Help required - Inverter earth seems undersize?

The "safe" answer is for the "safety grounds" to be the same diameter/AWG as the power leads (for rated current--We can oversize AWG to reduce voltage drop--Which can be a critical issue for lower voltage system--Such as a 12 VDC system).

The reason for the green wire safety ground is two fold (at least). The first is to keep metal boxes/conduit/appliance housing from becoming energized if there is a short circuit (i.e., Hot lead to metal box, cable falls loose in appliance, etc.).

The reality is the grounding conductor is not expected to carry "fault current" for more than a fraction or a few seconds, so it is OK to downsize the grounding conductor...

Handy chart:

https://learnmetrics.com/ground-wire-size-chart-nec-grounding-conductor-size-chart/

• For example, the fusing current for a 6 AWG cable is around 600 Amps... So sending 200 amps of "fault current" is not going to fuse the cable (at least short term in open air):

https://www.powerstream.com/wire-fusing-currents.htm

There is another issue to watch for... In solar power systems with long cable runs and low voltage... The resistance of the "ground conductor" has to be low enough to trip the upstream breaker or fuse... If the cable is too long and working voltage too low, the condutor may not carry enough current to "trip" the breaker/fuse (total resistance of Hot cable, and ground return)...

The other typical reasons for grounding (to "earth ground") are (I can speak for New Zealand--Where you are posting from?):

• In North American 120/240 VAC split phase residential power systems, we have L1, L2, and Neutral. L1-N=120VAC, L2-N=120VAC, L1-L2=240VAC. The neutral (return) is "bonded to" the cold water supply pipe, a 10 foot ground rod, etc... The idea is to keep Neutral near zero volts. People won't get shocked and no need for circuit breakers/fuses.
• Another reason is for Lightning--The ability to direct (nearby and direct) strike energy to ground.
• Other reasons can include grounding for flame detection circuits (gas stove/ovens), grounded fluorescent fixtures (tube lamps can start easier), and cathodic grounding (you don't want o energize underground metal structures/plumbing and increase corrosion).

In North America, the "good resistance" for a ground rod to soil is 25 Ohms or less... So, ground rods really do not carry 120/240 VAC fault current. The ground rod(s) are typically installed "next" to an exterior wall/foundation near the main panel, or in the case of solar, your equipment room. And if you have lightning strike possibilities, then you may want AC and DC surge suppressors (from "hot/neutral" wires to ground/ground rod(s)).

Anyway... Portable solar/power installations work just fine without being connected to a ground rod. However, if this becomes a fixed installation (and especially if you have lightning issues), then a ground rod is a good idea. I usually suggest 6 AWG cable minimum from ground rod to other system ground buses/connections. And if you have multiple locations (such as a "solar power shed" and your home/cabin)--A ground rod at the shed and a ground rod at the home is usually a good idea (lightning will not follow 6 AWG cable for more than 10-20 feet before finding a "better path").

The whole issue of single point vs multiple point ground bonding--Lighting control works best with multipoint grounding. 120/240 VAC is typically done with "single point" neutral+ground bonding. Details matter here--But I will stop now before I get more confusing. Please feel free to ask if you need more information.

I work with AWG here--And you know how to convert to your local mm or mm^2 standards.... Using your local building codes for wire diameter/insulation/temperatures/etc. for max current ratings is a good place to start.

The North American NEC (national electric code) is relatively conservative. Here is a simplified chart:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

You can also use other standards... For example, marine standards are much less conservative:

https://www.bluesea.com/resources/529/Allowable_Amperage_in_Conductors_-_Wire_Sizing_Chart

I like the NEC chart as it is relatively conservative... Using your local code limits is probably quiet safe too (and is based on wire/insulation/etc. that are available for your region).

One suggest for charging battery banks... A battery bank at 50% capacity can take hours to recharge to 100%... For those cases, the NEC has a 0.85 derating factor (1/0.80). If, for example, you have an AC battery charger that takes 15 amps @ xxx VAC, then I would suggest the branch circuit (and breaker) be rated for:

• 15 Amp battery charger current * 1/0.80 NEC derating = 17.6 amps ~ 20 Amp wire+breaker branch circuit

How this all applies to your question. 2,200 Watt inverter @ 12 VDC... Worst case is 10.5 volts (typical inverter battery cutoff voltage):

• 2,200 Watts * 1/0.85 typical AC inverter eff * 1/10.5 volts cutoff = 246 Amps worst case current draw

Using the NEC chart with high temperature insulation, that would be 4/0 cable (107,2 mm^2 metric)--Not a small cable. The reality is that most applications are not going to draw more than 1/2 the inverter's rating--Leaving room for motor starting surge current and such (note: most AC inverters are rated 2x Watt rating for surge power--a few seconds to a few minutes typically).

123 to 246 amps through 6 AWG cable--That is pretty agressive. 6 AWG copper cable has a fusing current of ~600 Amps--Running almost 300 Amps continuous (and 600 Amp typical max surge rating) is not a great start (hot/overheating wiring, possibly insulation failure and short circuit/fire).

The other issue is voltage drop. 246 amps @ 12 volts -- Look for 0.5 volt maximum drop for normal operation. Using a simple voltage drop caculator:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=4&necconduit=pvc&necpf=1.0&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=2&distanceunit=feet&amperes=246&x=44&y=14&ctype=nec

Results for 2 feet of 6 AWG cable @ 246 Amps voltage drop

Voltage drop: 0.48
Voltage drop percentage: 4.02%
Voltage at the end: 11.52

Running a DC inverter at 1,200 to possibly 1,800 Watts takes some heavy cabling and careful wiring designs.

Higher Wattage inverters would suggest 24 VDC (2,400 to 3,600 Watts) or even 48 VDC battery bus.

In any case--What to expect from your present hardware. 660 Watts of solar panels, 40 Amp MPPT charge controller, and 2x 6 volt @ 300 AH (basically) flooded cell lead acid batteries.

From the battery bank, suggest a cabin/home run 25% planned discharge for 2 nights maximum (i.e., stormy weather and no sun). 50% maximum discharge. The typical planned daily 240 VAC Watt*Hours per day:

• 12VDC * 300 AH battery bank * 0.85 AC inverter eff * 1/2 days usage * 0.50 max planned discharge (longer battery life) = 765 WH per day
• 765 WH per day / 5 hours per evening (example) energy usage = 153 Watts "average" load (5 hours per night, for 2 evenings)

And the typical maximum AC inverter suggested for a flooded cell lead acid battery bank is (rough rule of thumb) 250 Watts per 100 AH:

• 250 Watts * 3006 AH battery band * 1/100 AH (this is for a 12 volt bank) = 750 Watt typical max AC inverter for your present bank

So, a 300 to 750 Watt inverter would be "better" for your present battery bank (I know nothing about your loads/power needs--This are just typical suggested starting points for planning and design).

And then there is your solar array... Say you are around Auckland NZ. A simple solar harvest calculator:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Auckland
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 53° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
5.80	5.54	5.10	4.27	3.47	3.14
Jul	Aug	Sep	Oct	Nov	Dec
3.39	3.82	4.63	4.92	5.41	5.57

Say you have a backup genset for the "darker" three months of the year... Make August 3.82 hours of sun per day as your break even month:

• 660 Watt array * 0.52 off grid AC system efficiency * 3.82 hours = 1,311 Watt*Hours per average August day harvest

So somewhere around 765 WH to 1,311 WH per average August day is what to expect (again these are typically 20+ year averages for solar harvest--Your values will vary).

Any corrections to may guesses? Any Questions?

In general, we like to start with your power requirements (and for solar--Your loads and inverter should be the Most Efficient that you can find)--It is almost always cheaper to conserve energy than to generate it.

-Bill

Lead Carbon Batteries--Should be roughly similar Voltage and Current output as Flooded Cell Lead Acid (I think). They are supposed to last longer than FLA. However, I like to start from a "conservative" set of electrical assumptions. Yes, AGM and Li Ion can supply much higher surge/continuous current than flooded cell... But if you can only run for 30-60 minutes before your battery bank is "dead"--That usually does not work well for the average off grid "home" power system usage.

I don't know which brand of Lead Carbon batteries you have... If you have a data sheet you can make some estimates of available power/current. For example, say they can discharge at a 1 hour rate (that is pretty heavy discharge for most battery types):

• 12 volts * 300 AH * 1C discharge rate * 0.85 AC inverter eff = 3,060 Watts

That is probably more than the bank can supply in reality (most batteries, the higher the discharge current, the lower "apparent" capacity they have)... But you might be able to get 30+ minutes from your 12 volt @ 300 AH battery bank (purely a guess on my part).

I found a generic NPS manual:

https://waveinverter.co.nz/download/NomadNPSInverterManual.pdf

But nothing regarding the actual cabling requirements...

Regarding grounding... Again the manual was of no help.

The short answer assuming a typical Sine Wave inverter design (isolated AC output).

1. Run minimum 6 AWG cable from chassis ground of inerter to ground terminal/bus of battery bus
2. You can run parallel power cables... Not great, but is done when you have one "very heavy cable" that you can divide into two smaller cables (1/2 of your single mm^2 cable size) for + and - cables.
3. Manual warns not to connect AC neutral to battery negative terminal unless recommended by installer... Not very helpful.

For New Zealand, do you have "floating" 240 VAC power (L1 and L2), or do you normally have an L1 (hot) and a Neutral that is ground bonded (either in your main breaker panel or by the power company/etc.)?

Generally, for smaller AC inverters (and gensets), their AC output is floating (L1 and L2). And almost all of your AC loads should work just fine with them.

For safety, say you have a kitchen sink and use AC devices (blender, mixer, etc.) near the sink. You can use a (we call it) a GFI outlet at the sink. It helps reduce the chances of electrocution (GFI--Ground fault interrupt--Turns of power if there is current flow to ground).

You might know it as a RCP residual current device (?).

https://www.worksafe.govt.nz/about-us/campaigns/energy-safety-summer/rcd-safety/

This does (sort of) allow us to "ignore" ground bonding the the 240 VAC Neutral question.

Basically all the DC "devices", electrical boxes, etc. ground run to a DC Ground Bus. And the DC Ground bus runs to the battery negative bus (or terminal). The idea is for any short circuit (say + to metal DC box) runs back to the ground bus and back to the battery terminal. This will cause the circuit breaker/fuse to pop for the + circuit lead and stop any fire/other problem.

AC side is similar. All AC boxes, three wire outlets (L1, L2/N, and Ground) are all tied together And eventually the L1 comes from 240 VAC breaker/fuse, Neutral from the Neutral Bus, and Ground from the ground bus.

The inverter may tie L2 (Neutral) to chassis ground. Or may leave it floating. And you would tie N bus (in main panel) to Ground bus bar in the main panel (N+G bonding in one location).

Then you would run the DC ground to the trailer chassis connection. And run the AC ground to the same trailer chassis ground point.

If the trailer is fixed, you would run the 6 AWG minimum cable from the DC negative bus/ground terminal to the ground rod. And you would run the AC ground to the same ground rod (only one ground rod is needed). And you would probably run another 6 AWG cable from the trailer chassis to the same ground rod too.

You want (from an AC and DC perspective) to have solar copper wiring connections for all grounds so that if there is a short circuit/failure (250 Am[s @ 12 VDC or 10 amps @ 240 VAC) there is a solid copper ground cable connection to trip the breaker/fuses.

You don't want (for example) DC ground to one rod, and AC ground to a second rod. The electrical path between them can be upwards of 25 Ohms--If you had a short from 12 VDC to AC ground then the current flow through 25 Ohms would be:

• V=I*R
• I=V/R= 12 volts / 25 Ohms = 0.48  amps... Not nearly enough current to "trip" a 15 amp or 250 Amp protective circuit breaker.

Regarding the Fridge... Need a little bit more information... That could be a 240 VAC "bar fridge" that uses something like 600-800 WH per day... Or it could be a "DC Camper fridge" that uses closer to 250-500 WH per day.

In August, your 660 Watt array something like 1,311 WH a day--The "mini fridge" could easily use most of that harvest every days (refrigerators and computers are, relatively speaking, energy hogs).

I don't know if the "20AWG" ground stud wiring was a typo on your part or not....If 20 AWG, that is way to small of wiring for a safety ground... But you need to look at the chassis ground stud... It only needs to make connection to the sheet metal (and not fail with >250 Amps of current). The 20 AWG wire inside the inverter may just be carrying a small amount of current/signal level voltage and could be perfectly adequate in this case.

For example, from the manual:
I don't know if this 20 AWG wire is used for the detection circuit or not--But just an example of where this small AWG cable could be used (for signal level voltages).

I would not change anything inside the inverter.

Next inverter, probably get a copy of the manual first and see that these are detailed instructions and requirements for wiring. To get NRTL/UL/etc. safety certifications, the manual needs to include this information.

-Bill

Drive one ground rod and connect A ) DC ground to rod; B ) AC ground to same rod; C ) trailer chassis ground point to ground rod. Just one rod... The rod should be driven next to an exterior wall (outside of the wall/foundation). You do not want "lightning" ground runs to be in the "middle of the building". Because of the way lightning (and any "high frequency") power works--The current wants spread outwards from the center. In the case of a home, a lightning strike on an antenna and wiring down through the middle of the home will want to jump outwards to the walls/metal/electrical wiring.

This forcing of current from the middle is even present on the level of wiring and is called the "skin effect":

https://en.wikipedia.org/wiki/Skin_effect

The only time you may have more than one rod... If you have very dry/rocky ground (>25 Ohms of Ground to Rod resistance) and need more than one rod for "good grounding". Or if you have home and an "out building" sharing the same AC/DC solar power. If you have two or more ground rods, then each ground rod should be tied to the next ground rod with a 6 AWG minimum copper cable (to be able to array "short circuit" current.

In general use in the USA, I think 6 AWG is typically minimum connection (in some cases 8 AWG can be used for grounding connections).

And years ago, I read a study of German (European) Churches that had been struck by lightning. Any grounding that was lighter than 6 AWG ran a risk of fusing (burning out like a fuse). With 6 AWG or heavier wiring, there were almost no examples of wire fusing/failure (I looked for the paper years later--But could not find a copy again).

-Bill