I'm developing plans for a rural build. I intend on generating my own 200A service via solar power but would like to maintain the ability to hook up to the grid if necessary. This means I'll be soaking up the ridiculous cost of a 400A meter/main to over come the 125% rule.

The meter/main I intend to use is a Siemens MC3042S1400FCL. It seems this "400A" box actually splits to two 200A lines on the load side (possibly incorrect term).

If I'm reading the schematics correctly. The far left allows for 200A PV connection on the service side. On the right there's two Main service disconnects. One that feeds the included 30 slot panel and it seems like the other is not in use. This means I could install a 200A breaker main to feed a distant subpanel, correct?

Additionally, is there anything in code that prevents me from receiving 200A service with a 400A box? I'll need temp power for construction and considering erecting the outbuilding to house electric components first to save hassle of rerunning wires later. Once I get the solar panels up, I can simply cancel utilities but the cabling will already be there if I need it in the future.

Siemens MC3042S1400FCL "Solar Ready" Meter/Main

  • 2
    "Simply cancel utilities" is something you need to look at very carefully. You might well be better off renting a diesel generator for your construction power if your intent is to be off-grid. In some locations you will pay for the availability of grid service to your house forever. In almost all locations you will pay for the line extension to get to your location; if you cancel service and had it rolled into your monthly bill, you will still be liable for the complete cost of it. Diesel generators are easily rented, or you might want to buy one for low-sun backup.
    – Ecnerwal
    Commented Apr 19 at 11:18
  • You can, of course, purchase bio-diesel to run the generator if non-fossil-fuel is important to you (common among those looking to be 100% off-grid.) The price of a quality low-speed (1200 or 1800 RPM in 60 Hz land - 1000 or 1500 in 50 Hz land) generator will likely be less than the cost of a grid hookup, even if your location permits "cancelling" service in a way that doesn't leave you on the hook to pay for it being available to you. You'll have construction power and "month of gray cloudy days" power in one go.
    – Ecnerwal
    Commented Apr 19 at 11:31
  • 1
    Who is your electric utility? Commented Apr 19 at 11:46
  • 2
    All due respect, this design is very wrong-headed to the point where all I can say is "Solar… Does Not Work That Way". Do not even think about buying that panel unless you need a doorstop. And you're going to need more education than may be possible in a Q&A format. Commented Apr 19 at 21:11

2 Answers 2


Ok so your house's service load calculation is <= 200A. You want to install up to 200A of solar, and have that be usable in an off-grid mode very important part there that "flips the table" on the hardware choices. Most electrical hardware marketed as solar ready is talking about grid-tied solar ONLY.

Wait, what is grid-tied solar? It's the cheapest option. You have solar panels on the roof. They sync up to the utility grid and generate back to the utility grid only. They automatically shut down on grid loss because you don't want to backfeed a damaged grid, it'll cause arcing fires and electrocute people. Since the only goal is to backfeed the grid, this can wire in on the service side of the main breaker - or not! And that's what those solar-ready panels are all about - they actually prevent the use of the solar off-grid.

How I would do it

So, here's how I would set up what you want. It would be a sequence of boxes.

First, a plain jane 200A meter acceptable to the utility. Once we're past this point, the utility gets no say.

Second, an isolation switch/disconnect - slightly more than a disconnect because it signals the off-grid equipment that we are off the grid and it can create a microgrid locally. This is an essential piece of safety equipment.

Third, a $200 Siemens "trailer panel" that has a main breaker, and 8 breaker spaces, and 200A "thru lugs" to carry onward to another panel. All the solar goes into those 8 breaker spaces. A few other things can go there too - surge suppressor, simple sliding-plate generator interlock, or the off-grid hardware.

Fourth, a perfectly ordinary, common 40-space panel with another main breaker (wait, what??). 40-space because houses need that. And the off-grid equipment can tie in here.

And then also any NEC 230.85 disconnects not already covered by the above hardware (varies depending on the hardware's placement.)

The "off-grid equipment" includes a common "grid-forming" inverter and battery system, in the usual modern style a-la Tesla Powerwall.

The solar is any random commodity grid-tied solar. A modern grid-forming inverter will trick the solar into coming up and generating, and the inverter will catch that output into its battery.


Why not 400A? Because you don't have any aspirations to use more than 200A.

What about the 125% rule? I think you mean the "solar breaker must be 125% of solar capacity" rule plus the 120% rule, which says that utility breaker + solar breaker cannot exceed 120% of panel bus rating if fed from opposite ends of the panel with loads between them". However, that only applies when the sum of load breakers exceeds the panel rating. Look carefully at what's happening in the 8-space trailer panel and that second main breaker in the 40-space. All the solar sources are sandwiched between two 200A breakers. The "extra" main breaker in the 40-space panel assures that the sum of utility + solar cannot exceed 200A. There are no loads in the trailer panel. So the 120% rule is not relevant, applicable or necessary.

You can shove 200A from solar to grid, solar to house or grid to house, in any combination and proportion, and breakers guard it all.

Why don't I want an "all-in-one" for a neater installation? Because of that vital isolation switch (disconnect with extra contacts). It MUST tell the off-grid inverter "hey, the utility disconnect is open, you may form a local grid". Now it's possible to get them in the form of a meter collar, but not over 200A and in any case the utility can veto the use of a meter collar. And they probably will once they realize what solar/battery is going to do to their business model.

I suppose you could come off the utility plain meter, and then go to a "trailer panel all-in-one" which includes a second meter pan and put the meter collar isolation switch in that second meter pan. But really, being obsessed with minimum box count, and wanting off-grid power, are mutually exclusive goals.

  • Note that the architecture you've described only works for AC battery systems such as Powerwalls. For a system using a DC-coupled battery and multimode inverter (Outback Radian, Victron MultiPlus), the multimode inverter needs to be at the point of coupling, which means you wind up with the transfer/isolation point further downstream Commented May 20 at 1:21

You said “ This means I'll be soaking up the ridiculous cost of a 400A meter/main to over come the 125% rule.”

This rule is only for back-feeding an existing panel through a breaker and I think it is 20% more than the rating of the panel not 125%.

The reason for the 20% PV rule is to keep you from overloading the bus bars in the main panel that are rated at 200 amps. You are getting a possible 200 amps from the utility and backfeeding a 40 amp breaker maximum. This would expose the main panel to a possible 240 amps on the bus bars. The NEC allows the 20% back feed but no more. After that you have to feed in ahead of the main panel.

If you have a 200 amp meter socket and a 200 amp main panel then you can feed the PV power ahead of the main panel after the meter socket through a 200 amp exterior disconnect. This limits the main panel’s exposure to 200 amps since you are not back-feeding a breaker in the main panel.

That is a HUGE amount of PV power but if that is your plan so be it. It will be very expensive. Additionally, the utility may not let you install that much and grant you a net-metering agreement.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.