my breaker box is over 200 amps can I still use the 120% rule
Goallan
Registered Users Posts: 1 ✭
Hi everyone I'm new to this site hopes everyone is doing great. I have a question my breaker box is for 200 amp but all the breaker inside is over 200 amp. My question is can I still use the 120% rule if my breaker box is over 200 amp. My solar will be 8000 watts with microinverter and groundmounted. Thanks in advance for all the help
Comments

Welcome to the forum Goalian,
You might need to get a copy of the NEC version used by your town/local building inspectors to get the "answer"...
However, you can at least do some functional math and see if it is practical.
First, need to know exactly what solar microinverters you are planning on using and their AC circuit requirements. Say it is 20 Amps: 8,000 Watts / 240 = 33.3 Amps or 2x 20 amp circuits
 8,000 Watts * 0.77 panels+controller derarings * /240 VAC = 26 Am;ps or 2x 20 amp or 20+15 amp circuits
Since you can "over rate" a 200 amp breaker box by 1.20x or 240 Amps (200 amps main + 40 Amps of solar), I don't see you needing to install a smaller main breaker so you can add more solar circuits.
However, if your micro inverters are (for example) 15 amp circuits, then 3x 15 amps = 45 Amps... And you would need to derate the main breaker by 5 amps, at least. The typical next size breaker down is around 175180 Amps... Or 240 Amp total  180 amps = 60 Amps of "room" for solar breakers...
Then the next question is does your house need 200 Amp service, or are your peak loads small enough for 180 Amp service. Many residential installations can easily have more breakers (i.e. 250 Amps of branch circuit breakers) and a 200 Amp box and work perfectly fine (most homes have lots of outlets and lighting circuits which are small current loads). A commercial installation with lots of fixed equipment, pumps, forklift chargers, A/C, and large lighting loads that are near the maximum branch circuit capacity, the total actually current could exceed the maximum main breaker.
You have several methods available to quickly get an idea of what your loads are... For some, like an electric water heater and stove, you can add up all the wattages. For other loads you could use an AC Current Clamp Meter or a whole house energy monitor to see what your kWatt / kVA maximums are... Roughly: 180 Amps * 0.8 PF (average PF for motors, sometimes microwaves, and other equipment) * 0.8 derating of breaker = 115.2 Amp max continuous current
The 0.8 derating for circuit breakers... In North America, most circuit breakers and fuses are rated to "not trip" at 80% or less current. And will trip at 100% or more rated current. By running at 80% or less current (max continuousfor Minutes to Hours at time), you will not have nucent trips of your main breaker.
SImilar for your GT Inverter wiring... A 20 amp circuit * 0.8 derating = 16 amps maximum continuous current... GT Inverters can output max rated current for hours at a time, and if you ran 20 amps from a GT inverter through a 20 amp breaker, it would eventually trip.
Checking with code (or even your local building departmentDifferent locations use different codes or approved NEC 2xxx date). You should find a copy at your local library if you don't want to buy one... And you can see if or what the limits are for total number of breakers (amperage total) in a 200 Amp panel.
Remember that GT Inverters only output power when the sun is up... Your main breaker (if derated) needs to run the home the rest of the time without GT support (night, bad weather, etc.).
Bill "not a code guy" B.Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset 
I believe the 120% rule does not apply to the sum of the all the breakers in the box. It applies to a situation that might allow more then 120% rating on breaker box bus bars. The main breaker prevents too much breaker bus current if there is too much branch load current summation.
The 120% rule violation can be caused if you have a GT inverter pushing into panel bus and grid breaker pushing into the bus. If the sum of the two is more then 120% of bus rating then grid main breaker or PV breaker must be reduced to keep the sum less then 120% of panel bus rating.
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