UL 3141 and Power Control Systems Explained
Imagine, for a moment, what the grid of the not-too-distant future looks like. Should current trends continue, you’ll likely notice more electric vehicles gliding silently through the streets. You may see more buildings in your neighborhood topped with a solar array, many of them backed up with energy storage. Want hot water or cool air? There’s a good chance those appliances are powered by electrons instead of liquid fuels. Across the board, smart distributed energy resources (DERs) are becoming ubiquitous.
This is not today’s reality for most people, but momentum toward this imagined electric future is hard to ignore, which begs questions like: How do we smooth out the transition to a more decentralized grid with potentially millions of controllable assets? How can electrical systems of all scales–from individual buildings to regional grids–be optimized to handle more capacity?
One of the biggest pieces to the puzzle will be power control systems (PCS) capable of monitoring and controlling both sources and loads in real-time. Right now, there is no formal UL listing for PCS, only a limited “certification requirement decision” (CRD) looped into a separate standard. But that’s about to change with the release of the new UL 3141 standard, as we will explain in this article.
What is a PCS?
A PCS is a system–not necessarily a singular device–designed to maintain safe levels of current and loading on busbars. The system consists of a controller plus sensors and a method of communication between the controllable loads and sources. In many cases, the PCS will be its own product. In some cases, the PCS is integrated within an inverter.
There are many ways to configure a PCS. The example solar-plus-storage system below aggregates many PV and ESS inverters before interconnecting with a standard 200 A residential main breaker.
Current transformers (CTs) monitor current at the aggregation panel and the main service panel. Communication cables connect the controller to the CTs and all inverters. When the CTs sense that a busbar is approaching overload, they send signals to throttle down the PV or ESS inverters.
A properly configured PCS can enable the oversizing of generation resources well beyond the load-side source connection sizing constraints of NEC Article 705.12. Another common application is using a PCS to control power flows from the multiple inverters (PV inverter, energy storage inverter, etc.) that make up an AC-coupled solar-plus-storage system. The same logic applies to systems that integrate EV chargers or other controllable loads and sources. If we return to our original thought exercise, it’s easy to see how the increasingly complex electrical infrastructure of the near future will benefit from the capabilities a PCS offers.
PCS and the National Electrical Code
The term “power control system” first appeared in Section 705.13 of the 2020 National Electrical Code (NEC) and was only used to describe systems that control sources.
705.13 Power Control Systems.
A power control system (PCS) shall be listed and evaluated to control the output of one or more power production sources, energy storage systems (ESS), and other equipment. The PCS shall limit current and loading on the busbars and conductors supplied by the PCS.
For the circuits connected to a PCS, the PCS shall limit the current to the ampacity of the conductors or the ratings of the busbars to which it is connected in accordance with 705.13(A) through (E).
Subsections (A) through (E) go on to detail monitoring, overcurrent protection, and settings restrictions requirements. Most notably, access to the PCS settings is restricted to “qualified personnel” for security purposes. While not explicitly spelled out in the Code, in practice, this means settings will be configured and locked in at commissioning. Adjusting settings post-commissioning will often involve support from the manufacturer.
The 2023 NEC introduced new language to Section 705.13, replacing “power control system” with “energy management system” (EMS), a term that has been long defined in Code in Article 750. The key distinction between a PCS and EMS is that a PCS is programmed to optimize safety and performance, whereas an EMS was historically programmed to optimize economic outcomes. A PCS would adjust inverter output to limit overloading busbars; an EMS would adjust inverter output to maximize ROI through utility time-of-use rates. An EMS with PCS would perform both functions.
705.13 Energy Management Systems (EMS).
An EMS in accordance with 750.30 shall be permitted to limit current and loading on the busbars and conductors supplied by the output of one or more interconnected electric power production or energy storage sources.
Informational Note: A listed power control system (PCS) is a type of EMS that is capable of monitoring multiple power sources and controlling the current on busbars and conductors to prevent overloading. See UL 1741, Inverters, Converters, Controllers and Interconnection System Equipment for Use with Distributed Energy Resources, and UL 916, Energy Management Equipment, for information on PCS and EMS.
It should be noted that the language used to differentiate PCS, EMS, and “energy management” more broadly is still fuzzy. But if we squint a little, we can start to see where each technology is headed and how they can work together. The inclusion of “listed PCS” in the 2023 NEC created an allowance for an EMS that would perform safety functions in addition to traditional EMS functions. Expect further clarifications in the 2026 NEC. You can find a great overview of the topic in our most recent Ask Mayfield Anything webinar with Jason Fisher.
PCS Listing Standards
The Informational Note tucked into 705.13 includes a reference to UL 1741, the listing standard for grid-tied PV and energy storage inverters, converters, controllers, and other DER interconnection equipment. If we follow this breadcrumb trail, we’ll find that there is no explicit listing for PCS, only a Certification Requirement Decision (CRD – last acronym, I promise).
A CRD is an approximation of an upcoming standard for a cutting-edge product that does not currently have its own standard. It acts as proof that independent test labs have evaluated new technologies, and a bridge to a more formal standard in the future. For example, the second edition of the UL 1741 listing standard included a CRD for non-isolated (ungrounded) inverters. Those CRD requirements were later absorbed into the third edition of the UL 1741 standard once non-isolated inverters became the norm.
Today, A PCS manufacturer can decide whether to test their product to the UL 1741 CRD. Since it is the only test standard available as of this writing, many manufacturers will opt to do so and will include the “UL 1741” or “UL 1741 CRD-PCS” on their datasheets.
As of January 2024, UL has begun developing an entirely new standard specifically for PCS. The UL 3141 Outline of Investigation for Power Control Systems is largely based on the original UL 1741 PCS-CRD but is intended to address many other types of PCS equipment and applications. One application unique to PCS is service upgrade avoidance and feeder overload control, which evaluates a product’s ability to ensure feeder and service conductors are not overloaded through load and/or source control.
The development of the new UL 3141 standard will be important to provide equipment manufacturers and other stakeholders with confidence that PCS products in the future will be evaluated for reliability and safety as principal requirements for listing.
The second edition of UL’s Outline of Investigation for Power Control Systems will be published this year, followed by the formal, consensus UL 3141 standard in 2025 or early 2026. We encourage you to track UL updates and submit public feedback if you want to be involved in the drafting process.
Conclusion
At a certain point, the widespread adoption of distributed energy resources becomes an optimization problem that power control systems (PCSs) can solve. Whether a business owner wants to electrify operations without a major service upgrade or a public utility hopes to avoid costly transmission upgrades by controlling thousands of small, grid-tied loads and sources, electrical systems of all scales are becoming smarter and safer by integrating PCS. As these innovative technologies evolve, expect codes and standards to follow suit.
The solar-plus-storage industry is evolving quickly. Stay up-to-date with Mayfield Renewables online courses on codes, standards, and engineering best practices.
