Does Tesla powerwall have to comply with NEC 2014 702.4.(B)(2)?

[fandi]

Hello,
Section 702 is not just about prime movers. Reading 702.2 definition, this section applies to Energy Storage Systems. Does Tesla powerwall have to comply with NEC 2014 702.4.(B)(2)? Can it connect to the main panel and feeds bunch of loads of the main panel? I don't see load management/load shedding strategy here given that fact that a powerwall can only output 7kW at 240V. Tesla says they have an mobile app to turn on/off specific loads but how is it possible if the contractor connects the single powewall directly to the main panel?

Thanks.

 

[petersonra]

Hello,
Section 702 is not just about prime movers. Reading 702.2 definition, this section applies to Energy Storage Systems. Does Tesla powerwall have to comply with NEC 2014 702.4.(B)(2)? Can it connect to the main panel and feeds bunch of loads of the main panel? I don't see load management/load shedding strategy here given that fact that a powerwall can only output 7kW at 240V. Tesla says they have an mobile app to turn on/off specific loads but how is it possible if the contractor connects the single powewall directly to the main panel?

Thanks.

702.4 Capacity and Rating.
(A) Available Short-Circuit Current. Optional standby system
equipment shall be suitable for the maximum available shortcircuit
current at its terminals.
(B) System Capacity. The calculations of load on the standby
source shall be made in accordance with Article 220 or by
another approved method.
(1) Manual Transfer Equipment. Where manual transfer
equipment is used, an optional standby system shall have
adequate capacity and rating for the supply of all equipment
intended to be operated at one time. The user of the optional
standby system shall be permitted to select the load connected
to the system.

it seems to me the app idea would be compliant with 702.4 (B)(1) so 702.4 (B)(2) would not apply. Also the user could select what loads to use by turning on/off circuit breakers.

 

[fandi]

it seems to me the app idea would be compliant with 702.4 (B)(1) so 702.4 (B)(2) would not apply. Also the user could select what loads to use by turning on/off circuit breakers.

Thanks, Bob.

 

[jaggedben]

The Powerwall system does not use a manual transfer method. I don't think one can invoke that section.

 

[caribconsult]

Powerwall limitations

Powerwall limitations

You have very little control over the Tesla system with their gateway. The gateway is the center of the system: everything connects first to the gateway - the grid, the house and the panels. The gateway decides who gets what and from where. If your battery is charged and your grid is off or disconnected, the battery will run your house within load limits continuously for probably 24 hours, depending on load. Of course, at some point during those 24 hours, you should have sun or even just bright daytime light and that will be divided by the gateway - some to the house, the rest to the battery. After sundown, the gateway senses no solar and switches the load (house) back to the battery, almost seamlessly. Not much sun one day? Turn on the grid if the battery is low and the gateway will divide the grid power between the house and recharging the battery. If you have a 'net metering' sell-back arrangement with the power company, the gateway will try to feed the excess back to the grid once the batteries are charged. We use the phone app to monitor the battery level and just shut off the grid when charged into the 90's since we don't have nor want the net-metering system, for several good reasons.

What we are experiencing is that this system does everything it supposed to do, as far as what I describe above, but it is also doing something it SHOULD NOT DO, which is shutting down completely. battery off, no power to house, sun or no sun, high load or no load, battery charged fully or not.

System just goes dead. Usually it resets itself quickly but last week it shut down and won't restart under any efforts. I think I have a defective something here...gotta be either the battery or the gateway...what else could shut this down and leave no evidence, like sparks, burns, that smell of electrical stuff burning? None of that, just light out. It's a PW2 with Tesla Gateway, recently purchased and firmware recently upgraded to latest available. We're not overloading it, it's in a cool dry spot, our wiring is up to date, no wiring errors have been detected and the system has been doing this from the start, 5 months ago. Any ideas here?

 

[c_picard]

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)