All you need to know about the 120% rule is that it is grid-tied.
The grid tie solar system is a net-metering system.
The altE Store started selling solar systems in 2000.A typical solar system was 5000 watt.As the price of solar panels goes down, the size of the solar array goes up.A larger inverter is required for a larger solar array.Commercial solar systems used to be the most common in residential.The National Electric Code restricts the size of solar inverters once they reach 7680W.
The restriction is known as the 120% rule.We will cover ways to deal with the rule.
A new dedicated breaker is the most common way to connect a residential solar system to the grid.
The rating of your main breaker box can be determined by reading the label on the box.It will give you a rating.I have a 125A busbar in my main breaker box.Look at the photo of the Main Breaker Box Components and the big main breaker in the box.I have a main breaker.You can see in the photo that I have a 30A 2-pole breaker at the other end.
If there are two power sources, the sum of their breakers can't be more than 120% of the busbar rating.If you have a 200A main breaker in a typical box, you can't add a bigger one because it's the 200 A mainbreaker plus.To prevent the current from being concentrated at one end of the busbar, the breaker must be at the opposite end.The sum can only be equal to 100% of the busbar if both breakers are near each other.
The 125A breaker box can have a total of 150A.There are 125A x 120% source breakers.I am code compliant with 130A total (100A main breaker + 30A 2-pole inverter input breaker) with a 100a mainbreaker and a 30a inverter breaker.A 5700W inverter can be handled by that 30A breaker.
Continuous load is when the grid-tie inverter is running for more than 3 hours.NEC requires the breakers to be sized at 25% of the rated output.There is a 7680W inverter that can output up to 32A.That requires a 40A breaker.The next available size of 60A is 10,000W 240V.A 200A main breaker and a 60A breaker adds up to 260A, which is 130% of the rated busbar, and not allowed.
If you have a 100A box, you can only add a 20A breaker for a 3800W inverter.
The breakers that are used by your loads are not included in this restriction because they are only used for a power source.
If you want to install a solar grid-tied inverter in a home with a 200A rated main breaker box, what options do you have?There are pros and cons to each of the three solutions.All of the options require the electric company to turn off power to your house while the work is being done, so make sure you coordinate with them.
The goal for a 200A breaker box is to keep the sum of the main breaker and the inverter breaker at or under 240A.You could swap out your main 200A breaker for a 175A one if you need to add a 60Abreaker.That gives you 175A + 60A, which is less than 240A.
It is not ideal and may be the least expensive option.You will now restrict the amount of current allowed to and from the grid.If you have high energy loads all come on at once, you may have a nuisance trip on your main breaker, where it used to be able to handle the loads just fine with the bigger breaker.
You could replace the whole breaker box with a 400A box if you have high loads and can't reduce the main breaker's amperage.This will allow you to add up to 80A for the inverters, which will give you over 15 kilowatts of solar power.
This is the most expensive option, as you need to buy a new breaker box for thousands of dollars, coordinate with the electric company to temporarily turn off your service, and most likely hire an electrician to make the swap.If the existing wires are long enough to move the circuits from the old box to the new box, it will be a very labor intensive project.
A Line Side tap is a common solution.There is a supply side tap.The work is usually done by a licensed electrician.A fused AC Disconnect is used to connect the inverter to the grid between the utility meter and the main breaker box.Most restrictions imposed by the breaker box are removed if the main breaker is large enough to receive the power from the inverter.
Solar power can either be sent back into the main breaker box or sold to the grid.Line side taps do not interfere with net metering or FiT, although an additional meter may be required as a dedicated production meter.If you sell or use the solar power, this production meter will measure it.The net meter only spins forward when you make Wh with the sun.
Some companies don't like their clients doing Line-side Taps, so the electric company would have to agree to this configuration.
If the main breaker box is fed into a sub-panel, an additional twist can be presented.What should be included in the calculations?The sub-panel is fed by the main breaker.The sub-panel has the main breaker.There used to be confusion about this, but it was cleared up in NEC.The sub-panel has the breaker used in the calculations.If you picture a 200A main breaker box feeding a 125A sub-panel through a 100A breaker, you can see how that works.Under the 150A restriction, that sub-panel can have a 40A breaker added to it.It is possible to carry that 40A upstream to the 200A main breaker box.
Since 2007, Amy Beaudet has worked at the altE Store.She shares her passion for solar around the world as a sales rep, instructor, and solar evangelist.She talks about solar on the boat or the slopes when she isn't at work.