What Does the 120% Rule for Solar Mean?

If you’re wisely considering the choice of switching to solar, you’ve likely heard about the 120% Rule. This regulatory guideline set by the NEC (National Electrical Code) is intended to protect both life and property from electric hazards. Solar power systems deliver electric currents to the MSP, or Main Service Panel. Each panel has a specific capacity, rated in amps, and homeowners must ensure any additional current won’t overload their MSP’s capacity, and that’s where the 120% rule comes into play. Here’s what the 120% rule for solar means.

Computing the 120% Rule

Every solar system relies on an MSP to absorb electric current, and at the center of every MSP is a metal busbar that can melt when overloaded, creating a potential fire hazard. When a solar system injects additional current into the MSP, its capacity becomes overstretched. 

The NEC 120% Rule attempts to prevent home solar systems from being overwhelmed by an electric current. It states that photovoltaic solar systems must be installed in electrical boxes that carry up to 120% of the busbar’s label rating. So, if a home has a 175-amp electrical meter rating, there can be an additional 20%–or 35 more amps—coming from the solar system. For example:

175 amps x 120 = 210 amps

210 amps – 175 amps = 35 amps

The system operates within an acceptable limit of 210 amps. The problem lies in the fact that 35 amps isn’t sufficient for most PV solar installations and battery storage, which require an average of 60-80 amps. Most homeowners need to expand the system’s capacity, so the additional solar current can be absorbed safely.

How to Comply With the 120% Rule

If your setup exceeds the 120% Rule, some viable alternatives include:

  • Downsizing the MSP. In some cases, it’s smart to contact your electrical service provider to remove your main breaker and replace it with one with less capacity. A new MSP with a lower-rated breaker can seamlessly integrate a solar system without compromising the power load.
  • Alternatives to Downsizing. If it isn’t practical to downsize the breaker—like in cases where the electric system capacity is too large—you can either create a line-side connection between the meter and service panel; introduce a solar-ready service panel; or even feed the existing MSP into a sub-panel with a higher busbar rating.

Get the Right Solar Setup for Your Home With the Help of Solar Bear of Tampa

While you need to check all the boxes like capacity, safety, and compliance as you integrate solar energy into your home, the process is easy when you work with the solar experts at Solar Bear of Tampa. We’re here to answer all of your questions and ensure you get the best solar setup for your budget and your needs. It’s never been easier to make the switch to clean, renewable energy that saves you money on your monthly electric bills. Call us today at (727) 471-7442 to get started!