House/Garage

(1) Can I run 2 SMA inverters on the same line from different locations?

Yes

(2) Can I run 2 different sized inverters or does the garage inverter have to be a 7.7?

You can run 2 different size inverters

(3) Do you foresee any problems with this setup at all?

Is it a 200A main panel in house with 200A Buss?
If the 7.7 is backfed by a 40A breaker to a 200A panel with a 200A Buss, you can not backfeed any more.

Typically what we do is refeed the garage panel via a line (supply) tap in house using a 60A Fused disconnect.

We then tie the solar into the subpanel in the garage via a 60A AC disconnect with 20amp fused via tap above the main breaker in the panel If the panel is a 100A panel you can backfeed with a 20A breaker

Andy

No, no - the most back feed you can have on a 200amp service is 20% of buss rating which is 40 amps or 7.7kW, and that is only if the backfed breaker is on the main's buss. If you add another 3.8kW inverter (20amp breaker) in the garage you risk putting as much as 60 amps into your main panel which could theoretically add to the 200amp coming from the utility and could melt the service panel (according to the NEC code). The 60 amp panel can't handle the 3.8kW inverter anyway. Plus, your utility with their disconnect switch no doubt has a requirement for all the solar power going through one disconnecting means.
Welcome to the world of solar - where what you want to do would most probably work, but is not allowed because of all the safety first, second, and third rules.....

Thanks for the response. I neglected to mention I am in Canada. My main service is 100 AMP. I Have installed a 9.2 KW system with the 7.7 sma inverter and have no problems. My system has been passed by Fortis, and hydro inspectors. I think the reason you have so many restrictions in the US is because of GRID Capacity which is unfortunate. Our roof mount systems require Rapid Shut Down which is done through the AC disconnect after the inverter.

I am not canadian, but are you sure you are not subject to Canadian Electric Code 64-112? It looks very similar to NEC solar requirements for inverter interconnect to main panel with 120%/125% load side limitations. For what it's worth, here is a link of a recent paper on Section 64 .https://aetiinc.com/wp-content/uploa...Section-64.pdf

Most of the restrictions in the US come from the National Electrical Code which deals with issues behind the meter and have nothing to do with Grid Capacity. It sounds like there is a similar Canadian Electrical Code.

Section 64-112 refers to the breaker connect to the bus. My 7.7 inverter is rated at maximum output of 7660 watts, and according to SMA 32 amps should be the maximum output. However inverter s are mechanical devices and often are not subject to the laws of man. At 32 amps going by the 80% rule my over current device should be 40 amps, however my inverter was putting out 7860 this summer which means based on the 80% rule the 40 amp breaker is not large enough. So 64-112 64-112 tells me the overcurrent device (breaker) can be 120% or a 48 amp breaker which they don't make, so 64-112 (d) allows you to go to 125% which in term means a 50 amp breaker can be installed.

I hope I haven't made this to confusing.

That who article in the code deals strictly with installation, overcurrent protection, rapid shut down, grounding etc.

My house has a 100 AMP service. Based on the 80% rule I can draw more than 80 Amps without starting to overload the main breaker. Electricity is like water or air, it takes the path of least resistance, so when your electric supplier sends out power, it doesn't automatically come to your house, it waits outside on the street until you turn on a light and then it sends only enough power to run your light. So when you have solar and it's running in the day time anything you use in the house runs first from the solar and if it isn't producing enough then power from your supplier comes in to make up the short fall. Subsequently if you are producing more power than you are using the extra goes and joins your electric supplier and travels down the road to the next house.

The only way I can over load my system is if I had a solar array producing more than 80 AMPs.

Bi directional meters are mechanical devices which measure what's coming and what's going out and that's all they do, but if every house in the US put a 15K solar array on their roof I can guarantee you your suppliers lines would be toast.

Your 20% rule, is this in every state?

The 20% rule is listed various versions of the NEC but as far as I know it is to be followed in every US state.

I would think a state like Montana with a million people would have enough grid capacity for any size array. That's unfortunate.

The NEC really doesn't care who is on the grid. The rules pertain to each individual home panel and what can be self generated back into it.

This has nothing to do with grid capacity. It has to do with not melting the busbar in your breaker box and burning your house down.
Most 200amp breaker boxes have 2 to 3 times that much in breakers loading it. Yes, virtually all the time (in a typical home) they never actually pull more than half of the rated 200amps. But, worst case it could. With a potential of 200amps from the grid, if you back feed 32amps (7.7kW or 40amps ampacity) into your busbar with solar and have more than 168amps of load, then your busbar is in danger of melting.

The theory is that with both 100A available from the PoCo input breaker and 40A
available from solar, turning on enough loads to approach 140A would overheat the
distribution box 100A busbar. One way to make that impossible is to downsize the
input breaker to say 80A, so the total supply current possible is limited to 120A.

Another approach is to put the solar feed breaker at the opposite end of the busbar,
from the input breaker. This causes the currents from the 2 sources to flow in
opposite directions, so they cancel instead of add at any common point on the
busbar. Max current at any point on the busbar is limited to the largest source breaker.

The belt and suspenders approach is to use both of these methods for safety. I
expect your installer used at least one of these, otherwise your are in real risk
of damage or worse. Bruce Roe

The only way electricity comes into your house from the utility is if you turn on a light. Turn off everything in your house take the cover off your panel box so the wires going to the main breaker are exposed and put an amp probe over one of the wires. If everything is turned off in the house the amp draw will be zero, which means there is nothing from the supplier coming into your house. If you turn on a light then you will see the amp gauge move. If you have solar then nothing will be coming in from the utility unless the solar is not enough to supply the load.

I can't explain this any simpler. The only way you can burn your house down or melt your bus is if you are producing more than 80% of your main breaker. If you have a 200 AMP breaker then you would need a 45000 watt solar array.

Nope, it's not the breaker that's in danger, it's the bus bar, if you are pulling too much load from the grid ( maxed out 200A breaker) and then start adding solar from the PV to supply the rest of the heavy loads, the PV amps are added to the grid amps to supply your 250A of load, via your 200A bus bars. Smoke

Most of what you said is true but unless you have a way of automatically reducing your house load you run the risk of sending too many amps into the panel buss. Solar will send what it can and the house will use what it wants. Add that up and the total can exceed your panel rating.

The NEC does not presume you have any automatic load reduction equipment so it states how much solar amps can be sent onto the panel. Those are the rules like it or not.