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

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

When you connect a solar power system to the utility grid, your solar panels deliver an electrical current to your property’s Main Service Panel (MSP). The MSP connects the breakers that handle electrical current flowing between your property and the utility grid.

Your MSP is rated to handle a certain amount of amperage (current) from all sources. When adding solar power to your property, it’s important to make sure the additional current from solar does not overload the capacity of the MSP.

During the system design process, we check to make sure that the total amperage of solar + grid power does not exceed 120% of your MSP’s rated capacity.

This 120% rule is outlined by the National Electric Code (NEC), and it must be followed for your system to receive permission to operate. Overloading the MSP will cause your system to fail inspection, as it presents a serious fire and safety hazard.

Here’s how to safely size your MSP for solar, and derate (downsize) the Main Breaker if necessary to accommodate a solar power system.

Why is this important?

Properly sizing the main breaker is necessary to adhere to electrical safety standards. Overloading your busbar will cause your system to fail its inspection.

How Your Main Service Panel (MSP) Works

The heart of the MSP is the busbar. All of the breakers attach to it, and the Main Breaker protects it from being overloaded.

There is only a certain amount of metal in the busbar, and if it is overloaded it can melt, potentially presenting a fire hazard. If the breakers draw more amperage than the rating of the Main Breaker, it will trip as a safety precaution.

For example, if the Main Breaker is rated for 200A, then the maximum amount of utility power that will reach the busbar will be 200A. As long as the only source of power is the utility, this is fine. However, solar power will also be running through the busbar (on its way back to the grid), which generates more current running through the MSP.

Solar Power and the 120% Rule

To determine the total amount of power that can potentially reach the busbar, add the solar + utility power together. If your solar array is 40A and your busbar is rated for 200A, the total running through the MSP is 240A.

Under the guidelines of the NEC, this total of solar + utility power must not exceed 120% of the busbar’s ampere rating. Going over this 120% threshold is a surefire way to fail inspection.

In our example:

200A MSP x 120% = 240A

Since the solar + MSP ratings add up to 240A, we are right at the acceptable limit.

Does this mean the amount of solar power can only be 40A? No, but the total still can’t exceed the 240A mark. If we want more solar, we need to “make room” for it by reducing the amperage from utility power.

Derating Your Main Service Panel For Solar

If the solar power is to be above 40A, then the Main Breaker must be reduced (derated) by an equal amount.

For example, if the solar power is to be 60A:

(60A - 40A) = 20A

Here, we would need 20A less utility power to reach the busbar. By lowering the amperage limit from utility power by 20A, we can raise the limitations of solar by 20A to stay under the threshold.

Under this example, the 200A Main Breaker would need to be replaced with a 180A version. However, Main Breaker amp ratings are only available in increments of 25A (200A, 175A, 150A, etc). Therefore, a 175A Main Breaker would be used, since a 180A Main Breaker is not manufactured.

Replacing the Main Breaker

If you need to derate your MSP for solar, contact the utility concerning “pulling the meter.” As you might suspect, this means yanking the meter out of the socket. The Tamper-Evident seal must be broken to achieve this, which requires the utility’s involvement.

Do NOT attempt to replace a Main Breaker on an energized MSP! Electricity needs to be paid the respect it is due. Doing this could be dangerous and potentially fatal. If you are not comfortable working inside electrical equipment, secure the services of a licensed electrician to replace your Main Breaker for you.

Sourcing a Main Breaker Replacement

If the MSP was recently manufactured, chances are good that a Main Breaker is available in the rating required. Start with the make and model of the existing Main Breaker. Use that to perform a search. The site that lists it will likely have the derated version you need.

If the MSP is made by Square D (Schneider Electric), their Tech Support can help cross-reference a part number. They can be reached by phone at 888-778-2733.

Here are links to two vendors that we are familiar with:

If the MSP is old, or the manufacturer is no longer in business, then it may require more research to find the right solution. However, chances are slim that one can be found. There are other solutions, like replacing the entire MSP, though that is more expensive.

What if I Can't Derate My Main Breaker?

In some cases, you might not be able to derate your breaker, either because the system is too large or you need the full current capacity coming from the grid.

In that case you may need to consider alternatives, such as:

  • upgrading to a larger MSP
  • installing a solar-ready service panel
  • connecting directly in between the meter and the service panel (commonly called a line-side connection)

If you are not sure which option makes the most sense for your project, consult with your solar designer for guidance.

Breaker + Solar Sizing Chart

For reference, here's a sizing chart for the most common breaker + solar scenarios. If you need a different setup, be sure to check with your solar designer to ensure your breaker is properly derated for solar.

Common scenarios for breaker + solar sizing

Need Help Going Solar?

If you are stuck with the system design process, please connect with us for a free PV design consultation. Our permitting service provides a custom plan set for your solar kit that is safe and code compliant, complete with a 100% money-back guarantee if your permit doesn't get approved. If you're ready to go solar, reach out to us - we'll set you up with a custom solar kit that is tailored to suit your needs.

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