7/20/2020: Initial Post
The ABYC definition of an inverter is “an electronic device, powered by batteries, designed primarily to provide AC current at a required voltage and frequency.” In North America, inverters produce 120V AC (or 240V AC) at 60 Hz from energy stored in 12V or 24V batteries. On boating forums that I follow, there have recently been many questions about selecting and installing inverters on boats, so in this article, the topic is “Inverters on Boats.”
There are two types of inverter installations found on boats. The first case is the stand-alone inverter. These are usually smaller inverters used for charging cell phone batteries or powering portable computers. Larger stand-alone inverters can be installed alongside, but separate and isolated from, the built-in AC system of the host boat. Stand-alone inverters are limited in features, requiring manual intervention each time they are needed. They are turned “on” manually when needed and turned “off” manually when no longer needed. Their un-shared outlets are often mounted on the unit itself.
The second case is inverters installed within the host AC power system of a boat. When installed fully-integrated within a boat’s AC power system, inverters offer boat owners a whole-boat “Uninterruptible Power Supply” (UPS), and commonly function as battery chargers while external AC power is available. Inverters installed within the host electrical system must comply with cUL/UL-458 per the ABYC Electrical Standards E-11 and A-31.
In 2020, most inverters sold for installation on boats are Pure Sine Wave (PSW) devices. Older inverters were Modified Sine Wave (MSW) devices. Some 120V household devices did not work well, sometimes not at all, on MSW inverters. Generally, PSW devices are to be preferred for overall compatibility with consumer electronics in household equipment and appliances.
Figure 1 shows a stand alone inverter. Inverters in operation can demand a great deal of DC current from batteries. Regardless of stand-alone or fully integrated installation, the B+ and B- cables from the batteries to the inverter must be sized for the maximum current the inverter can draw from the battery. The B+ feed must be fused to protect the cables, and should have a disconnect switch rated for continuous use at or exceeding the maximum demand of the inverter. The device itself must be “grounded” to the grounding buss of the host boat. Unfortunately, I too often see stand-alone inverters that do not meet these ABYC electrical standard requirements, which apply to all DC devices.
The ABYC electrical standard, E-11, “AC And DC Electrical Systems On Boats,” July, 2018, treats stand-alone inverters in the same way it treats any other DC device (windlass, winch, thruster, water pump, instruments, auto-pilot). The AC output of a stand-alone inverter is entirely separate and isolated from the boat’s host AC power system. Thus, there are no specific ABYC requirements for the AC output of a stand-alone inverter. These devices are easy to install, relatively inexpensive, and can meet basic AC power needs. Some stand-alone inverters do not comply with North American residential electrical system requirements (grounded-neutral). Stand-alone inverters enable bad user practices, such as extension cords running across the floor of a boat, and wiring that is too small for the loads. A common “operator error” is to forget to turn the stand-alone inverter “off” after use, which can damage or destroy batteries. These “owner errors” are common as fire and personal safety concerns.
The ABYC E-11 electrical standard does apply to this AC system. In a previous article, I discussed the E-11 Standard as it correlates to Sanctuary’s AC system.
There is an important US National Electric Code/Canadian Standards Association “rule” to remember about all end-user AC power systems in North America. For fire and shock safety, AC power sources are grounded at their source. The result is called a “grounded-neutral” system. The neutral conductor itself is a current-carrying conductor that returns current from the load to its source. To automatically disconnect electrical faults, the neutral conductor is held at zero volts by a connection between the neutral conductor and the facility’s ground conductor. The connection is called the “neutral-to-ground bond,” or “System Bonding Jumper.” So in Figure 2, the shore power neutral conductor is “bonded to” the shore power ground conductor before these conductors come onto the boat, in the electrical infrastructure of the marina/boatyard. The neutral of the boat’s onboard generator is “bonded to” the boat’s AC safety ground network at the metal frame of the generator.
The “grounded-neutral” requirement is the reason the “energized” (“hot”) Line conductor AND the “grounded” Neutral conductor must BOTH be switched by the Generator Transfer Switch (GTS). When the GTS is in the “Shore” position, the neutral-to-ground bond comes onto the boat from the shore facility, via the shore power cord. When the GTS is in the “Generator” position, the neutral-to-ground bond is at the generator, as shown in Figure 2. To eliminate a ground fault path, the generator’s neutral-to-ground bond CANNOT also be in the active circuit when shore power is feeding the boat. So, it is switched “out” of the active circuit by the GTS, which switches both the hot and neutral conductors.
Figure 3 shows the case of an inverter that is fully-integrated into the host AC system of the boat. In this case, the inverter is not stand-alone, as in Figure 1, but is installed within the host AC system, between any other AC power source(s) and the boat’s AC distribution panel. Here, it can be operated manually, or it can operate automatically, changing modes as incoming AC power comes and goes. Automatic operation is helpful when commercial power fails, or when a dock neighbor inadvertently turns “off” the pedestal breaker of another boat.
STANDBY mode – passes power that originates upstream of the inverter through to attached downstream loads (“passthru”); in Figure 3, all of the boat’s AC loads are fed via the inverter.
INVERT mode – draws energy from the onboard batteries in order to create AC output at the rated voltage (120V, 240V) and frequency (60Hz) to feed downstream loads.
Figure 4 shows a similar system, but here some loads are powered via the inverter and other loads are powered only by upstream AC sources. On Sanctuary, our onboard utility outlets are powered via our inverter, but our hot water heater, genset battery charger and fridge only receive AC power from upstream sources. That arrangement greatly conserves our available battery capacity.
Note that Figures 3 and 4 refer to the Underwriter’s Laboratory’s UL-458 Standard, which is entitled, “Power Converters/Inverters and Power Converter/Inverter Systems for Land Vehicles and Marine Crafts.” Recall that all AC power sources in North America must be grounded at the source (grounded-neutral), and so shore power is grounded in the facility infrastructure and the generator is grounded at the generator. To accomplish automatic ground switching, inverters intended for use on mobile platforms (ambulances, trucks, airplanes, RVs and boats) MUST comply with cUL/UL-458.
This is a good time to digress for a moment to look at the ABYC portfolio of electrical safety standards. These standards fall broadly into two categories. The first is standards that apply to the design and construction of individual electrical components, such as:
- A-16 Electric Navigation Lights
- A-27 Alternating Current (AC) Generator Sets
- A-28 Galvanic Isolators
- A-31 Battery Chargers And Inverters
- A-32 AC Power Conversion Equipment And Systems
- E-10 Storage Batteries
The second is standards which apply to joining individual component parts together to work within a unified boat system, such as:
- E-11 AC And DC Electrical Systems On Boats
- E-30 Electric Propulsion Systems
- H-22 Electric Bilge Pump Systems
- TE-4 Lightening Protection
- TE-12 Three Phase Electrical Systems On Boats
All of these standards make reference to other Industry Standard sources for detailed specification of performance requirements. Typical outside references are to established by industry standards organizations including IEEE, IEC, ISO, cUL/UL and eTL.
So as applies to inverters, there is an ABYC standard (A-31) that is specific to the design of the unit itself, and a second ABYC standard (E-11) governing the system into which the unit is installed. For inverters, the design reference is UL-458 in the US (and CSA C22.2#107.1 in Canada).
When a UL-458 compliant inverter is in “Invert” mode, a relay inside the inverter automatically creates the inverter’s neutral-to-ground bond. When the inverter is in “Standby” mode, that same relay automatically removes the inverter’s internal neutral-to-ground bond so both AC power and the source’s neutral-to-ground bond are “passed through” the inverter to the boat’s AC power panel. Functionally, this is what a GTS does in the case of a generator; i.e., when the GTS is set to “Shore Power,” the neutral-to-ground bond at the generator is switched out of the system. The GTS transfers both hot and neutral, and transferring the origin of the neutral is what changes the origin location of the Neutral-to-Ground bond.
Figure 5 shows a simplified drawing of a UL-458 inverter in “Standby” mode. AC power passes through (“passthru”) the inverter from an external AC power source, whether that be shore power or generator. The relay shown in the red circle is “energized” (“picked”) by the presence of external AC power, so it connects the incoming power hot and neutral conductors to the output load circuits. The green circle shows the inverter’s ground connection, but since external power is present, the relay is “picked,” so the neutral-to-ground bond that is located at the incoming source is “passed through” the inverter to protect downstream branch circuits.
Figure 6 shows the same inverter operating in INVERT mode. In this case, incoming AC power is absent, so the inverter’s internal relay (red circle) is de-energized (“down”). Because the relay is “down,” AC output from the inverter is created by the inverter’s electronics from energy stored in the boat’s battery bank.
The green circle highlights the inverter’s internal neutral-to-ground bond, which in this mode is connected via the relay. That connection is required because the inverter, in INVERT mode, is the actual “source” of the AC power being delivered to the boat.
Following in Figure 7 is a complete circuit diagram of the AC system aboard Sanctuary. Our 120V, 30A, two inlet AC System is fairly common on boats of our size class, and consists of eight AC branch circuits serving the equipment on the boat. Other than completeness, our system is just like the simplified view portrayed in Figure 4. Boats with 240V, 50A shore power service (3-pole, 4-wire cords) will look slightly different on the front end, but 120V inverter installations will be the same as shown here.
Sanctuary’s generator is in the upper-right corner of the drawing. Note the generator’s neutral-to-ground bond, highlighted there in green.
Our fully-automatic, fully-integrated inverter/charger is in the lower left-center of the drawing, in the small red circle.
On the right middle, in the dotted red circle, is our house, “Shore 1,” AC distribution panel, containing the eight branch circuits. The top four branch circuits are fed only from either shore or generator power, whichever is selected by the GTS. The bottom four branch circuits are fed via “Invert” or “Standby (passthru)” power via the inverter. Our inverter is always part of our outlet distribution circuit, 24x7x365-1/4.
In Sanctuary’s system, at inverter installation-time, the hot buss feeding the branch circuit breakers on the AC power panel had to be divided into two parts (blue ellipses) in order to accept two separate feeds from 1) external power and 2) the inverter. Dividing the hot buss required modification of the OEM electrical panel. Also at installation-time, the neutral buss (red ellipses) had to be divided in order to separate the neutrals of circuits that are not fed via the inverter from the neutrals of circuits that are fed via the inverter.
The need to separate the neutrals stems from the requirements of the 2011 NEC and 2012 ABYC E-11 standard, adopted in coordination to reduce/eliminate dangerous ground fault currents flowing into the water from docks and boats. (See the article on Electric Shock Drowning for more information.) If the neutrals are not separated, an unintended ground fault leakage path can be present. The day the boat arrives at a marina or boatyard where pedestals are fit with ground fault sensing shore power breakers is the day that boat may trip the shore power breaker, and will not be able to get shore power. The dock attendant will tell the unhappy boat owner that “there is an electrical problem on your boat.” The unhappy boat owner will think, “but it’s been working for many years!” Both statements are correct. It had worked for many years, but there is “an electrical problem on the boat!”
A fundamental rule of all electricity is, current will flow on all available paths to get back to it’s source. If the neutrals from one AC circuit on the boat are cross-connected to the neutrals of another AC circuit on the boat, power will divide at the cross-connection (neutral buss) and flow back to the source via all available paths. That situation is, by definition, a ground fault.
Following are two relevant and important excerpts from ABYC E-11, July, 2018:
184.108.40.206 Isolation of Sources – Individual circuits shall not be capable of being energized by more than one source of electrical power at a time. Each shore power inlet, generator, or inverter is considered a separate source of power.
220.127.116.11.1 Transfer of Power – The transfer of power to a circuit from one source to another shall be made by a means that opens all current-carrying conductors, including neutrals, before closing the alternate source circuit, to maintain isolation of power sources.
Ordinarily we think of cross-connected neutrals as a situation that affects boats fit with two 120V shore power inlets; indeed, the neutrals from those two inlet circuits must not be cross-connected on the boat. But more subtly, the separation requirement also applies to distribution circuits fed from generators and inverters. UL-458 is the design standard that specifies that the needed neutral-to-ground bond in an inverter be “established” and “removed” based on operating mode. If the inverter neutrals and non-inverter neutrals are cross-connected (as, for example, all sharing a common neutral buss on the boat), the terms of 18.104.22.168.1 may not be met, resulting in a short ground fault condition. In that case, there can be a duplicate path, if only momentarily, for shore power to use to return to the pedestal. The following events happen in a fraction of a second. Just “milliseconds (mS).”
At the instant (time=0.000) shore power is applied to the boat, any AC current that comes onto the boat via the hot conductor should also return to the pedestal on the shore power neutral conductor, and ONLY the neutral conductor. Period! Full stop! Fundamental rule!
But… At the instant shore power is applied to the boat (t=0.000), the inverter is in “Invert” mode with its internal neutral-to-ground bond still in place. For the time it takes the inverter to respond to shore power and transfer its internal relay from “Invert” mode to “Standby” mode, there are two paths for the newly applied shore power to take to get back to its source at the pedestal. The first path is via the shore power neutral, as intended. But with unseparated neutrals, there is also a second effective (ground fault) return path. The ground fault path starts at the neutral buss, where the returning current divides. Some current will return as intended, on the shore power neutral conductor, but some will divert to the shore power cord’s ground conductor, through the inverter’s as yet unbroken neutral-to-ground connection. That diversion path is a true ground fault. One half of the total current will flow in each path. The pedestal ground fault sensor expects the outgoing and returning currents to balance (within 30mA), but in this case, that sensor will see much less current returning on the neutral conductor than what was delivered on the hot conductor. The pedestal breaker will want to trip. How fast will it take for the trip to happen? Usually between 30mS (t=0.030) and 50mS (t=0.050), but in all cases, less than 100mS (t=0.100), the maximum specified for the pedestal circuit breaker to trip.
In any case, we now have a “race” condition. The race “contestants” are 1) the inverter relay against 2) the ground fault sensor. The intent is for the inverter relay to “win.” My inverter’s spec for transfer time is 18mS (t=0.018). But, if the time it takes for the shore power Ground Fault sensor to trip is less than the time it takes the inverter’s relay to transfer into “Standby” mode, the pedestal breaker will indeed trip. Furthermore, turning the inverter “off” will not eliminate that ground fault condition because the inverter’s internal relay would still be de-energized (“down”), and therefore, even with the inverter set “off,” its internal neutral-to-ground bond would still be present, creating the ground fault path. Regardless, if the neutrals are separated, no cross-connection, so “no problem!” So yes, it really is necessary to separate the branch circuit neutrals of the inverter-fed circuits from the neutrals of circuits that are not fed from the inverter. Elimination of the cross-connection of these neutrals is what eliminates the unintended, unwanted ground fault path.
Although I have not implemented an Inverter Bypass Switch aboard Sanctuary, I have drawn up a circuit diagram for such a switch, for those interested. In Figure 8, the bypass switch is shown in the “Bypass” position.
When in “Bypass,” the switch’s external AC “power in” (red lines) comes from the hot and neutral lines that also feed external AC to the inverter. Note that the “hot” feed for the bypass switch is upstream of the inverter’s power switch on the “Shore 1” AC panel. This arrangement allows for bypassing the inverter while at the same time enabling a service technician to apply AC power to the inverter for diagnostic testing and repair verification.
When planning for the installation of an inverter, two pre-purchase considerations are, 1) what branch circuits will be powered from the inverter, and 2) what does the capacity of the inverter need to be in order to support the load of those circuits? Aboard Sanctuary, we determined that we wanted to have AC power in the galley and at other utility outlets while underway. That allows us to use our coffee maker, microwave, toaster and crockpot (not all at the same time), keep our DVR and AC lighting active, and occasionally charge utility batteries for my power tools. We selected a 2kW inverter/charger to do that, which provides a maximum continuous AC output of 15A, shared by our four utility branch circuits. That has served us well for 12 years.
Following is a “cut ‘n paste” from my “project plan” for the installation of our UL-458 compliant inverter/inverter-charger into Sanctuary’s DC and AC electrical systems, and timeframes based on my personal DIY-install timeline. My need to “reconfigure” the B+ and B- DC busses on Sanctuary was because I consolidated the batteries from two separate banks (“house” and “start”) into a single bank at the same time, and updated the battery monitor from a stand-alone Xantrex monitor to a Magnum BMK. Combining banks greatly simplified battery charging from both the inverter/charger and the engine alternator. Those steps are not specifically necessary for the inverter installation, but I like the consolidated battery bank. Click to see my article describing that change.