Off Grid Solar and Grounding questions

From an AC wiring point of view, having the 300 Watt Suresine inverter powering your 120 VAC lines as "floating" is not going to be a fire hazard. You are using 14 AWG or heavier wiring, so there is no chance of having over current (>15 amps) on the neutral/white/return line(s).

If you are worried about shock hazard, you can use GFI outlets where needed (near sinks, outside outside outlets, etc.

With the 9 kWatt AC genset, you really need the Neutral+Ground bonding. This ensures that "Neutral" is never above Zero Volts, so there is no chance of a short between Neutral and Ground elsewhere in the system. "True floating" AC (and DC) outputs should have breakers/fuses on both "Hot and Return" to prevent overheating of wiring in some short circuit cases.

You have several options here... You need to connect a green wire ground from your AC ground bus bar to the genset frame (or through the 120/240 VAC genset cord (assuming N+G bond inside genset). This N+G bonding when genset is running, and N floating with AC inverter running...

Then the transfer switch... Some transfer switches only transfer H+N (or H+H+N) and others will also switch the ground (i.e., floating ground when set to Genset, and N+G bonding when connected to AC inverter).

OR--Lift the N+G in the genset and connect N+G bond inside the main panel. That is the traditional method. The only downsides are a) genset should have N+G connection made when moved genset is moved elsewhere/stand alone operation.And b) not all genset have an obvious/easy access to N+G bond connection.

The details matter here... Is the genset 120 VAC only or 120/240 VAC? Most 9 kWatt gensets (guessing) are 120/240 VAC. Some have switchable AC output (i.e., 120/240 VAC at 120/240 @ 40 amps or 120 VAC only @ 80 amps).

A 9 kWatt genset uses a lot of fuel--And is pretty fuel inefficient below ~50% (to 0%) of rated load (i.e., if using 300 Watt load, 9 kWatt genset will use almost as much fuel gallons/hour as running a 4.5 kWatt load). It would seem like using a much smaller genset (1-2 kWatt) would be more fuel efficient unless you plan on a major upgrade to the cabin AC power (larger AC inverter) in the future.

If 120/240 VAC split phase power--What are you doing at the panel? Everything on 120 VAC circuits (use 1/2 of main panel breaker slots, or wire Red+Black 120/240 panel to 120 single phase all breakers)?

The transfer switch, how many switching contacts (1, 2, 3, 4)?

Depending on the specific transfer switch, you can have the transfer switch doing the switching (N+G bonding for inverter, let genset N+G bonding running on genset).

Don't connect the Green wire from system ground to the negative solar panel lead... That gives you parallel current path back to battery through the negative solar wiring and the green wire wiring--It is a redundant connection, and we "don't want" normal load currents flowing through the safety ground systems. Plus there are a (very?) few solar charge controllers where wiring negative panel wire "around' the charge controller can cause issues (those few solar charge controllers measure current in the negative solar lead).

If you have lightning in the area, it would be better to use good quality surge protection on wiring that enters the cabin (solar panels, genset, AC mains--if ever):

https://www.solar-electric.com/search/?q=surge+protection

Or for the AC power side, install a surge protector in the main breaker panel.

Sorry for the complicated answer (and confusing, I am sure). This is not a simple yes/no question.

-Bill

1. GFI outlets will work fine on "floating AC" power... They measure the "sum of the current' on H+N is less than 0.010 amps. It does not "care about" grounding and such... Only the Sum of the Current between the two power lines is near zero.

You could run into issues with MSW (modified sine/square wave inverters) and other possibly "noisy" AC inverters causing false GFI trips.

Where you can run into GFI issues is the example of a genset with GFI... N+G bonding inside genset connected to your AC panel with N+G bonding... You now have a path for current flow in the Green Wire and the H+N is >0.010 amps (rest is flowing through the green wire in parallel). That is why N+G in only one location is important with GFI protection.

Connecting to the cabin with the twist lock / non-GFI generator output is fine... Use GFI inside the cabin where needed...

I suggest not using GFI to cabin power connection... Use the 30 Amp 120 VAC twist lock (my suggestions). Don't genset GFI if N+G in generator and cabin. And personally, I don't want to "kill" AC power to the whole cabin... A GFI trip (10 milliamps is not much current) that trips the genset GFI cuts AC power to the whole cabin--And kills your lighting too--Don't want that to happen at night.

GFI where needed (sinks, outside outlet, wet areas) only trip limits the outage to where it happens--And you don't have to hunt around the whole cabin to find the the fault location (or do that in the dark).

Note that your green wire/ground rod should also be connected to cold/hot metal water pipes, and even to metal gas/propane lines (for example, s short from 120 VAC Hot to sink finds its way back to green wire safety ground and trips a breaker/fuse/inverter. If sink is not grounded, then could become "120 VAC hot" and somebody could get a shock from sink/plumbing to something out--Such as wet ground, grounded power tool, etc.).

2. The transfer switch "works". If you are only using 120 VAC single phase with N+G bonding in AC main panel (and lift/disconnect bonding for N and G in genset)--Then you only need to switch the one 120 VAC "Hot" (black or red) wire. The neutral does not need to be switched.

That N+G bonding in panel, and lifted/floating genset is the simplest method with transfer switch (one or two wire switching, don't really care).

Floating your 9 kWatt works. And if you have 3 kW or smaller other gensets--They generally "float" their AC output anyway (always check manual and/or with an Ohm meter to be sure if in doubt).

3. 4 circuit with single phase 120 VAC works. N+G bonding would be "to plan".

4. The Suresine is 300 Watts continuous, and 600 Watts for something like 15 minutes (as I recall)... The DC current would be:

• 600 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff voltage = 67 Amps worst case DC current to inverter

You don't say what AWG your DC wiring to inverter is... But remember fusing/breakers are their to protect the wiring (not the device directly). From NEC table (simplified):

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

4 AWG cable is a nice fit for inverter power.

Note that DC cable to inverter should be kept short (lower voltage drop). If cable is longer than 10 feet (one way run), then may need to revisit and look at heavier cable (to keep voltage drop to 0.5 volt max @ 67 amps).

The continuous current derating (this is really worst case worst case):

• 67 amps * 1.25 NEC derating = ~84 amp rated branch circuit (80 or 85 or 90 amp rated fuse/breaker).

And using a circuit breaker vs fuse... Makes it easy kill all power to the inverter before heading home from the weekend/for winter. Instead of fuse+main battery switch.

In the main panel, suggest a jumper wire from Neutral bus bar to Ground bus bar (N+G bonding). Some panels, the Neutral bus bar has a "Ground Bonding Screw" integrated into the bar.

-Bill

That is correct... The PV- bus is separate from safety/equipment ground.
I suggest that the green wire ground from the solar panels be physically separated from all other wiring (DC and AC) from solar panel/mounting grounds to the ground rod (outside of foundation next to array/exterior wall) if there is any reasonable chance of getting a lightning strike in your area.

You don't want to bring lightning energy inside of your home. If you connect the array ground to your AC inside your AC panel--Who knows which path(s) the lightning energy will find to ground rod/earth.

Because you will be connecting all three grounds (array, AC, and DC) grounds to the ground rod (or multiple ground rods tied all together with 6 AWG cable--The 6 AWG stitching all ground rods together ensures a path for AC/DC short circuits--To provide electrical path to trip breakers and fuses in case of any "hot to ground" shorts.
In your case the Combiner Box is mounted on the side of the house closer to your DC/AC power systems than the solar panels (I think). I would suggest not to "Bring" the array grounding wires to the DC combiner box--You are bringing the lightning energy next to your DC circuits after they have been separated at the array.

Generally you will combine all panel ground wires (either a single wire "stitching" panel frames together, or separate grounds to each panel) and bring one wire from the array down the outside of the building directly to the ground rod.

Obviously, separating DC/AC from lightning ground to ground rod by a few feet does not matter to a bolt of lightning that just traveled several miles from sky to earth--But avoiding co-mingling ground and electrical wiring at your home is something I would suggest avoiding.

The above is just based on my personal understanding of lightning. For example, routing lightning cabling should have "soft rounded" corners vs a sharp "right angle" turn because the increased impedance of the "sharp corner" can cause lighting to "leave the wire" and find a different path to ground.

Bringing such energy inside a "box" on the wall of a home with DC (and/or AC) wiring--Is just something I would avoid.

-Bill