Help connecting inverter to sub panel with ATS

Big Cali,

The answers really depend on the details of your needs/installation. For example, there are AC Off Grid Inverters that have their own internal transfer switch--And you can take a standard AC inverter (or even genset) and connect to your sub-panel for backup power.

First... What is grounding. Grounding is tying all the metal boxes (AC panels, electrical J-boxes, water plumbing, gas plumbing, sinks, etc.) together with the "Green Wire" ground. This is to both ensure if there is a short to ""something" metal, that that piece of metal has a connection back to the "power source" (main panel, genset, utility power, AC inverter, etc.) and shunt that current to the power return (DC Negative ground, Ground bonded neutral in the USA) and pop a fuse/circuit breaker. The grounding is also there for shunting lightning energy to "ground" too.

Next.. The above says that the green wire needs to shunt the energy to the "return" of the energy source. With DC wiring, that is (typically) tying the Negative battery terminal or Ground Bus to the "chassis ground" of a car/RV/trailer. You put fuses/breakers in the positive wires leading from the positive bus (sized based on the AWG of the wiring--Fuses/breakers are there to protect the wiring, not the "devices/loads").

For North American Power systems... We have 120/240 VAC 60 Hz split phase power... What is that? Think of a transformer with L1 and L2 leads... And a "center tap" (Neutral) in the middle of the transformer. You get 240 VAC from L1 to L2, and 120 VAC from L1 to neutral or from L2 to neutral). Again in N.A. power, we tie (bond) the AC Neutral to the Green Wire grounding (typically in the main panel for homes, sometimes in the AC inverter or the Genset--Typically, less than ~3,000 Watts have their AC neutral "floating", and larger units tend to have their AC neutral bonded to chassis/frame ground in the inverter/generator). When you Ground Bond the Neutral, you only need to put your breakers in the L1/L2/+ DC terminals. No fuse/breaker in the neutral wiring (and not needed as the neutral always is at "zero volts" because of the neutral+ground bonding.

The above takes care of short circuits for the AC and DC power... Any short finds its way through the ground wire/conduit back to the "return" Neutral/DC Negative terminals--Popping the breaker/fuse.

Ground Rods--They are not there to provide 12 VDC/120 VAC grounding. They are typically pretty high resistance (NEC is typically 25 Ohms to earth (dirt) maximum). At 120 VAC that is only (120v/25o= ) 4.8 amps. Not enough to even trip a 15 amp 120 VAC breaker on a normal home circuit.

Ground Rods are there to provide a path for lightning strikes (xx,000,000 volts--25 Ohms is not going to slow anything down). Generally, if you are in a home--If you need multiple ground rods (lightning needs short/relatively straight paths from strike point such as a solar panel frame to the ground rod, right at the base/outside wall of the structure). If your panels are 40 feet away, then you would want a second ground rod for lightning path to "earth". However, to provide "electrical safety" (xx VDC, 120/240 VAC), you want to tie all the ground rods together with (typically) a minimum of 6 AWG solid wire--So that a short circuit at the "remote" thing (such as at the solar array/frame and a shorted security light) can find its way back to the main panel AC Neutral/Green wire ground bond (you don't want a solar framework to become hot, and somebody walking through wet grass get electrocuted if they grab the frame).

So--The above talks about the "green wire" grounding and the "single point" AC neutral/ground bond.

Next, the AC neutral (and DC Grounding)--The "return" wires are there to conduct the current (return flow) from the "hot" leads (L1, L2, DC +).  Could you have multiple Neutral+Ground bonds (such as a N+G bond in main panel, and in the Genset electrical box)--The first answer is sort of "yes" you could. However it is to be avoided form multiple reasons. The NEC (and general electrical design) is taking multiple reasons into account. The first--No load current flow through the green wire ground/conduit. It is a "backup" electrical path for short circuit current. If you have multiple current paths, you could "over current" a segment, or even overheat an electrial box (if you put AC current "out" through one knockout, and the "return"/ground current through a second knockout--That is like a transformer and causes "circulating current" in the sheetmetal of the box--and can overheat). 

Another big reason is because we are using GFI (Ground Fault Interrupter) outlets and circuit breakers. GFIs can and does save lives where a person grabs an electrical appliances (drill, blender), and puts the other hand in the sink water, or steps in a puddle. The GFI measures the current between Lx and Neutral--If the current Lx=N, then all is OK. If there is more than 5-10+ mAmps of "difference"--It is assumed that there is an alternate flow of current and trips the GFI (through a person, an extension cord falls in the pool, etc.).

If you have (for example) a generator with GFI outlet, and plug that into the home (through a transfer switch) and connect to your main panel. A larger genset has N+G (frame) ground in the generator, carries L1+L2+N+G to the main panel, which also has N+G bond in the panel... This allows current flow through both N and G wiring... And if you have a GFI in the genset, the genset GFI will trip open.

There also have been reports where N+G in genset plus N+G in home causing generator voltage regulation to fail... Don't know if that was the cause--But recommend not doing this.

Now to address N and G with an ATS/TS... Recreational vehicles are a very common source of confusion... You have Shore Power (N+G back at the RV park AC power). You have a backup Genset (N+G Bond?), and an AC inverter (N+G Bond?) and the AC panel in the RV (typically no N+G bond)--Some transfer switches will switch L1+L2+N leads, not G. This is done so that, for example, "lift" RV N+G bond when running from shore power. And connecting N+G when running from Genset or AC inverter.

Back to the hardware--What ATS do you have an can it "switch" the AC Neutral? Typically, the switch would transfer from AC Main panel (L1+N and carry ground through) to AC inverter (L1+N plus ground). In this case, we either assume (verify) that N+G bond inside inverter, or your run a separate N+G bond on the "inverter side" of the electrical system.

You might also wish to decide if you want a manual or automatic transfer switch for a backup AC genest too for that subpanel (use a cord for physical transfer, or a second transfer switch (as an example--And you need to check N+G bonding for both inverter and genset to make sure they are compatible).

Your thoughts?

-Bill