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The 120% Rule: Derating Your Breaker to Make Room For Solar

DIY Solar/
Under the 120% rule, solar + utility power must not exceed 120% of the amperage rating on the main service panel's busbar

Making accommodations to install your home solar panel system involves a few calculations and assessments.

First, you’ll measure your roof or available land to determine how many solar panels you need. But did you know you also have to evaluate your main service panel (MSP)? Doing this determines how large your solar PV system can be based on the 120% rule.

In this post, we’ll explain the 120% rule, how to calculate your threshold, and how to derate your main service panel to accommodate the solar system you want.

What is the 120% Rule?

The 120% rule means that the total amperage of your solar + grid power cannot exceed 120% of your MSP’s rated capacity.

Why Is This Important?

This rule is essential to follow, as it is in National Electric Code (NEC). Every building department in every U.S. state must adhere to this code. While some states have specific code books, their regulations still mirror NEC standards.

When the building department reviews your plans and inspects your system after installation, you can bet that they will check your system against NEC codes, including the 120% rule.

When Is This Important?

The 120% rule is critical for all solar PV systems installed as a load-side interconnection. A load-side connection involves a circuit breaker connecting the solar inverter output to the main service panel.

Suppose you install your system with a line-side tap (also known as a supply-side interconnection) where the solar inverter output is tapped between the main service breaker and the electric meter. In this case, the 120% rule does not apply. This is because the main service breaker is the only overcurrent protection device protecting the busbar where there is a line-side tap.

Calculating Your Threshold

For all load-side interconnections, the 120% rule states that the main service breaker rating plus the sum of 125% of the inverter ampere rating shall not exceed 120% of the main service panel busbar rating.

To calculate how much solar you can install with a load-side interconnection and no alterations to your main service panel, you must first determine the busbar rating and the rating of the overcurrent device protecting the busbar (typically the main service breaker).

A service panel for a typical modern house has a 200 amp busbar with a 200 amp main breaker protecting. We’ll use that as our example in the following calculation.

For the 120% rule calculation, do the math to determine 120% of the busbar rating (which is just 1.2 x the busbar rating). Per our example, that would be 1.2 x 200 = 240 amps. So, all the power sources feeding the busbar shall not exceed 240 amps.

The grid is just one power source regulated by the size of the main breaker (which is 200 amps in our example). So, your other power source can be 40 amps (240 – 200 = 40 amps).

With solar, the rating of what the solar could feed the busbar is always the sum of 125% of the inverter output (or 1.25 x inverter output). If your inverter has an output of 32 amps, you can calculate 1.25 x 32 = 40 amps. That would be the limit for the solar inverter you can install on the 200-amp main service panel without making any adjustments.

Derating Your MSP for Solar

Next, we’ll go over what it means to derate your MSP.

Replacing Your Main Breaker

Derating your main service breaker means replacing the 200 amp main breaker with one rated for a lower amount (typically 150 or 175 amps). You would do this to increase the amount of solar you can install. By derating the main breaker in our example above, the math comes out to 240 – 175 = 65 amps of solar that you can install.

What If I Can’t Derate My Main Breaker?

It’s not always possible to derate your main breaker. If your main breaker is too small, you won’t be able to power all your loads. Downsizing your main breaker to 100 amps, for example, would not be realistic.

One option is to do the line side tap. This would likely solve your problem but is not typically allowed by building departments.

Another option is to upgrade the whole main service panel. You can install a regular 200 amp main service panel (like in the example above). Or, go above and beyond with a “Solar Ready” main service panel with a 225 amp busbar and a 200 amp main breaker that can take up to 70 amps from solar. Here’s how that calculation would look: 225 x 1.2 = 270 amps and 270 – 200 = 70 amps.

Other types of “Solar Ready” service panels provide a place for a solar breaker. It is separate from the busbar that contains loads for the house, so the 120% rule no longer applies here.

Breaker + Solar Sizing Chart

Here is a chart showing what’s allowed in common scenarios.

Breaker and Solar Sizing Chart

Need More Help Going Solar?

Not a math person? No worries — we understand that the 120% rule can be a bit confusing. But don’t let it stop you from installing solar! The experts at GoGreenSolar are ready to assist you with your solar panel kit selection, design and installation — every step of the way.

If you are stuck in the system design process, please contact us for a free PV design consultation. Or, reach out to us today with any questions.

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