prefer the ICOM and Standard Horizon brand units.  Some of the newer units also have AIS capability built in, which is a good idea.

Autopilot—These are a godsend when singlehanding the boat.

If your boat is tiller-steered, you can get either an integrated unit, which is usually less expensive, but more likely to get damaged, or a component-based system that has a separate control head, fluxgate compass,  course computer and linear drive—more expensive and a bit more complex to install, but likely to last longer.

For wheel-steered boats, you again have the choice of a wheel-drive unit or one with a below-deck drive system.  The wheel-drive units are easier to install and less money, but the below-deck systems tend to be more reliable and robust.  A below-deck system is really the only choice for many larger, heavier boats.


Fans—I prefer the Caframo Boras and the Hella Turbos, depending on where the fan is being mounted.  The Hella Turbos are bit better if being mounted on a bulkhead, since they pivot at the central axis of the fan.  The Boras, which are a nicer fan IMHO, tilt at the base of the fan housing, and are better mounted on railings or on top of half-height bulkheads.

Solar Fans—These I’ll mention, but they’re really a separate topic, since most do not wire into the boat’s 12 VDC system. They are really useful and make a huge difference in keeping a boat well ventilated. I use several Nicro solar vents aboard my boat.


For most boats, there are two choices for refrigeration.

The first are engine-driven systems.  I don’t really care for these, as they’re really not useful for long-term cruising or liveaboard boats.  In general, minimizing the amount of time you are required to run the engine is a good idea.  Also, on boats like my s/v Pretty Gee, there isn’t an option for engine-driven refrigeration since she is outboard-powered.

The other choice is a 12 VDC system, or a 12 VDC/110 VAC system. These generally require installing an insulated box, a cold plate, a compressor and a condensor or heat exchanger. On some systems the heat exchanger uses raw water to cool it and requires a raw water intake through hull, a raw water circulation pump and a raw water exhaust through hull.

If you want a self-contained system 12 VDC or 110 VAC/12 VDC system, rather than one that requires a separate condenser, compressor and cold plate, then the Engel or Norcold units are pretty good choice.  They’re fairly efficient and work quite well and come in a variety of sizes and designs. These have consistently been ranked as best for 12 VDC small refrigerators.  They’re actually made in the same factory. I have an Engel MT27 22-quart refrigerator on my boat.  The units are dual voltage and will switch from 12 VDC to 110 VAC automatically.

Safety Gear

Propane Solenoid and Fume Detector—I highly recommend getting and installing one of these if you use a propane powered galley stove/oven, barbeque grill or heater. It will help give you some additional protection and safety.

Carbon Monoxide and Smoke/Fire alarms—should be a requirement on all boats, but I prefer the battery-powered units, rather than powering them off the boat’s 12 VDC system.

Sizing the Batteries

Properly sizing the batteries is something that takes a bit of work. You really need to make a rough electrical budget before you can really start. An electrical budget consists of figuring out what the amperage of each item you will use is and estimating how many hours you will use it per day. By doing this, you can get a rough estimate of the total amp-hours you will need per day.

Now, I generally recommend doing two budgets. One budget is your “At Anchor” budget. This will generally consist of the anchor light, the cabin lights, the refrigerator, the stereo, the VHF, and such. The other budget will be the “Passagemaking” budget. It will consist of the “At Anchor” budget minus the anchor light but adding things like the GPS, the autopilot, the radar, navigation lights, etc. The “Passagemaking” budget will generally be higher than the “At Anchor” budget.

A good compromise number is to use the average of the two, since you’ll generally be at anchor more than on passage, and on passages, you will often be under power for some portion of it which will give you a chance to recharge the batteries.

Multiply this by the number of days you want to go without plugging into shorepower or running the engine to recharge the batteries, and you’ll have a base estimate for your needs. Double that number and you’ll have a rough estimate of the size of the battery bank you will need, since, as a general rule, lead-acid batteries do not like being deeply discharged very much and should not be discharged past the 50% state-of-charge level on a regular basis.

Here is a sample electrical budget. This assumes the boat uses a manual head, alcohol stove and is equipped with LED-based lighting for both cabin and navigation.

Sample 12 VDC Electrical Budget

Sample 12 VDC Electrical Budget

In the case of the sample budget, we’re looking at six Trojan T105 golf cart batteries for the house bank. That will give us about 670 amp-hours at a 20-hour rate, since each pair is rated at 225 amp-hour at 12 VDC.

A Bit About Batteries

Getting the battery bank size right is a bit tricky. Unless you have been running on a battery monitor and have a good idea of what your actual electrical usage per day is, it is better to over-estimate the usage and have more capacity than it is to underestimate the usage and have not enough.

One reason this is a very good idea is that the higher the load, relative to the battery size, the lower the effective battery capacity will be.  This is why the 20-hour rating is always higher than the 5-hour rating for the same battery. 

For instance—the 20-hour rating on your example battery is 120 amp-hours or a 6-amp load for 20 hours.  If you were to put a 20-amp load on the same battery, it would not give you six hours of use.  It would probably give you something more like four-hours of use, or 80-amp-hours total.  This is due to the Peukert factor or rating.

Now, lead-acid batteries generally do much better if they are not deeply discharged. Discharging them beyond the 50% level will generally shorten their effective lifespan. So, an 120 amp-hour battery really should only be used for 60-amp-hours or so at the 20-hour rate of discharge. This is one reason I recommend getting a good battery monitor.

If you know that you’re constantly running the batteries down below the 50% level, you will realize that you really have to increase your house battery bank size to get the maximum life out of them. Likewise, if you realize that you’re not using as much electricity as you thought, you might be able to spread out running the generator an extra day and save a fair amount of wear/tear and fuel from running it less frequently.

Of the three types of battery chemistries—wet-cell, AGM and Gel, I would avoid Gel, as it has the undesirable characteristics with no real benefits compared to the other two in my opinion. AGM is nice, but for most people, the cost of wet-cells makes them the most feasible and most affordable.

Charging batteries—or why you need an intelligent battery charger

As for how long you can go between recharging the batteries.  That all depends on the size of the battery bank and the loads on it.  Lead-acid batteries don’t charge at a constant rate. They charge in three basic phases.

Bulk Phase—Up to about 80–85% charge level, they charge fairly quickly.  This is the bulk charging phase, and on wet-cell batteries is about 20–25% of the battery bank’s capacity in amperage.  So, for a 200 amp-hour bank, it would charge at 40-50 amps until it reaches this point.

Absorption Phase—Then the amount of current the batteries will accept will drop drastically.  This is called the absorption phase.  The voltage of the charger will also drop as well. It may well take longer to get from 85% to 99% than it did to get from 50% to 85% because of this.

Float Phase—This is the third charging stage or phase, and the batteries are essentially nearly fully charged at this point.  The battery charger drops the voltage down to about 13.5 VDC and the amperage is minimal. The battery charger’s main purpose at this point is to keep the batteries topped off.  Most rechargeable batteries self-discharge and without the float phase, would slowly discharge over time.

A non-intelligent battery charger won’t check the voltage/state of charge on the batteries and will stay at the same voltage and boil off the electrolyte on the batteries as they approach the fully charged state. An intelligent battery charger will drop the voltage and amperage as the batteries approach being fully charged and the really good ones will essentially shut off and then run an occasional maintenance charge to keep them topped up.

The high bulk phase current acceptance rate is why many cruisers will run their batteries from the 85% state-of-charge level down to about 45% and then back, with an occasional full charge up to 100% about once every three weeks. This gives them far more efficient use of their batteries, saves fuel and wear on their engine/generator, and probably gives them more effective amp-hours overall than running them from 100% to 50%.

Planning your electrical system

I highly recommend drawing up a wiring diagram for your boat. Many smaller boats have a very small electrical panel with only 6 or 12 circuits. That can make planning the system a bit more complicated, since you have to make compromises.

On S/V Pretty Gee which has a six-breaker main 12 VDC electrical panel, I gave myself a bit more flexibility by running some of the circuits from the main panel to switch sub-panels. For instance, I have one breaker that is marked electronics and it runs to a fused switch panel that has the VHF, GPS, Instruments, Primary Autopilot, Secondary Autopilot, and Autopilot Remote on it. Another circuit breaker for the navigation lights runs to a panel that has the anchorl light, tricolor light, bow bicolor, steaming light, stern light and foredeck light on it.

After you drawn up your wiring diagram, you should write down how long each run is. You will want to confirm this by actually checking the wiring runs that you are planning to use on your boat. Try to avoid running wiring in the bilge if you can, since wiring in the bilge is going to be exposed to more moisture and water than wiring higher up in the boat.

If your boat doesn’t have proper wiring conduits, you can often make them by installing PVC pipe in the appropriate locations. I like using 1-1/2″ Schedule 10 PVC pipe for conduit if it is possible. It has a fair amount of space for running wires and protects the wires from chafe, water and vibration.

If you don’t run the wiring in conduits, you need to support it every 18″ or so to prevent it from work hardening from vibration and fatiguing. This often requires installing cable tie mounts and cable ties at each point. This makes adding or modifying the wiring much more difficult.

If you have to install cable tie mounts to secure the wiring instead of using a wiring conduit, I highly recommend using the WeldMount system to install them. They also make useful fasteners for mounting battery retention straps, electronic components and such. Their Executive kit has a fair amount of very useful hardware and mounts for the average boatowner.

Calculate the wire gauge and amount you will need based on a minimum of 14 AWG wiring for all branch circuits, and upsize the wiring as necessary for heavier loads. Remember to order it in the appropriate color insulation.

Here is a table with the recommended minimum AWG wire size based on the round trip distance of the circuit and the amperage load.

12 VDC Wire Size Table

12 VDC Wire Size Table

Buying Wire and Terminals

Purchase the wire and battery cable you will need for your project. Get the battery cables cut to length and pre-terminated with the appropriate diameter battery/terminal lugs. Get the wire/cable in bulk spools. Don’t forget to order a bit extra, because having a bit surplus beats having to re-order in the middle of a project. Also, having some spare wire aboard for repairs and modifications is never a bad thing.

You can buy duplex cable, which has a yellow and a red (or a black and a red) wire in an external jacket which makes running wire a bit simpler logistically, but can be harder to run than individual wires due to the greater bulk and lower flexibility. It also does reduce the wire’s maximum amperage slightly.

Fastenal sells a good heat shrink terminal kit with torch for marine wiring. They also sell a good crimper for heat shrink terminals.

Main Panel Design and Placement

The main DC electrical panel should be mounted fairly high above the cabin sole. On my boat, the main electrical panel is mounted above the navigation console and that’s a pretty reasonable choice on most boats. I do not understand why so many boat manufacturers insisted on mounting them next to the companionway and often as low as possible. This almost ensures that they will be damaged by water at some point.

Ideally, the panel will either be hinged or mounted on a hinged plate that swings open to allow access to the back of the panel. The panel should have space behind it or near it to allow you to mount the bus bars and any terminal strips you need. Any switch panels and the main battery switch and fuse for the main panel should be located nearby as well.

The main battery switch should be visible from the companionway, so you can quickly and easily check to see if you’ve shut it off or not.

Fuses and Circuit Breakers

Just remember, the fuses and circuit breakers on the panel are generally there to protect the wiring, not the equipment. If you want to protect the equipment from overcurrent, then you will generally need to either run the circuit through a fused switch panel, which I have done on s/v Pretty Gee for many things, or use an in-line fuse holder.

I prefer circuit breakers over fuses, but use both on my boat. I highly recommend trying to standardize the fuses you use on a boat to the minimum types you can. I currently use three different types of fuses—ATO/ATC blade-type fuses, Mega fuses and Maxi blade-type fuses.

You should have a large fuse in each battery bank positive cable. This is to protect the wiring for the batteries in the case of a short at the main battery switch or such. It should be as close to the battery as possible. The best solution is to use a battery terminal fuse block that mounts at the battery terminal. I did not do this because the battery boxes on my boat did not have the vertical clearance necessary to allow this.

Running the Wire

Running the wire, especially if you have installed messenger lines and conduits, is a relatively simple concept. Unfortunately, most boat manufacturers cheat when they wire the boat and run the wiring before the boat is fully assembled. This gives them options and ways of running wire not available to the owners of said boats.

Try not to put the wire under a heavy strain as that can damage the fine strands and the insulation. If a wire doesn’t seem to want to make a run, figure out why rather than just pulling harder on it.

Don’t forget to leave drip loops anywhere a wire passes through a exterior bulkhead or the cabin top and before attaching a wire to a panel, terminal block or electrical fixture. A drip loop is a small section of slack wire that is formed into a low hanging loop that any water coming in along the wire’s insulation will follow and then drip off the wire at. This prevents the water from following the wire all the way into a piece of electrical equipment or into the back of the electrical panel.

I got this image of a drip loop off of Captain Pauley’s Virtual Boatyard blog.

An electrical drip loop.

An electrical drip loop.

I also recommend leaving at least a foot or two of slack at the outboard end of each cable run. This allows you some room for error and for future equipment replacement and such.

Remember to wrap each wire with a bit of white electrical tape and label exactly what the wire is for. Do this on both ends. It makes your life a lot simpler when you go to troubleshoot or repair the system later. You can also use a labelmaker instead, but I find the 3M White Electrical tape and a ultra-fine tip Sharpie works pretty well.

Terminating the Wires

Again, you should read Maine Sail’s excellent article on terminating electrical wire connections. It goes into far more detail than I probably would, and is an excellent tutorial on how to properly terminate marine electrical wiring.

To be continued….

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