• Rooftop and Solar Controller Wiring
• Solar Array Wiring Considerations
• MC Connectors
• Battery to Inverter Wiring
• Interfacing to Your Loadcenter
• AC Wire Types
• Grounding
• Neutral Bonding 
• Installing a Sub Panel
• Powering the Entire Loadcenter
• "Splitting" a 50-amphere Loadcenter
• Monitoring and Control
• Recommendations

Some inverters have a wire pigtail internally that you connect your AC lines to (not many have this these days). If the inverter does not have terminal blocks for the AC input/output connections, use  twist-on wire nuts, like in a residential electrical connection. If you have to use twist-on connectors, make sure you tape them to the wires securely to prevent loosening from vibration. You can also use the newer "push in" connectors with solid wire - these actually secure the wire better.

It is important to completely design your system before you start implementing it. If you want to phase in the implementation, that is OK, but you need to design the entire thing first. You also need to consider where you are going to mount components, and their layout. On the left is a picture of the "first generation" electrical center in my 2000 Newmar Kountry Star (done in 2000). It is placed on a piece of 3/4" plywood, which makes it convenient to mount components. The battery bank was moved to the front of the coach from the rear storage tray, and the lines directly wired to the battery bank by Newmar were moved to the distribution hubs. This required some effort to find the wires and to move them to the front of the coach. Getting the design done first exposed all these issues and allowed for proper planning.

Rooftop and Solar Controller Wiring 

For now, I will assume you are connecting your rooftop panels in parallel, and that you are using 12-volt panels (nominal rating). We will discuss higher voltage panels and serial wiring a little later.

Just like batteries, solar panels come in 12 volt (nominal) and "high voltage" versions. The 12 volt panels have a Vmp of around 17-19 volts (no more, typically). The high voltage panels vary in Vmp from around 26 volts to more than 35 volts. Why would you use higher voltage panels? Two reasons: first, larger panels typically come in higher voltage, so if you want 175+ watt panels you have more choice in higher voltage. Second, higher voltage panels mean less voltage drop on the way to the controller. A technically superior design would be to use higher voltage panels and a solar controller that can convert the output voltage to 12-volts. This allows you to use smaller wiring, or have longer wire runs from the roof to the controller. 

With 12-volt panels all wiring is parallel. You simply interconnect all the + and all - lines. You can do this two ways; daisy chain them from panel to panel, with the last panel having the wire that comes down to the solar controller; or with a distribution hub on the roof. These are typically called "combiner boxes". Use of the combiner helps in several ways. First, each panel's wire runs directly to the combiner, so wiring is easier. Second, adding a panel later is easier, since you don't have to modify or lift any of the existing panels. Third, you can use smaller wire to interconnect the panels, and run a larger wire from the roof down to the controller. If you have panels grouped together, you can daisy-chain between panels within a group, and then run a single line to the hub. This makes wiring a little easier.

One caution on daisy chaining the wire runs between panels. This is, in general a bad practice if you have a lot of panels. The reason is that you are using the terminal on the panels to pass all the current from one to the other....the last panels will have current across the terminals that they are not rated for (in some cases). It is better to use a combiner box and "home run" all the wiring directly from the panel to the combiner.

The alternative to building your own is to buy a simple ready-made combiner box from AM Solar. Check the product section for combiner boxes. The new AM Solar combiner box is the 4/2 box. It will take much heavier wire than the previous boxes available from them. The old boxes would only take #6 wire - these boxes will take #2 easily. You can also find combiners from Outback and Midnite Solar that contain DC-rated breakers in them. These are weather-tight enclosures, but are intended for vertical mount. They can be mounted at up to a 45 degree angle on the roof without a problem (with the back facing the front of the RV, so water is never forced into the lid). The advantage of using the combiner with the breakers is that you can easily test each panel, or string of panels. Plus, each panel is protected from a short or catastrophic failure of another panel. I think the $100 or so that you will spend on the combiner  with breakers is well worth it. When adapting these breaker boxes to roof-mount I often position them behind and air conditioner cover to protect them from wind-forced water.

An alternative to putting the combiner on the roof when you are using a high voltage system is to place the combiner in a "close" storage compartment and run the #10 wire from each of the panels (or strings) down to it. This is especially nice with the breaker boxes, since it negates any water entry to them. This works well ONLY IF the voltage is high enough so that there is not more than 2% voltage drop to the combiner. Use the voltage calculators.

For the home-built and AM Solar combiners, I try to bring the wires in from the sides of the box that will face the sides of the RV, or from the rear. That way if the entry holes are not perfectly sealed there is less chance of wind forcing water into the box when driving. I bring the single entrance wire (going to the controller) out the back.  To hold the box to the roof, use adhesive caulk compatible with your roof material. If you have a fiberglass or aluminum roof you can use adhesive silicone caulk. If you have an EPDM rubber roof, or a vinyl roof use the Dicor adhesive caulk designed for that application - do not use silicone on an EPDM roof, because it will not stick. With Outback or Midnite Solar combiners the wire routing is the same as a standard electric subpanel - from the bottom.

Use a waterproof connector on the home made boxes to secure the cables into the sides or back (as described above). These connectors are available at Home Supply stores. In the pictures above, the combiner  to the far left is a Midnite Solar 6-position combiner. It can take six parallel-wired panels, or 6 strings of series-wired panels. There is a water-resistant cover that is not shown. The next two boxes are from AM Solar. The middle one is the small CB combiner box that can take four panels in (or double up on the terminal strips for more). With more than four panels I do not recommend the CB - it is too tight to wire. The third box is the new (larger)  combiner box from AM Solar - it is their best box and I highly recommend it if you are not doing breakers. There is plenty of space for lots of wires, and it will easily handle any size up to 1/0.

To hold single wires on the roof I use "puddles" of the appropriate adhesive caulk - embed the wires in the puddle. I put the puddles about every 3-5 feet along the wire run. Once the caulk sets, add a little to the top. I've been doing this for years, and have never had a wire come loose. For the "bundle" of wires you might have if you combine the individual wires of multiple panels on the way to the distribution hub (tie wrap them together), use a combination of caulk and one tie-wrap with a screw slot to secure them. Cover the single screw with caulk. Use caulk alone to secure the rest of the bundle to the roof.

Some RV's have "solar prep" packages. These typically have 10 gauge wire installed by the manufacturer from the roof to the solar controller location. This is marginal in most circumstances plus it may not terminate in the location that you want it. You will have to decide if you want to use this wire, or run another wire that better maintains voltage. Consult the voltage drop calculator and estimate the length of the wire run. In some cases it is easy to add a second wire - in which case you could run a second 10 gauge wire in parallel to the "solar prep" wire supplied by the manufacturer. Or you could just run the 4 gauge wire (or approriate size) and abandon the manufacturer's wire.  Wire size and connector quality are particularly important when using an MPPT controller. Heat and bad wire connections will cause an MPPT controller to operate far below its rating, negating any advantage to using it instead of a non-MPPT controller. The voltage-drop calculator will tell you what you need to use. Without using the voltage drop calculator you are simply guessing. Wire for a 2% or less voltage drop - you should strive for 1%.  If the input terminals on your solar controller will not accept the wire size you use, simply clip some of the fine wire strands off until it fits. This won't affect anything.

From the solar controller to the battery bank I often am able to use the same #4 AWG welding wire, depending on the length of the run. It is critical to minimize voltage drop from the controller to the battery bank. Make sure that your wire is heavy enough. You MUST use the voltage drop calculators - especially with an MPPT controller where the current coming out of the controller is far greater than what is going into it. Aim for a 1% drop. There is a fuse installed in this line. On small systems (one or two panels), I use automotive "Maxi" fuses instead of the glass fuses usually supplied in solar installation kits or with some controllers. They are easier to install, and easier to insert fuses. It is also easier to "pull" them if you want to service the lines. This should only be done on smaller systems. With larger systems you can use an air conditioner disconnect box with two legs of service on it (shown on the left). One leg  (and fuse) handles the input side of the controller - the wire from the roof. The other leg (and fuse) handles the output side of the controller - the wire to the battery. So the wire goes from the roof into the disconnect, then to the controller, then back through the disconnect and on to the battery.  This allows you to isolate the controller from all power by simply pulling the disconnect handle out. You can find these boxes at any Home Depot or Lowes in both 40 amp and 60 amp ratings. The SquareD boxes are DC rated.  Make sure you use the appropriate fusing and that it is DC-rated; it is likely a larger fuse on the output side.

A neater solution to the requirement to be able to isolate the controller is to use a DC-rated breaker box and fuses. These days I almost always use a Midnite Solar Baby Box for isolation of the solar controller, or a PV6 combiner box. The small extra expense is well worth it in my opinion. Wiring of these devices is covered in detail in the article Solar Controller Disconnect and Combiner , which is a Google document that will open in a separate window. It will provide a great amount of detail on designing and wiring the run from the panels to the battery. It has design alternatives and detailed examples in it.

With  higher output (larger) systems, the #4 cable you used from the roof to the controller may not be enough for the run to the battery bank. Make sure you run your numbers through a voltage drop calculator. On a 60 amp controller you should use #2 or larger to ensure that there is no voltage drop. Even if the current implementation does not require the larger wire, you may want to use it so that you don't have to rewire if you add panels - in other words, wire for the max output of the controller you are using. Use the voltage calculators. You need to ensure that you meet BOTH ampacity standards and voltage drop goals. Ampacity charts do NOT account for voltage drop! Remember that an MPPT controller can boost the amperage quite a bit under ideal conditions. Make sure you understand how to figure this and take it into account. It helps if your solar controller is located close to the battery bank. An MPPT controller can make up for voltage loss from the roof fairly effectively, and properly output the correct voltage for the battery charge stage. But if it is too far from the battery bank you will have voltage drop between the controller and the battery bank, and there is no "machinery" like the controller to compensate for this.

Solar Array Wiring Considerations

In a small solar system that is typical of an RV it is pretty simple to design the array configuration. Most systems on RVs are 1-4 panels, and typically 12-volt panels. For discussion purposes lets say they usually have an output of about 135watts each, with an Isc of about 8.4A. Isc is the "short circuit" current (I) of the panel in ideal test conditions. It is the very max a panel can output, and is used for calculating wire sizes and controller sizing. On RVs, these panels are often wired in parallel - all plusses are joined together on the roof (and the negatives) at a combiner box and a single pair of larger wires is used to bring the power down to the solar controller. When wired in parallel in this fashion the voltage stays the same (Vmp of 17.7), and the amperage is cumulative. In this example, 8.4A*4 panels=33.6 amps. You have to send this amount of current (power) down to the controller. At this low a voltage (12-volt nominal) you have to be careful of the wire size so that the voltage is not reduced too far over the length of the run - especially with an MPPT controller that "likes" high voltage. However, the advantage of the MPPT controller is that the current is lower and thus there is less voltage drop over the same distance than the lower voltage/higher current of a 12-volt nominal panel.

Lets take an example of 20' of wire run to the controller from the combiner box on the roof - which would not be atypical on an RV. Using the voltage drop calculators for 35A and 21' and you will find you need #2 wire. Not good, since that is a pretty big wire size and expensive. But now lets look at reality: your panels are flat on the roof, and not well ventilated. They are not going to output the Isc, or likely even the Imp (current at "max power"), but lets use Imp at 7.63A * 4 = 30.52A. You are now pretty close to the (minimum) #4 cable that I recommend as the minimum. And in reality you will probably never see 30A off the array - you might out of the solar controller after boost though. But that should be a short run from the controller to the battery.

Since we are marginal in the configuration above, lets look at the effects of running these same panels in series with a MPPT controller. Remember, all of the larger MPPT controllers are rated to handle a max of 50-150 volts. In reality, if you follow NEC codes you are constrained to less - say 145 volts.

With the same four panels in series (by the way, these are Kyocera KD135GX-LPU panels in the examples) the voltage combines, but the amperage stays the same. Exactly like with batteries. So in this case: Vmp = 17.7V * 4 = 70.8V, and the current stays at Imp 7.63. So you are sending 7.63A at 70.8 volts to the charge controller. And that fancy MPPT controller can take that high voltage, perform some magic on it, and output it at nominal battery voltage (lets say 14. 8 Volts on bulk charge with flooded cell batteries) and whatever amperage is appropriate - generally the max it can push out in bulk mode on a small array like this. In this case lets say it is outputting the 30A from above and boosted it 10% to 33 amps.  It is easy to see that with a wire run of 20' and a current of only 8 amps (I rounded up) you have no issue at all with #4 cable to the solar controller from the roof. So there is a lot of benefit to the higher voltage. And you can only do that with an MPPT controller.

But there is an additional complication. The MPPT solar controllers are most efficient when the voltage coming in is about 2-2.5 times the nominal battery voltage, more or less. They lose efficiency in down converting really high voltages on input to really low voltages on output. So for the most efficient array configuration we might want to reduce that voltage some and get it closer to 35V. We can do that with two panels in series instead of  four (this is referred to as a "string") which will output Vmp * 2 = 17.7 * 2 = 35.4V. Much better. Now we have two strings of serial panels on the roof, and we will parallel them together at the combiner box: 35V @ 15A. How did I get that? Each string is 7.63A, since the two panels are in series. And when you parallel the strings the voltage stays the same, but the Amps adds: 7.63A*2=15.26A. Consulting the calculator we see we have no issue with the distance or the cable size using #4.

You should only put similar (exact) panels in series.  Voltage specs (Vmp) for all panels in the string (and all strings in the array) should be within 0.2-0.3 volts of each other (note the decimal point). Otherwise you introduce inefficiencies and reduce output.

When wiring panels in series you must match the IMP ratings. The rating of the entire string of panels is the lowest Imp in that string. For series strings the voltage adds and the current is limited to the lowest Imp rating. Example: two 180 watt panels with Imp=10 and Vmp=18. One 90 watt panel with Imp=5 and Vmp=18. If you series these three panels you will have a string with 54 volts @ 5 amps. You can see how that affects the expected output of the string. In the example it would NOT be 180+180+90=450 watts. Instead it would be 54*5=270 watts. Not what most people would expect. 

And finally, series-wired panels that are tilted must all be tilted or you will kill your output. It is, in effect, similar to shading a panel. So if you intend to tilt a series string, make sure you tilt them all or you will be surprised at how much you did NOT improve solar harvest by tilting.

When wiring in parallel the current will add: Imp+Imp+Imp. Your voltage is then limited to the lowest voltage in the array. So, like in the example above, if you put a low voltage panel together with higher voltage panels (say a 16 volt panel with a bunch of 18 volt panels), then what you get is effectively a16 volt array.

All of this applies to strings of panels, as well. Consider a string of panels as a single panel, electrically. So you need to be concerned with matching strings. Mostly, strings are joined to other strings in parallel, so the "parallel requirements" apply. The current of each string will add, and the voltage will be limited to the lowest voltage of the strings. This may not be what you expect. Make sure that your strings are balanced, or, alternatively, on a large system an "odd" string can be run to a separate controller and not paralleled to the other strings.This works fine, as long as the controllers are networked.

You can see that it is not real simple to properly design a higher-powered system. But if you take your time and understand the concepts it is not rocket-science.

MC Connectors

MC connectors are a push-in type connector found on almost all higher-output solar panels today. They are now typically MC4 locking connectors, although there may be some MC3 connectors still on the market. MC connectors are integrated with the panels themselves - they are on the ends of the wire pigtails coming out of the panels. There are no longer junction boxes on panels, except in rare exceptions. The pigtails vary in length, but all are long enough to allow two adjacent panels to be interconnected without extending the pigtails.

MC connectors are a mixed blessing, IMO. They greatly simplify wiring, but they also add cost. Personally, I'd rather have a J-box; that is just me...Regardless, we are stuck with MC connectors. They do make for a fail-safe connection and are far less prone to installer error.

There is a positive and negative MC connector on each panel. You use MC extension cables to extend the wire run back to the combiner box. Buy an extension cable twice the needed length and cut it in half - that gives you a separate male and female MC connector, with a bare wire end to connect at the combiner box. If you are running strings of panels then for the serial connections between adjacent panels you can just plug the panels in directly to each other (as appropriate), and then run the opposite end(s) +/- back to the combiner. The extensions come with 12 gauge and 10 gauge UV resistant wire - make sure you get the 10 gauge. The pigtails from the panels are almost always #10.

If you parallel all the panels (or use two strings) then you might want to reduce the wire runs to the combiner. You can do that with MC "Y" connectors. They come in male and female.

This permits a 2-1 reduction.

With the MC4 latching connectors you need an unlatch tool, or you will struggle getting the connectors apart. This can be bought with the connectors. The MC4 connectors are waterproof, but it is still a good practice to wrap them with tape when finished.

Cables and Battery Connections

I always build my own cables, and I recommend that you do the same. First, it is cheaper and you generally get a better product. Second, it is difficult or impossible to get the wire lengths and orientation of the lugs correct if they are built commercially.

It is not difficult to build your own high-amperage cables, but you do need the correct tools and parts. For tools, you need a cable cutter that is capable of cutting at least 2/0 cable. Klein makes a compact cable cutter that will work - available at electric supply houses and Home Depot/Lowes for about $25. This will cut 4/0 with a little grunting. Believe me, it is worth buying. If you decide not to use a cable cutter you can cut the wire with a reciprocating saw or hacksaw - clamp it in a vice first. If you go this route you will have ragged ends - use a grinder to smooth the edges out. If you don't you will never get them into the lug - the lugs are pretty much the EXACT size of the wire. You can also use a dremel tool with a small cutoff wheel or even high-quality pruning shears work OK.

The best cutters are the ratcheting cable cutters. You can find them on Amazon for around $35. They cut up to 240mm cable - which is larger than 4/0. Search on "ratcheting cable cutter". I'd put in a link but it will go bad....

You also need a large crimper for the magna-lugs you will use. The picture on the left is an example of a hammer crimper that works acceptably well. You put the lug into the anvil and whack it with a maul. The alternative to crimping is to solder the lugs (more on soldering below). If you decide to solder I recommend Fusion lugs. These have solder and flux in the barrel of the lug. You stick the lug in a vice, heat it with a torch and when the solder melts insert the wire. The problem with this is that it is often difficult to get the wire in, and you have to fool with it. Difficult when you have a hot lug and limited time to get the wire in. I prefer to crimp.

Note that these lugs have closed fronts, and are tin-coated for corrosion resistance. They are pure copper underneath. The lugs, crimper and battery extension posts (see below) are available at Solar Seller, The Solar Biz, or at Wrangler Northwest Power. You will find it useful to call Wrangler Power (800-962-2616) and order their catalog. They have high quality parts, regulators, high output alternators, isolators, lugs, 12-volt fuse centers, etc. described in the catalog. Their website is very difficult to use. Solarseller has better prices than Wrangler, if they have the part. If you can't find wire locally you can get it from Welding Supply. They have colored wire for a reasonable price.

You also need an antioxidant, which is used on the wires before crimping. This helps prevent corrosion. Apply to the wires and rub in. Squirt a little into the lug before crimping. You should put antioxidant on all wires - no matter the size - before crimping. One brand name available at Home Depot/Lowes and Ace Hardware is Ox-gard.

After crimping you apply an adhesive heat shrink tube (color coded, of course) over the lug. Once melted, the adhesive totally seals the barrel of the lug and greatly minimizes future corrosion. You will probably have to mail order the adhesive heat shrink tubing .

For battery interconnections and the run to the inverter from the battery bank you can buy regular battery cable, or the highly flexible battery cable. For runs from the rooftop solar combiner to the solar controller the industry norm is to use welding cable - since it is highly flexible and far easier to handle. This is easily obtained at any welding supply house. Buy it uncut and cut it yourself when building the cables. You can also use welding cable for battery interconnects and the run to the inverter.

Hooking up the inverter cables is not difficult but there is only one correct way to do it. The positive feed originates from one side of the battery bank, and the negative feed from the opposite end (battery 1 and battery 4, in a 4-battery system).  Diagonally loading the bank ensures that all batteries are drawn down equally. If you hook both leads to one battery - no matter which one - that battery will be supplying more of the load than the others, and will get more charge than the others. Rub a little Oxguard on the lug before bolting it down. You may have to drill the lug to a larger size, depending on the lug and the battery. You might want to measure the battery terminal bolt size before ordering lugs.

On the positive battery terminal feeding the inverter you need to insert a fuse of an appropriate size - 25% larger amperage than your largest load (or possible load) but also within the ampacity of the cable (this should not be a problem if you use 2/0 or 4/0 cable). Your inverter installation instructions should tell you the appropriate size. You want a Class T DC-rated fuse.  I use ANL DC rated fuses because they are a medium speed (blow) fuse, and are what is typically used in this application. This prevents accidental welding or other catastrophic shorts. If using an ANL-type fuse, mount the fuse in a fuse holder. On vehicles this is often difficult to do and still keep the fuse within a (max) of 18" from the battery. If you have no mounting location then you can use a different type fuse and  mount the fuse directly to the battery post on vehicles (look at the JJN type). On RV's I almost always use a fuse holder, since there is usually plenty of mounting room. Either approach is acceptable. More detail on fuses and connections - including some innovativefuse holders - can be found in Fuses and Breakers for RV Electrical Upgrades, which is a separate document that will open in another window.

Route the cables from the battery bank to the inverter either parallel to each other and touching, or twist them around each other. This minimizes interference from the magnetic field that will emanate from the cables.

Hints on Cable Building

When you go to build the cables, build them one at a time - do not try to cut them all to length and then mass-produce the ends. You need to take into account twists and turns in the line - the lugs do not necessarily orient in the same direction. I have found that if you put one lug on and then take the entire cable bundle and lay the uncut cable out in its final position (or approximate it), then place the uncrimped lug on its bolt and actually lay the cable across the lug you will get the exact length you need. Make sure to place an orientation mark on the uncrimped end so you know the angle of the lug on the wire. Otherwise you may find you have a very twisted wire because the lug is rotated into an inconvenient position. Repeat for each cable. Typically, you do not leave much extra length in inverter-feed cables. Every foot counts against you for voltage drop, so make the cable runs as short as possible. Also, when building battery cables leave some extra length on the interconnect cables. When you replace the battery bank, the next brand of batteries may have the terminals in slightly different places - you do not want to be forced to build new interconnect cables. An inch or two extra is enough.

When you are done connecting the battery wires put a corrosion eliminator on them. The one shown is available at Backwoods Solar (and other places). It dries solid so is not slimy. Use this only on the battery connections - do not place it elsewhere.

Solder or Crimp?

Many people will tell you it is always best to solder, but that is not true. It depends on the circumstances. In dealing with a mobile environment that is full of vibration soldering can be problematic. Soldering a wire makes it stiff and inflexible. It can easily break over time where the solder joint meets the unsoldered wire. Look at marine and aviation wiring - it is rarely/never soldered.

Soldering is also not UL approved. The issue is that if the wires heat up the solder joint will melt and the joint will fail - often causing a fire.

Here is my advice. Take it for what it is worth. I solder all small wires that absolutely need a good connection - brake wires in trailer brake controllers are an obvious choice for soldering. In this case the benefit overweighs the potential downside. I would never solder a wire bigger than #8, and generally I avoid soldering lugs on any wire. For large solar or high-amperage wires (#6 and bigger) do not even think about using a soldered connection. It is far better to PROPERLY crimp the wire.

There are some major considerations in interfacing to the load center (the circuit breaker box) that can drive the entire system design and complexity.

AC Wire Types

RV manufacturers all use regular house wire for the AC feeds in RV's. The exception is the actual shore power cable coming in, which is usually stranded wire. Type NM wire is solid copper wire and is generally what is used for the AC distribution wiring.

A better wire is boat wire, which is stranded and tinned. Stranded wire stands up to vibration better, but is much more expensive. It is required for marine use by Coast Guard regulation. In the past I usually continued to use standard house wire because the coach  is already wired with it, so upgrading just a portion of it is not usually justified. However, I have recently reconsidered this due to the number of burned and loose fittings I have seen in RVs with solid wire.

If you do decide to use solid wire in an RV make sure you tape the wire nuts to the wire (when using wire nuts). A better solution than standard wire nuts for solid copper wire ONLY is the newer "push-in" wire connector shown in the picture. These are available at all the home stores. I use them exclusively for electrical work now. Using tape or the push-in connector will prevent vibration from loosening the connection, which it can, over time.

Grounding

Unless your inverter manufacturer states otherwise, you may directly ground the house battery bank to the chassis. All other DC system ground will be carried back through the chassis ground. There is a DC ground point on the inverter itself ( a safety ground for the case). It must also be grounded to chassis at any convenient point. Make sure you use the proper size wire.

Some inverter manufacturers specify that the battery bank not be directly grounded to the RV chassis. All DC grounding is to originate at the inverter and the DC loadcenter. If your inverter manufacturer specifies this method of grounding, you need to follow it.

Neutral Bonding

Most high-power inverter chargers intended to be hardwired have an AC neutral-to-ground bonding system. This bonds neutral to ground while inverting, and disconnects neutral from ground while on shore power. The purpose is to satisfy code requirements that specify neutral-ground bonding can only occur at one location. The utility power feeding the inverter will have neutral bonded at the electrical panel; therefore the inverter must not have neutral bonded when on shore power.  This is the same reason that RV's NEVER have neutral bonded to ground in the RV electrical panel. Neutral and ground must float in an RV electrical system (be isolated from each other). When doing the AC wiring to the inverter, do not connect the AC input neutral directly to the AC output neutral; use the separate connection lugs provided. Otherwise, you will circumvent the neutral bonding system.

I mention this mainly because installation of some inverters can cause an anomaly when hooked to shore power circuits protected by a GFCI or AFCI. That is, the GFCI may be tripped by the inverter neutral-to-ground bonding relay. This occurs because the GFCI relay that detects a neutral-ground short (potentially a dangerous condition) is "faster" than the inverter neutral bonding relay. When shore power is connected, power passes through the (normally closed) inverter bonding relay before it can be activated (opened), and back to the GFCI. This causes the GFCI to detect the neutral-ground short and disconnect the power. All this happens in milliseconds, and is typical in "driveway boondock" situations where you may be plugged into a friend's garage outlet, or on an outdoor receptacle (including those 20 amp outlets in RV pedestals) - all of which are required to be GFCI protected. There is no way to circumvent this, other than to find a non-GFCI outlet. Look at the garage door opener outlet; code does not require that to be GFCI-protected (but it may be anyway). Note that some inverters have provision to circumvent the neutral bonding system. Do not do this unless you know what you are doing. Also, some newer inverters do not exhibit this behavior.

Installing a Sub Panel

An additional advantage to using a sub panel is that shore power is not being fed through the inverter transfer switch for your high-draw appliances, like the air conditioner.  At least in theory this should prolong the life of the transfer switch, since it is handling less power in normal use. (Remember, all power is passing through the transfer switch for the inverter circuits even when the inverter is not in use.)  In practice, it is unlikely to make a difference, since transfer switches are typically tested at 100,000 cycles at rated power.

Use of a sub panel also allows you to "mix" shore power and inverter power use. Even when hooked to shore power, you can flip off the 30 amp breaker that feeds the sub panel and your inverter will then supply power to the circuits in the sub panel, while shore power will supply the heavier loads like the air conditioner. Why would you want to do this?  Well, to save power when you are being metered. Or to use your converter, which you wisely left on the main panel, to supplement solar when hooked up to marginal shore power - like at a rally or parked in a friends driveway.

So what if one side of your load center does not contain all the circuits you want to power with the inverter?  You will have to re-organize the circuit locations in the box so that the circuits you want to have inverter power are on the leg supplied by the inverter. But in doing so, you have to make sure you maintain some degree of balance between the two legs. In a split box,  just as when you power the entire load center, it is up to the user to manage the electrical loads - don't turn on high-draw loads or you will kill your battery bank.

Monitoring and Control

The components of the monitoring and control system can vary, based on the solar controller and inverter you select, and the functions that are required. In many cases you will need a separate battery monitor, in addition to the control instrumentation for your inverter. At a minimum, you need the following:

• The ability the see cumulative amp-hours into and out of the battery bank, in DC amps, to tenths (e.g. 13.6 amps used). This "running amp-hour" meter function is the heart of your system. It is the most accurate way to determine your battery DOD (depth of discharge), and to monitor your usage habits. The "fuel gauge" type displays present in most of the monitors can supplement this capability, but is not a sufficient measure. If you don't design for this feature to be present, you will be buying it later, at a greater cost. This is also typically available to be viewed in watts. Either work, but you might become more comfortable with watts if you start using that setting.
• An instant amp-hour measure. How much current (or watts) am I putting in, or taking out, of the battery bank right now. This is the measure you will probably display on your monitor as the default. It is what we look at first, and it will allow you to measure the draw of all AC and DC appliances, the output of your panels going into the battery bank, and if any lights are left on at night.  By watching this measurement you will quickly get a feel for your power usage and will be able to identify and diagnose any problems that might be occurring.
• Battery voltage. You won't use this as often as you might think. State of charge of the bank is primarily determined from the number of amp-hours (or watts) you have consumed. Voltage is never a good indicator of state-of-charge in a bank that is under load.  Primarily, you will watch the voltage being applied to the batteries change as the different stages of charging occur. Once you learn and understand this, you probably won't refer to voltage very often.  
• Control functions: you need to be able to turn the inverter on and off, turn the battery charger on and off, and control the genset, if you have one. Usually, the genset is already in place, with its own controls, but if you are adding a generator on the tow vehicle, and want remote start then you need to design for this. Some monitor systems have generator start and management functions.

These are the minimum functions you need, in my opinion. Anything else is optional, but you may feel like you need it. Personally, I like to know everything that is going on, but realistically it is not required. For example, it is nice to have a monitor for the solar controller, but since there is really nothing to control (other than possibly equalization), it is really not necessary. You can see the charge amps and voltage on your battery monitor.  The only measurement that is missing is input amps and voltage coming into the regulator. This is interesting if you want to see how much "boost" you are getting from an MPPT controller, or just to see how much current is lost in wire runs. My solar controller display is behind a door, and I rarely look at it.

You need to place the displays where you will see them. They are no good hidden away, unless you discipline yourself to look at them. Our displays have varied based on the RV we have had. In many rigs they were on a side wall,  where they are easily seen. In another rig they were in the hallway to the bath. In our current coach our controls are all behind a door. You will find you will refer to the battery monitor often, especially when you are learning the operational characteristics of the system.

In my opinion, and it is shared with many industry experts, you absolutely need to measure cumulative amp hours of your battery bank. There is simply no other way to effectively know the current state of the bank. If the instrumentation that is available for the inverter does not provide for this, then you need to augment it. If you are going to augment your inverter monitor/control, then buy the cheapest control panel available for the inverter (making sure that it allows control of all functions). Augment the control functions with monitoring functions provided by a TriMetric TM2030 monitoring system. This provides all the monitoring functions you will need for your entire system, and is commonly available for around $150, including a 500 amp shunt. You can check out the TriMetric at  Bogart Engineering. You can buy the TriMetric at Solar Seller. If you want to monitor more than one power source individually, check out the Pentametric battery monitor from Bogart. It can monitor 3 charging sources at once. But it is not cheap. Don't view the purchase of the TriMetric as spending an "extra" $150. If you can not determine the state of your system accurately you will have continual problems down the road. You might also look at the Victron battery monitor. It has a very nice display and is around the same price as the Trimetric. Both are excellent - I'd probably buy the Victron if choosing today (2017).

I'm often asked if multiple instruments can be wired to a single shunt. In general, the answer is "yes". Simply follow the instructions for wiring the (new) instrument onto the existing shunt.