CODE CORNER: Energy Storage Systems: Article 706

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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.

Comments (2)

Edward-Dijeau's picture

The NEC is making it harder and harder for "plug and Play" systems to be built, tested and installed before they change the rules. 18 volt panels with 12 volt batteries with no voltage higher than 30 volts puts MPPT Charge Controlers out of business, Micro inverters requiring 36 volts and without remote disconects out of business. Remember, the lower limit used to be 50 volts potential and that was lowered to 30 volts just to get the Micro inverter croud. What the NEC wants is "protectionism" on profeshionaly intalled, high voltage systems rather than a simple 12 to 18 volt micro inverter system like they have in Asia, Africa and Europe. Micro-inverters built in Asia and shipped all over the world can work on 18 volt panels and output 120 volts or 220 volts 50 to 60 Htz and match to the grid but they can not get UL or NEC approval no matter how safe they are being such low voltage. If you were a fireman, would you want to go onto a roof with 240 to 360 volts potential or just 18 to 36 volts maximum potential? In Asia and Europe you would face just 18 volts to 36 volts of PV power but our systems in the USA you get 220 volts AC or up to 360 volts DC depending on the system.
These battery systems are also high voltage so they can put a replacement 360 volts DC onto the on grid inverter rather rather than the 360 volt panels to get on grid output through the same inverter. Plug and Play can use a larger micro-inverter that could use just 18 volt panels, 24 volts of Battery or 36 volts of either like the Micro inverters from Asia can use. E-phase uses panel mount 36 volt 300 watt micro inverters that boost the power to 220 volts up on the roof where ground mounted (wall mounted) Asian built microinverters bring down safer 18 volt or 36 volt panel power off the roof so fire fighters don't have to wory about high voltages and conversion is done within 6 feet of the home circuit entry point on a dedicated 20 amp circuit(s). 24 volts battery output gives the same output of a 18 volt solar panel through the inverter when wired through a DPDT 30 Amp Contactor and the recomended maximum panels are used for daytime use. You charge the 12 volt batteries at 18 volts durring the day, then series them to 24 volts at night to power the micro-inverter. The cost of the larger copper cables and contactor is a small price to pay for real safety for the average homeowner and the firefighter on the roof. It would also allow homeowners to plug in a "Plug and Play" system using lower, safer voltages.

Mr. Bruce Arkwright, Jr.'s picture

Thing they are forgetting is that with high voltage you need all the panels to be in series. Series panels are only good in Zero Tolerance shade! Too many places, locations, & homes are shaded throughout the day, lower voltage means multi parallel panels will keep power flowing! Even 48 volts is too high for me, I have to keep it to 24 volts panels so just one panel will not bring down the system! Yes, the amperage sucks for going the distance but there is no other way for me....
That is not to say, today we now have similar 300 watt panels that are 60 volts - LG neon panels (72 cell) voc 47 v or Panasonic hit (96 cell) voc 69 v, will make a difference! If you know about them beforehand....

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