Rapid shutdown for solar: What you need to know

If you've just installed a solar panel system or you're looking at a picture of one, you may notice a box with an on/off switch that says "rapid shutdown." But what does rapid shutdown mean, and why is it essential for a solar panel system? We'll give an overview of rapid shutdown requirements, how they vary by state, and list some popular inverter options that meet with rapid shutdown requirements.

Rapid shutdown is an electrical safety requirement set for solar panel systems by the National Electrical Code (NEC). Simply put, it provides a way to quickly de-energize a rooftop solar panel system.

The National Fire Protection Association (NFPA) wrote rapid shutdown requirements into the NEC to keep first responders safe. Hopefully, your home never catches on fire, and you don't even have to worry about rapid shutdown functionality – but if it does, firefighters can use rapid shutdown solutions to de-energize your solar panel system just in case they need to go up on your roof. Surprisingly, simply turning off a solar inverter doesn't always do this on its own: with some inverter setups, wires and circuits can remain energized even if the inverter is turned off, increasing the risk of shock for the firefighters working on top of your roof or in your attic. A disconnect switch that enables rapid shutdown allows firefighters to physically flip a switch to reduce the electrical voltage of your solar panel system to safe levels in less than a minute.

The rapid shutdown requirements most familiar to installers today were first introduced to the NEC in 2014; among other things, this code set the standard to quickly reduce the voltage of any conductors (i.e., the material in wires or cables that electricity flows through) that rest more than 10 feet away outside of the array or are at least 5 feet of length inside of a building. Newer regulation, NEC 2017, takes these standards a step further: the more recent code decreased the shutdown boundary requirements to include any conductors within 1 foot of your solar array or more than 3 feet of length inside your home.

Effectively, the changes in NEC 2017 stipulate that rapid shutdown needs to occur at the individual solar modules rather than at the solar array as a whole.

Importantly, NEC is not federal law – individual states can choose to adopt and abide by the code, as can individual cities or municipalities. This means that the rapid shutdown requirements for your state can vary from your neighboring state (we'll dive into that a bit later).

Some inverter systems inherently comply with module-level rapid shutdown (NEC 2017 and later), while others require additional components to enable rapid shutdown.

Microinverter and power optimizer systems–like Enphase and SolarEdge–are the most popular inverter options for residential solar panel systems in the U.S. Fortunately, both microinverters and power optimizers operate at the panel site and have built-in rapid shutdown capabilities.

If you install a string inverter independently, you may need module-level power electronics (MLPE) to comply with NEC 2017 or NEC 2020. Pairing power optimizers with a string inverter (as with SolarEdge) should do the trick. However, power optimizers aren't your only option for rapid shutdown compliance – as more states began adopting NEC 2017, many string inverter manufacturers or other solar companies built new MLPE technologies with rapid shutdown capabilities. In many cases, these MLPEs created specifically for rapid shutdown can be a more cost-effective option than installing power optimizers or microinverters.

SMA America, one of the most popular string inverter manufacturers, has its own rapid shutdown solution (a SunSpec-certified TS4-R-F) that can be installed on the back of panels. Similarly, Tigo manufactures power optimizer solutions, but they also offer inverter-agnostic MLPEs–fire safety modules–explicitly geared towards rapid shutdown.

Generally speaking, most states have some form of rapid shutdown requirement. However, the requirements will vary depending on which code is in effect.

NEC 2017 and later (i.e., NEC 2020) require rapid shutdown at the panel module level. As of July 2020, these codes are in effect in 37 states. On the other hand, four states do not have a statewide NEC in effect (Illinois, Arizona, Missouri, and Mississippi), though it may exist in some cities and municipalities.

Here's a summary of the National Electric Code adoption by state:

You can find an up-to-date map of the NEC in effect by state on the National Fire Protection Association's website.

The first step towards ensuring your solar panel system meets the necessary safety and electrical codes is to find a qualified installer. On the EnergySage Marketplace, you can receive up to seven custom solar quotes from local installers. These quotes will include information about the proposed equipment, including the number of panels, type of inverter, and more. Want to start with a quick estimate of potential solar costs and savings? Try our Solar Calculator.