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What is the solar 120% rule?

The NEC, 120% rule states that solar PV systems should be installed in electrical boxes up to 120% of the busbar's label rating. For example, if the home's electrical meter rating is 175 amps, the rule allows an additional 20%, an equivalent of 35 amps from the solar system.

solaroptimum.com - What Is the 120 Percent Rule and Will It Affect My Solar Panel Installation?

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How To Compute the 120% Rule

At the center of the MSP is a busbar made of metal components that can melt on overloading, a potential fire hazard. In addition, a circuit breaker trips when power exceeds the rated capacity. However, the MSP capacity becomes overstretched when a solar system injects additional current. Therefore, the 120% rule is a welcome code covering the additional risk.

Here’s how the 120% rule comes into play.

The NEC, 120% rule states that solar PV systems should be installed in electrical boxes up to 120% of the busbar’s label rating. For example, if the home’s electrical meter rating is 175 amps, the rule allows an additional 20%, an equivalent of 35 amps from the solar system.

175 *120 = 210 amps

210-175= 35 amps

At 210 amps, the system is within the acceptable limit. However, is 35 amps sufficient for the typical solar PV installations and battery storage? Keep in mind that a standard PV installation and battery require an average of 60-80 amps.

So, will it limit the solar PV capacity to 35 amps to remain compliant with the 120% rule?

Let’s explore how to expand the system’s capacity to absorb the additional solar current safely.

How To Navigate the 120% Rule

If you exceed the 120% rule, the electric panel receives more energy beyond its capacity, exposing life and property to fire risks. Remember, you need to observe the system capacity and conform to the NEC code.

Some of the viable alternatives include:

Downsizing the Main Service Panel For Solar

If the expected solar power is 60 amps, vis-a-vis an allowable limit of 35 amps, the main breaker needs an additional capacity of 25 amps.

60-35= 25 amps.

In such a scenario, your service provider can remove the 175 amp main breaker and replace it with a 150 amp unit. The new main breaker will seamlessly integrate the extra 60 amp without compromising the existing power load.

Alternatives of Derating Main Breaker

In some cases, it’s not practical to downsize the breaker. For instance, when the electric system capacity is too large.

In this case, you can:

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