CODE CORNER: Energy Storage Systems: Article 706

Enphase is one of several manufacturers offering self-contained energy storage systems. Its AC Battery products have received UL 9540 certification.
sonnenBatterie’s eco series ESS is made up of individual NRTL-listed components designed to work well together.

Most energy storage for PV systems has been comprised of lead-acid batteries. Newer technologies and applications—flow and lithium-ion batteries; all-in-one storage plus power conversion “appliances;” peak load shaving; and grid stabilization—are changing the face of energy storage. Article 706, “Energy Storage Systems,” addresses this in the 2017 National Electrical Code (NEC).

What’s Covered

Article 706 addresses permanently installed energy storage systems (ESSs) that operate over 50 volts AC or 60 VDC. An ESS could be a battery connected to separate power conditioning equipment—all of the configurations in Figure 690.1(b) that include an ESS show it located after the PV system disconnect. (The line between PV and ESSs is clearly defined—a PV system isn’t included, ever, although a PV system may be installed at the same location.) Or it could be a “self-contained” system, such as an AC-coupled ESS. There are many options and applications, and Section 706 applies to stand-alone, interactive, and multimode systems.

To comply with Section 706.5, an entire self-contained ESS can be (but does not have to be) listed by a nationally recognized testing laboratory (NRTL) as a complete system. Otherwise, individual components (charge controllers, overcurrent protection devices, power conversion and energy storage equipment, etc.) must be listed. The only exception to this listing requirement is lead-acid batteries.

This can be confusing, as products and systems may be listed to the same standards, but have different functionality. Sections 706.3 and 706.8 refer to Article 705 for ESS interconnection with primary sources, such as the grid. Stand-alone systems have specific requirements, and must also comply with the new Article 710—the rules are taken from the 2014 Code Section 690.10.

Some of the language in 706 was, and still is, in other articles, including Article 480, “Storage Batteries.” In future Code cycles, some shared definitions will be moved to Article 100, but expect Article 480 to continue to be relevant for battery installations.

Types of ESSs

Sections 706.2 and 706.4 define three categories of ESSs.

  • Type 1. Self-contained ESSs are assembled, installed, and packaged into a single unit. Typically, this type of ESS is manufactured and sold as a product and listed to UL 9540. The Enphase Model B280-1200-LL-I-US00-RF0 consists of a UL 1973-listed stationary battery packaged with a UL 1741-listed inverter, with the entire system listed to UL 9540. Tabuchi and JLM Energy also manufacture UL 9540-listed ESSs. Typically, self-contained ESSs use lithium batteries.
    A listed, self-contained ESS is a box with internal components that are not field-assembled and thus are not subject to field inspections. However, input and output wiring, overcurrent protection, and disconnect(s) connected to a listed ESS are subject to inspection by the AHJ.
  • Type 2. Pre-engineered of matched components ESSs are field-assembled, but the components are from a “singular entity” and are intended to be installed together on-site. Components in these systems are either NRTL-listed individually or as an assembly. An example is the sonnenBatterie eco series (an OutBack Radian inverter with Sony lithium batteries).
  • Type 3. Other ESSs are not self-contained or pre-engineered, and consist of various components that are individually listed to relevant NRTL standards, designed to work in a system, and field-assembled. Any system comprised of various components, likely from more than one manufacturer, that together allow for energy storage, would fall into this category. This category includes most of the batteries installed with PV systems until recently, and many of the ones still being installed.

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