If you’re going to use more than one inverter to power your home or business you might know what I’m talking about, and if you’re doing a three phase system with three inverters than you almost certainly do. There’s a lot more to building a multi-inverter system than buying inverters and batteries. They need to be connected in very specific ways to pass inspection and for important features like anti-islanding to work as required. When I first looked at this diagram I had no idea what they meant by combiner boxes or a PV interactive 3-pole disconnect. And if you’re just doing multiple inverters to better suit your house load, don’t get smug about the complexity of the three phase diagram–your requirements are almost identical.
When I sat down with the EG$ 18KPV Manual and traced every conductor from the utility meter to the shop panel, I noticed that while inverters take up maybe a fifth of the drawing. The rest of it — the majority of the copper, the majority of the cost after the inverters and batteries, and probably every issue you’re going to have with your permitting authority or utility— is boxes. Steel boxes full of breakers with no screens, no firmware, and not a lot of explanation about what you need, how to configure it, where to buy it, and how to make sure you get the right stuff. And yes, the cost for some of this will probably shock you. Not cheap. Oh and remember that the drawings above are simplified–no neutral or ground wires, and no information about communications. The dense, complicated drawing in the manual are what you need to follow–this drawing is just to help you understand what you need.
Four of these boxes are particularly important:
- The dedicated inverter grid input combiner panel
- The inverter load output combiner panel
- The 3-pole manual transfer switch
- The PV interactive system 3-pole disconnect
Two of those four are, functionally, just a typical electrical distribution panel and some breakers. The other two are switches. And yet if you get any one of them wrong, you either fail inspection, damage equipment, or — the interesting failure mode — build something that works perfectly right up until the day a lineman gets hurt. This is the post I wish I had three months ago.
A note on where this drawing comes from: EG4 publishes a standalone document called EG4 Inverters in 3-Phase Configuration (v2.0, 2025), separate from the 18kPV manual, and it’s dramatically more readable than the diagrams buried at page 42 of the manual. It’s on their Additional Downloads page, filed where nobody will find it. Drawing #2 is the one for 18kPV and 12kPV units. Everything below refers to that drawing.
First, How do split phase inverters become three-phase?
Before the boxes make sense, you have to fully understand that the 18kPV is not a three-phase inverter. It’s a split-phase, 120/240 V machine. Three of them do not become a three-phase inverter in any deep sense. What they become is three single-phase sources, each one deliberately assigned to a different pair of the wye, synchronized 120 degrees out of phase and cooperating closely enough to look like a three-phase source to everything downstream.
That’s why every inverter connection in the drawing is a 2-pole breaker — the drawing’s note reads “For Inverter Grid and Load Breakers use a 2-pole breaker for each inverter.” (Everything else on the drawing is properly three-pole: the main service breaker, the feeder tap breaker, the transfer switch, the PV disconnect, and the 200 A mains on both combiner panels.) Each inverter’s GRID port lands on two of the three phases. Each inverter’s LOAD port lands on two of the three phases. Inverter #1 gets L1–L2, #2 gets L3–L1, #3 gets L2–L3, wired and configured so each machine delivers 208 V across its two legs and 120 V from either leg to neutral. When the grid is live the phasing is automatic, synced to the grid. When the grid is down the internal configuration keeps the legs 120 degrees out of phase and the anti-islanding system opens the connection to the grid, preventing the system from backfeeding a grid that is supposed to be dead.
It also explains the single most counterintuitive line in EG4’s drawing: the phase settings are not in alphabetical order. Inverter #1 (master) gets phase “U”, inverter #2 gets “W”, and inverter #3 gets “V”. Not U-V-W. U-W-V. EG4 flags this in red because the naming convention in the parallel system menu doesn’t match the U.S. convention for phase rotation, and if you set them logically you get it wrong.
Here’s the part that makes me nervous. Set the phases wrong and the system will appear to work. On grid, the inverters auto-detect and correct the phasing — EG4 is explicit that this happens in on-grid mode only. Everything hums along. Then the power goes out, the inverters drop to island mode with no utility reference to correct against, and if you got it wrong your three-phase output goes out of phase with itself. Now you’re feeding a rotating-field motor a scrambled sequence. EG4’s phrasing is “could damage equipment,” which I suspect is the polite version. My rotary compressor would probably do it’s best to rip out the drive link. Get it right and your equipment won’t even stutter.
The same table specifies the parallel DIP switches — inverter #1 both UP, #2 both DOWN, #3 both UP — and the wire colors, #1 black-red, #2 blue-black, #3 red-blue. The DIP switches are arguably the easier thing to get wrong, because unlike the phase setting there’s no menu to double-check them against. Set them with the covers off and photograph them before you close anything up.
The other quiet killer in that drawing: “Provide a properly sized Neutral conductor direct to each combiner panel. Do not rely on a single N conductor nor daisy chain neutrals.” Three single-phase inverters on a shared wye neutral can put real current on that conductor — the phase currents only cancel to zero when the loads are balanced, and in most shops they never are. My whole load-balancing spreadsheet exists because of this sentence. And I know it’s never going to be perfect. When I fire up my ancient Miller Synchrowave 250 TIG, that houses a transformer that wouldn’t fit in a minivan, the lights in Portland probably flicker. It’s on one 208V leg. When I’m welding anything, or using any heavy single phase machinery, the system isn’t balanced and the neutral conductor is passing current.
And one more from the cover page, which will save somebody a purchase order: the EG4 GridBOSS does not work for three-phase systems. It’s the first thing the document says, in bold, underlined.
Let’s look in those boring boxes
Box #1: The Dedicated Inverter Grid Input Combiner Panel
What it is: a 200 A three-phase load center whose only job is to gather the three inverters’ GRID ports onto one three-phase bus and hand that bus to the utility side of the system.
Where it sits: this matters. The path is main service breaker → feeder tap → feeder tap breaker → PV interactive disconnect → grid input combiner panel → the three inverters’ GRID ports. The utility disconnect is in series in that run, not off to the side. Hold that thought; we’ll come back to it and why it works that way in Box #4.
How it’s built: one 2-pole breaker per inverter connected to the breaker bus in a specific pattern so the three lines (L1,L2,L3) are each fed by two Grid lines from the three inverters. The pattern is inverter #1 — L1–L2, #2 — L3–L1, #3 — L2–L3. If you are using a panel that will have lots of open spaces you can spread them out to make working in the panel easier, but if you do that you have to be very careful to have each breaker connected to the right line. EG4’s table in the drawing shows a 70 A minimum for the 18kPV, with 100 A shown as the maximum practical breaker in a three-phase setup. The drawing itself shows 70 A as the example. You can go larger up to the rated pass-through, but your conductor ampacity has to follow — a bigger breaker protecting undersized wire is just an expensive way to start a fire.
Why it exists at all: you have three inverters and one point of interconnection. Something has to combine them. You could theoretically land the three 2-pole breakers in the main panel, but then you’re managing three separate interconnections, three sets of conductors have to run from the inverters to the main panel, and the busbar calculation will be complex and will vary with loads–it’s likely your inspector won’t like that.
And now the important part, which EG4 prints right on the drawing with an arrow pointing at this panel and no other: Do not add additional loads this Dedicated Sell-Back Panel
The missing “to” is theirs. I’m quoting it exactly because I want you to be able to find it on the sheet. This is the instruction most likely to be ignored, because it looks like a suggestion and the panel has thirty empty spaces sitting there being useless. Maybe the panel is in a cold, dark spot. Can’t I add a breaker to add a heater and one for local lights? NO! It isn’t a suggestion. Three reasons:
One — it’s a power production panel, not a distribution panel. Under the interconnection rules this bus is a source, not a load center. Every calculation anybody has done about this panel — conductor sizing, tap rules, whatever the AHJ signed off on — assumes power flows one direction through it and nothing else is connected. Hang a load off it and you’ve changed the problem into one nobody has calculated and that varies depending on the external loads.
Two — you’d break the metering. The system uses CTs on the service conductors — one CT per inverter, back to the CT1 port on each — to know how much power is flowing which way. That’s how the inverters decide whether to export, hold, or charge. A load hanging off the grid input bus sits in a blind spot: it draws power the CTs attribute to the wrong side of the equation, and your export logic quietly goes stupid.
Three — anything on that panel is not backed up. The grid input side is upstream of the islanding boundary. When utility power fails, the inverters disconnect from it and start supplying the load side. Everything on the grid input panel goes dark. If you’d put your well pump there because it was the closest empty slot, you have a well pump that only works when you have grid power.
Leave it empty. It’s a junction box that happens to look like a load center.
Box #2: The Inverter Load Output Combiner Panel
What it is: the mirror image. Another 200 A three-phase load center, same 2-pole-breaker-per-inverter arrangement, gathering the three LOAD ports into a single backed-up three-phase 208 V bus.
What makes it different: this bus is the island. When the utility drops, this is still powered. Everything downstream of it stays alive on solar and battery; everything upstream of it does not.
The load output combiner is also where the system’s hard ceiling lives. Three 18kPV inverters can pass through a lot of current, but they cannot pass through infinite current, and in an outage the island is limited to what the inverters and battery can actually deliver. On grid, pass-through covers you. Off grid, the inverter limits rule–but you expected that.
One detail from the drawing worth repeating: this panel needs its own properly sized neutral run direct to it — not shared with the grid input panel, not daisy chained from anywhere. Same rule as the grid side, same reason.
Box #3: The 3-Pole Manual Transfer Switch
This exists purely to protect you from your own equipment. You may have experienced automatic transfer switches that work with a generator to quickly and automatically restore power in an outage. This isn’t that. Your PV system can and generally does backfeed the grid when the grid is live. If the grid goes down the anti-islanding features sense the grid is down and disconnect it. See our other post: https://expertamateur.com/how-the-eg4-18kpv-knows-the-grid-is-down-and-why-it-doesnt-need-a-transfer-switch/
What it is: a 200 A three-pole switch with three positions — On, Off, On — feeding the main breaker panel. One “On” takes the main panel from the utility, through the feeder tap breaker. The other “On” takes it from the inverter load output combiner. The “Off” in the middle is the entire point.
Why center-off matters: the middle position makes the switch break-before-make. There is no rotational path, no wiring error, and no moment of clumsiness that can connect the utility and the inverter output to each other. You physically cannot parallel the two sources, because passing through “Off” is the only route between them.
Think about what that prevents. Your inverters are grid-interactive — they want to synchronize with utility and push power back through the meter, and they have anti-islanding logic to stop doing that when the grid dies. That logic is UL 1741 certified and generally excellent. But the transfer switch doesn’t rely on it. The transfer switch is a piece of steel and copper that says: whatever the firmware thinks it’s doing, these two sources are not going to touch.
Why manual instead of automatic: an automatic transfer switch is for a generator, where you want the thing to start when the grid dies and cut over at 3 a.m. while you sleep. It’s generator to load only–it never backfeeds the grid. Your grid-interactive battery inverter already does the seamless part in software — outages should be a flicker, or nothing at all. What you actually want from a physical switch is bypass: the ability to run your loads only on the utility while the inverters are down because one died, or for a firmware update, a battery swap, or a repair. That’s the real justification, and it’s the one to put in front of your electrician if he asks why you’re spending money on a switch you may never touch. Without it, the only way to safely service the inverters is to depower the entire system. With it, servicing the inverters is safe and easy. Over a twenty-year system life that switch pays for itself the first time something breaks.
Why 3-pole and not 4-pole: it switches the three ungrounded conductors and leaves the neutral solidly connected straight through. That’s deliberate. The neutral-to-ground bond happens at exactly one place in the whole system — at the main service, where the drawing says “Bond Neutral to Ground here and only here.” A switched neutral would either break that reference or, worse, create a second bond and start putting current on your equipment grounding conductors. Solid neutral, switched phases. Don’t get creative, follow the drawing in the manual that shows everything.
Box #4: The PV Interactive System 3-Pole Disconnect
A very fancy name for a knife switch. What the drawing tells you: a 200 A three-pole disconnect labeled “PV Interactive System 3-Pole Disconnect,” sitting in series between the feeder tap breaker and the grid input combiner panel, and the drawing says — locate near outdoor service entrance for access by utility and first responders.
That’s all EG4 says about it. Everything else below is what you’ll actually have to satisfy, and it comes from your utility’s interconnection agreement and your AHJ, not from the drawing or instructions in the manual. I’m separating the two because I spent time assuming the drawing was the spec.
Who it’s for: not you.That’s the thing to understand about this one. Every other box in this system serves your building. This box serves a lineman or a first responde. Here’s the scenario it exists for. A wind storm takes down the line. The utility sends a crew. They open the appropriate devices, ground the line, and start working on conductors they have every reason to believe are dead — because they are dead, from the utility’s side. But your building has PV and a battery bank, and it’s sitting there fully capable of energizing your service conductors.
Anti-islanding is supposed to prevent exactly that, and it does. But “supposed to” is not a thing you bet a lineman’s life on. So you give them a disconnect they can find, reach, operate, and lock open with their own padlock — without entering your building, without your permission, and without trusting a single line of your inverter’s firmware.The NEC requires a readily accessible disconnecting means for a power production source. The utility-accessible, utility-lockable, visible-blade specifics are terms of your interconnection agreement with the POCO (your power company). Two different authorities, two different documents, and the utility’s version is usually the stricter one.
What that means practically, and these are the things that get you red-tagged:
- Outside, on the exterior, near the meter. Not in the container with the inverters. Not behind a locked shop door. A crew at 2 a.m. has to find it.
- Lockable in the open position, with a hasp that accepts the utility’s lock.
- Permanently labeled. Something like PHOTOVOLTAIC SYSTEM AC DISCONNECT, in weather-durable text, plus a placard at the service identifying that there’s a power production source on site.
- Visible break, if your utility asks for one. Some POCOs require a disconnect where you can physically see the blades open. Ask before you buy — this is a specific-utility question, not a code question, and the answer determines which product you order.
- Working clearance around it, per the usual rules. An outdoor disconnect wedged behind a propane tank is a disconnect nobody can operate.
What opening it actually does: it kills your backfeed to the utility’s line. That’s the job. It does not shut the inverters down, does not de-energize the load output island, and does not necessarily kill the main breaker panel — which the transfer switch can still be feeding from the load combiner. A disconnect that opens the interconnection is exactly what the lineman needs and exactly what it’s named for; don’t mistake it for a master off switch for the building.
What it is not: it is not your rapid shutdown device, and the two get conflated constantly. Rapid shutdown deals with the DC side, and the requirement is more nuanced than “turn it off” — conductors outside the array boundary have to drop to 30 V or less within 30 seconds, while inside the boundary the limit is 80 V or, alternatively, a listed PV hazard control system that is usually code-compliant wire management. That’s generally a code requirement for roof mounted systems, not ground mounts. The logic behing the RSD requirement is that Firefighters need to put out your roof fire and could be exposed to deadly DC voltage when they do. They can, and probably will, let your ground mount burn and just prevent it from starting a bigger fire. That’s the RSD initiator on the drawing, located near the service entrance, and on supported EG4 batteries it also initiates the ESS disconnect.
So for my build specifically: rapid shutdown under 690.12 applies to PV systems on buildings. Mine is a ground-mounted rack, which may put it outside that requirement entirely — a question for the AHJ, not for me. I just hope they agree with the NEC. Either way, the AC disconnect and the RSD are different code sections, different hazards, different devices. Where both are required, they are not substitutes.
A brief word about the GEN port, since it’s on the drawing
The 18kPV drawing also shows a generator or AC-coupled input combiner panel, fed from three-phase AC-coupled solar or a generator, with a 30–90 A main shown. The per-inverter GEN feeds are drawn as 2-pole breakers like everything else, sized by a rule of thumb I like: take your generator’s breaker size, divide by three, round up to the next available size.
I’m flagging it rather than explaining it, because on my build it’s an open question and I have no intention of installing a generator or AC couple solar. My reading of the 18kPV manual is that generator input isn’t supported at 120/208 V, and the GEN port is also the AC-coupling port, so if that’s right it takes both features off the table for a three-phase system. The three-phase drawing shows the panel anyway. Those two documents may not agree, or I may be misreading one of them.
I’ve got the question in to EG4 and I’ll update this post when I have an answer worth trusting. If you’re planning on AC-coupling an existing array or backing up with a generator on a 208 V three-phase system, don’t design around it until you’ve confirmed it in writing. I’d rather tell you I don’t know than tell you something confident and wrong.
The code, roughly
I’m not an electrician and this is not code advice — Oregon runs the OESC, which is the NEC with amendments, and your AHJ has opinions. But it helps to know which articles you’re arguing about, so here’s the map:
- 705 — Interconnected Electric Power Production Sources. Governs how your system connects to the utility, and 705.20 is the disconnecting means requirement behind Box #4.
- 690 — Solar PV Systems. 690.13 is the PV system disconnect; 690.12 is rapid shutdown on buildings.
- 706 — Energy Storage Systems. Easy to forget, and the article that governs my 43 kWh of battery. Your AHJ will not forget it.
- 702 — Optional Standby Systems. 702.5 requires transfer equipment designed to prevent inadvertent interconnection of sources, which is the entire argument for center-off in Box #3, stated in code language.
- 240.21(B) — Feeder Taps. This is what “feeder tap” in the drawing title refers to.
- 250.24(A)(5) — Load-Side Grounding Connections. The “and only here” half of the bonding note. (250.24(B) is the half that requires the bond at the service in the first place.)
One correction to something I said earlier, because I nearly wrote it wrong: the famous “120% rule” — the busbar math in 705.12 for backfeeding a panel through a breaker — is not what governs this design. That’s rather the point of the feeder tap. EG4 titled the drawing Whole Service Backup with Feeder Tap precisely because tapping the feeder ahead of the main panel sidesteps the busbar problem instead of solving it. If you’ve been reading about the 120% rule and wondering how it applies here, the answer is that it doesn’t (I think). That’s the whole trick, and it deserves its own post. It’ll get one once I know for sure.
The cheat sheet
If you skimmed — as I would have— here’s the whole thing in a nutshell:
- Grid input combiner panel — combines the three GRID ports into one interconnection. 2-pole breaker per inverter, 70 A minimum for an 18kPV. Put nothing else in it. Not backed up, breaks your CT metering, changes your interconnection math.
- Load output combiner panel — combines the three LOAD ports into the backed-up 208 V island. Everything you care about during an outage lives downstream of this box. Design your load layout around which side of it things land on.
- 3-pole manual transfer switch — center-off, break-before-make, 200 A. Selects utility or inverter for the main panel. Its real job is bypass, so you can service the inverters without shutting the building down. Switch the phases, never the neutral.
- PV interactive 3-pole disconnect — outdoors, near the meter, lockable open, labeled. Sits in series between the feeder tap and the grid input panel. Exists so a utility crew can kill your backfeed to their line without trusting your firmware. Not a master off switch for the building, and not the same thing as rapid shutdown.
- Phases are U, W, V — not U, V, W. Wrong settings work fine on grid and go bad the moment the grid dies.
- DIP switches: up, down, up. Inverter #1 both up, #2 both down, #3 both up. Photograph them before you close the covers.
- Dedicated neutral direct to each combiner panel. No daisy chaining. No single shared run.
- GridBOSS doesn’t do three-phase. Page one, bold, underlined. Don’t buy one for this–I’d love to have one, can’t use it.
None of these boxes will ever show up in a thumbnail. Every one of them is the reason the system passes inspection, survives a service call, and doesn’t hurt anybody.
Next up: the feeder tap, and why the 120% rule sends most whole-building backup projects straight into a service upgrade they didn’t need. Maybe