Homeowners considering installing solar PV systems frequently ask, “What is the 120% rule?” To achieve its goal of protecting people and property against electrical hazards, the NEC (National Electrical Code) has established various chapters, including the 120% rule.
Your home’s main service panel (MSP) receives electricity from your new solar power system. MSPs have a predetermined capacity which is measured in amps. It is crucial to ensure that the additional current from the newly installed solar does not overload the capacity of the MSP as a safety measure.
This crucial regulatory principle known as the “120% rule” ensures that the use of additional solar power will be safe.
How to calculate the 120% rule
Solar PV systems must be connected to electrical enclosures up to 120% of the busbar tag rating, per the NEC’s 120% guideline. The rule allows an additional 20%, or the equivalent of 35 amps, of the solar system if the home’s electric meter is 175 amps.
175 *120 = 210 amps
210-175= 35 amps
The MSP busbar, which can ignite if overloaded, is located in the middle of the equipment and constructed with metal components that can melt. In addition, when the power exceeds the rated capacity, a circuit breaker trips.
As a solar system injects more energy, the capacity of the MSP is strained. The 120% rule is a necessary code that solves this additional danger.
Now the system is operating within the allowable limits at 210 amps. However, would 35 amps be enough for a standard solar PV installation? In many cases, the answer is no.
By following the 120% rule, would this cap solar PV capacity at 35 amps? Not necessarily.
Let’s see how to safely increase the system’s ability to absorb the extra solar current.
Navigate the 120% rule
If you exceed the 120% limit, the electrical panel will absorb more energy than it can handle, posing a fire hazard to people and property. Remember that you must follow the NEC code and respect the capacity of the system.
Here are some strong alternatives:
Reduction of the main service panel
If the expected solar system wattage is 60 amps but the allowable limit is only 35 amps after using the 120% rule, the main circuit breaker requires an additional 25 amps capacity.
60-35= 25 amps.
Your service provider can remove the 175 amp main breaker and install a 150 amp unit in this situation. The additional 60 amps will be efficiently integrated by the new main circuit breaker without affecting the current power demand.
Derate main circuit breakers without downsizing
It is not always possible to decrease the circuit breaker. For example, when the capacity of the electrical system is excessive. You can in this situation:
Increase the capacity of the current MSP
Make a line-side connection between the service panel and the meter.
Provide a solar ready service panel.
Use a sub-panel with higher busbar amperage to connect the current main breaker.
Install your system with confidence
With DIY Solar Power, not only do you get premium products to install, but you also have the opportunity to partner with our experts who provide solar services for your project. solar services The package includes a site survey of your home which consists of a preliminary inspection of the following:
- Roof: including complete measurements, condition, available space, vent locations, possible obstructions, skylights, and determining your system orientation
- Electrical service panel adequacy and required clearances by code
- Location option for system components including AC Disconnect and J-Box
- Shading analysis report to determine the ideal location of the solar system
Get ready to install your home’s solar system today. Your quick quote is right a few clicks away. And if you have any questions, contact a DIY solar energy expert at 877-349-1020.
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