EC&M Asks — How to Apply the 120% Rule for Interconnecting PV Systems

May 23, 2024

Episode 36 features Ryan Mayfield who answers reader-submitted questions about PV installations and Code compliance.

In Episode 36 of “EC&M Asks,” a video series featuring subject matter experts (SMEs) answering reader-submitted questions regarding popular electrical topics, Ryan Mayfield, founder and SME of Mayfield Renewables, discusses how to apply the 120% rule in the PV industry. He explains how to make a solar interconnection to a subpanel by applying this rule — and if the 120% rule applies to the subpanel only or if installers need to look upstream to the main as well.

“EC&M Asks” is a video series that enlists the technical expertise of our brand’s subject matter experts to answer our readers’ most pressing electrical questions. Posted twice per month, these quick videos offer Q&As on topics related to various installations, applications, and troubleshooting scenarios. When our readers ask, EC&M answers.

About the Author

Ryan Mayfield

An engineer by training and a licensed Limited Renewable Energy Technician, Ryan has been widely known for his commitment to understanding and demystifying codes and standards for the PV and energy storage industry for decades. Starting in 1999, his career has blended technical expertise, designing and consulting on solar PV projects, with an affinity for writing and education. He founded the engineering consultancy Mayfield Renewables in 2007 to provide much-needed guidance and expertise to the solar and storage industries. Ryan was also the PV Systems Technical Editor for SolarPro Magazine from 2010 to 2018, and authored the PV Design & Installation for Dummies in 2010. He is passionate about bringing up the next generation of experts through teaching nationwide PV courses on topics such as: the National Electrical Code, commercial and utility-scale PV systems, energy storage systems best practices and electrical testing O&M best practices. He is also an alternate member of NEC Code Making Panel 13 tasked with charged with reviewing NEC sections 480 and 706.