Tecktalk - DC/AC Inverter and the Cruising Sailboat

Telltale Article - by George de Witte.

My previous Telltale article on the Ammeter generated enough appreciative comments by NSC members, that I got inspired to write a similar article on a different subject. This time it is about the DC to AC inverter and its application and installation on a small or medium size cruising sailboat.

It is probably best first to explain what an inverter does. As the name implies, it converts the 12 Volt DC (Direct Current) from the boats battery system to 120 Volt AC (Alternating Current) standard household current.  The only difference is that most midrange inverters produce a quasi square wave as opposed to the sinusoidal or wavelike voltage delivered by the electric utility to your home.  This difference in waveform has little impact on most electrical devices, however when you measure the output from the inverter on a DVM (Digital Voltmeter) you will read something like 95 Volts. Don’t worry about this as your DVM is calibrated to read sine-wave AC signals accurately. The inverter documentation usually cautions you that certain devices are not compatible with this different waveform.NiCad battery chargers for instance are frequently mentioned, but I have never found it to be a problem.

The attractions for an on board inverter are numerous. Past NSC boards so far have resisted suggestions to have shore power on our docks. Don’t get me wrong as I fully support this policy. So the availability of an inverter allows you to operate small power tools like drills and sanders or an electrical soldering iron dockside without having to make a run to the service dock to find AC power. Once your mid season maintenance issues are under control and you dropped the hook in your favourite anchorage, it is time to think about the galley. You guessed it. The chef can now operate mixers and blenders to produce haute cuisine that competes with anything chef Aaron can produce.  Come to think of it: you can actually work on your “Do List” at anchor without “free advice” from your dockside neighbour. The long term cruiser will find the inverter useful for things like PC laptops including DVD movie play, freshly ground coffee, electrical shavers, rechargeable flashlights etc.

Once convinced that life without an inverter is impossible, you need to consider the big question: how big is big enough. Inverters present large loads to the battery, so some form of compromise is necessary. It is probably easiest to play around with a few numbers. From the Ammeter article you may remember that
Current = Power ÷ Voltage
Or in electrical units
Amp = Watts ÷ Volts.

So a 400 W load with an inverter efficiency of 90% and a 12 Volt battery will draw 400 ÷ 12 ÷ 0.90 = 37 Amp.  Therefore it does not make sense to rate the inverter for things like 1200 Watt toasters or hair dryers as they will quickly deplete the battery charge. It is a personal choice, but I would recommend something in the range of 400 or 600 Watt. They are fairly compact and allow you to run most medium size AC loads without running down the battery in a short time.

Installation Challenges

Installing an inverter on a sailboat requires some careful planning. For starters as I explained before, inverters are current hogs, which suggest they should be installed as close as possible to the battery system. This will minimize voltage losses in the DC wiring without going to excessively heavy wire sizes. In most cases that means somewhere under a settee. But this immediately creates 2 problems. First the On-Off switch is located on the unit itself, so you will have to remove cushions and covers from the settee in order to access the inverter.  Not very attractive to the chef who is just whipping up a Mousse Viennoise.  Secondly the AC connection consists of standard household receptacles, implying that you need to run extension cords from underneath your settee to wherever you want to operate your AC device.

Actually what you would really like to do, is wire the inverter into your existing AC outlets. However the manuals of most inverter makes that I have been able to read strictly prohibit connecting the inverter directly to your boats AC wiring. This has to do with the fact that in AC wiring standards the neutral (white wire) and the ground (bare copper or green wire) are normally tied together in the distribution panel. The way inverters work both line and neutral are hot, therefore the hot inverter neutral cannot be wired directly into a grounded AC neutral as this will result in a big short circuit. Another problem with direct wiring is that if the boat is plugged into shore power, the shore AC will reach the inverter output circuits, for which they are not designed and not surprisingly will destroy them according to the manufacturer.

But don’t get discouraged by all this. The next section will solve all this.

Inverter mounting and Wiring Details

Most boats should have an AC wiring diagram similar to that shown in Fig 1. (This diagram and the next one in Fig 2 are easier to follow when admired in colour if you are reading the Telltale online).  The AC boat wiring is pretty straightforward. The shore power enters the boat via a hull mounted inlet and is fed to the AC panel. The main Circuit Breaker provides the first level of overload protection and then feeds a number of secondary circuit breakers to provide power to the individual loads. It may seem that the AC and DC systems are completely isolated, but keep in mind that the Battery Charger (not shown in Fig 1) connects the 2 systems and in particular can tie the 2 grounds together. There may also be other intentional or unintentional AC to DC ground connections.

The additional wiring and components to install the inverter are shown in Fig2.  Let’s start with the DC wiring. After locating a suitable mounting location as close as possible to the house battery, you need to determine a minimum fuse rating. For instance a 400 watt inverter at 10.7 Volt and 90% efficiency draws 42 Amp, so a 50 Amp fuse or circuit breaker will do. Similarly a 600 Watt inverter draws 62 Amp, so a 70 Amp fuse/CB will suffice. After mounting the inverter and the fuse/CB block, determine the roundtrip length of DC wiring in feet. Allowing for say a 0.3 Volt drop across the DC wiring, calculate the minimum DC wire size.  For example the 600 watt inverter at 62 Amp and 6 feet of wiring allows a max wiring loss of 0.3 ÷ 62 ÷ 6 = 0.00081 Ohm/ft or 0.81 Ohm/1000ft. Fromdata on wire specifications like the Ancor website, we can determine that #8 AWG will suffice. (#10 AWG = 0.98 Ohm/1000ft, #8 AWG = 0.62 Ohm/1000ft and #6 AWG = 0.40 Ohm/1000ft)

Now comes the good part: the transfer switch. As you can see by studying Fig 2, it allows the selection of power for the boat outlets to come from shore power or the inverter by simultaneously switching the line and neutral AC connections to either source. This simple action gets around all the problems I outlined in the previous section. Technically this switch is called a Double Pole Double Throw (DPDT) switch. They are readily available in electronic supply stores and should be rated at 10 Amp AC. However I found them to be less than reliable in the outdoor marine environment. My solution was to buy 2 so called 3 way toggle switches, a common item in the electrical section of hardware stores. A 3 way switch is really a Single Pole Double Throw (SPDT) switch and is used in house wiring to operate a single light from two different locations.  Drill a small hole in the toggle end of the switch and after side by side installation, connect the two toggles together with a piece of (teak) dowel and some small wood screws. While in the hardware store also get a twin plastic electrical utility box. They last much longer in the marine environment than the metal variety.  So with a bit of carpentry and some house wiring practice, this should take care of the transfer switch and give it a very professional look.

The last part involves the On/Off switch. The switch itself is any simple SPST switch that can be bulk mounted anywhere on the settee bulkhead for easy external access. The current is negligible, so use twin 18 AWG wire for the connection. The challenge is to connect it in parallel with the inverter switch. To do this, you need to remove the inverter cover, feed the switch wire through a suitable opening and solder it to each of the two switch pins on the circuit board. I am afraid that this action likely voids the warranty on the inverter, but sometimes a small risk is worth the adventure. Leave the original switch in the “Off” position and the new remote switch will now act as “On/Off” switch.


This takes care of the installation. As usual one word of caution:  use only marine grade wiring for these projects. It is stranded and tin plated and will last a lot longer than bare copper landlubber wiring, which will corrode and lead to very hard to diagnose problems. From this project you can also figure out why the ABYC (American Boat & Yachting Council) has recommended that yellow be used for DC ground in lieu of the historic black: it can easily be confused with the Line side of AC wiring.

Run a load close to the maximum specified for the inverter for 10 minutes or so and make sure that the inverter and DC wiring do not run excessively hot. That’s all. Hopefully I have given you an excuse to visit the gadget section on the Toronto boat show. And in the best tradition of NSC: if you have any questions regarding this project, feel free to give me a call and I’ll try to help to the best of my knowledge.