The 120% Rule

Homeowners planning to install solar PV systems frequently ask, "What is the 120% rule?" To carry out its purpose of safeguarding life and property from electric risks, the NEC (National Electrical Code) has established various chapters, including the 120 % rule.

The Main Service Panel (MSP) of your home receives electricity from your new solar power system. MSP’s have a predetermined capacity that is measured in amps. It is crucial to make sure the additional current from the newly installed solar doesn't overload the MSP's capacity as a safety precaution.

This crucial regulatory principle known as the "120% rule" ensures that the use of additional solar energy won't pose any dangers.

How to Calculate the Rule of 120%

Solar PV systems should be connected to electrical boxes up to 120 % of the busbar's label rating, according to the NEC's 120 % guideline. The rule permits an extra 20%, or the equivalent of 35 amps, from the solar system if the home's electrical meter rating is 175 amps.

175 *120 = 210 amps

210-175= 35 amps

The MSP's busbar, which may catch fire if overloaded, is located in the middle of the equipment and constructed of metal components that can melt. Additionally, when power exceeds the rated capacity, a circuit breaker trips.

As a solar system injects more power, the MSP’s capacity is strained. The 120% rule is a needed code that addresses this added danger.

Now the system is operating within permissible limits at 210 amps. Would 35 amps be enough for a standard solar PV setup, however? In many cases the answer is no.

By complying with the 120 % rule, would this cap the solar PV capacity at 35 amps? Not necessarily. 

Let's look at how to securely increase the system's ability to absorb the extra solar current.

Navigating the 120% Rule

If you go over the 120% limit, the electric panel absorbs more energy than it can handle, causing a risk of fire to people and property. Keep in mind that you must adhere to the NEC code and observe the system capacity.

The following are a few strong alternatives:

Main Service Panel Reduction

If the predicted solar system power is 60 amps but the allowed limit is only 35 amps after using the 120 percent rule, the primary breaker requires an additional 25 amps of capacity.

60-35= 25 amps.

Your service provider can take off 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 breaker without affecting the current power demand.

Derate Main Breakers Without Downsizing

It's not always feasible to decrease the breaker. For example, when the capacity of the electrical system is excessive. You can in this situation:

Install Your System with Confidence

With DIY Solar Power, not only are you getting top of the line products to install, but you also have the option to partner with our experts who provide solar services for your project. The Solar Services package includes a site survey of your home which consists of a preliminary inspection of the following:

• Roof: including full measurements, condition, available space, vent locations, possible obstructions, skylights, and determination of the orientation of your system
• Electric service panel suitability and needed clearances per code
• Location option for system components including AC Disconnect and J-Box
• Shading analysis report to determine the ideal solar system location

Get ready to install your home’s solar system today. Your fast quote is just a few clicks away. And if you have any questions, reach out to a DIY Solar Power expert at 877-349-1020.

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