I know this thread is 3 years old, but have an AHJ referencing it so figured I'd respond here as well. This is based on the 2020 NEC and a similar compliance path exists in 2017. If you're trying to apply the 2014 NEC to a listed ESS you'll come up short every time. UL 9540 didn't even exist when the 2014 NEC was written.
The underlying concern addressed by 702 is that when a power source is transferred into an
overload condition there is the potential to cause an electrical or mechanical failure. This issue stems
largely from rotating generators, or ‘prime movers’, that historically did not have the types of engineering
controls in place to prevent this. Although the code is not written as a design or performance standard,
there are also power quality issues that are addressed by ensuring the power source is adequate to supply
the connected load.
A UL 9540 listed ESS does not pose these same fundamental hazards or power quality issues. The safety standard addresses overload, short-circuit, voltage and frequency abnormalities that could result in a hazard. The NEC recognizes this and provides a path to compliance, separate from the guidance given for traditional power sources and the associated transfer equipment. The Powerwall ESS does not actually include a transfer switch at all, simply an additional UL 1741 anti-islanding device found in the Backup Gateway component.
The AC Powerwall includes integrated UL 1741 utility-interactive power conversion equipment and is therefore subject to the rules defined in Article 705 for connection to other power sources. Starting with 706.16(C) we can see this reinforced by the pointer to 705.40. In the last paragraph of 705.40, it is stated that an interactive system is also allowed to be operated in isolation from the primary power source, typically the utility grid: Emphasizing this even further 706.16(F) makes it noticeably clear that Article 710.15 applies. 710.15 is specific as to sizing and system capacity.
Attempting to use the sizing guidance found in Article 702, Optional Standby Systems quickly becomes problematic when applied to the Powerwall. A prerequisite for determining size is choosing which type of transfer switch is being employed. Automatic or manual? In the case of the Powerwall the component that connects to the primary source is the Backup Gateway. This device is not a transfer switch at all, simply a UL 1741 listed isolation device that disconnects the Powerwall and selected loads from the utility grid. In short, there is no underlying safety concern that warrants sizing the Powerwall for the total premises load.
There is some confusing and conflicting in the commentary associated with 706.16 and can say that CMP-13 did not universally agree with that take. In summary:
- Powerwall will simply cease output when connected to a load that exceeds the rated output, this is a customer education/expectation issue and not related to any safety hazard within the scope of the NEC.
- Powerwall system capacity must meet the requirements of Article 710.15 while operating in Stand-alone mode, i.e., equal to or greater than the largest single utilization equipment. (Note that on-site PV or other sources may contribute to the total or overall system capacity)
