Energy Storage Systems, based on the 2023 NEC

Figure 01

Note: This article is based on the 2023 NEC.

The high energy levels in Energy Storage Systems make them especially dangerous if they are not installed and maintained per Code.

Article 706 applies to energy storage systems (ESS) that have a capacity greater than 1 kWh and that can operate in stand-alone (off-grid) or interactive (grid-tied) mode with other electric power production sources to provide electrical energy to the premises wiring system. ESS can have many components, including batteries and capacitors. They include inverters or converters to change voltage levels or to make a change between an alternating-current and a direct-current system. Figure 01

Only qualified persons may install or maintain an ESS [706.3]. Each ESS must be listed [706.5] and have eight bits of data marked on a nameplate, for example rated frequency and rating in kW or kVA [706.4].

Multiple ESS are permitted on the same premises [706.6]. As with PV systems, ESS may be composed of multiple pieces of equipment assembled into a single system, or each piece of equipment may be considered an ESS on its own. The best way to identify an ESS is to look for a nameplate and review the instructions, both of which are part of the equipment's listing.

Commissioning and maintenance
For other than one- and two-family dwellings, ESS must be commissioned upon installation [706.7(A)]. NFPA 855, Standard for the Installation of Stationary ESS provides guidance on commissioning.

For other than one- and two-family dwellings, ESS must be maintained in proper and safe operating condition in per manufacturer and industry standards [706.7(B)]. A written (this includes computerized) record of repairs and replacements must be kept.

For information on developing an effective electrical preventive maintenance program, see NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, or ANSI/NETA ATS-2017, Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.

Disconnecting Means
A disconnecting means must be provided to disconnect the ESS from other sources of power, utilization equipment, and premises wiring [706.15(A)].
In other than one- or two-family dwelling units, the disconnect marking must include the identification and location of the circuit source that supplies the disconnect unless located and arranged so the identification and location of the circuit source is evident [110.22]. The marking must be sufficiently durable to withstand the environment involved.

The ESS disconnect must be readily accessible and located either within the ESS or within sight of (and not more than 10 ft from) the ESS [706.15(B)].

Where the disconnect is not within sight of the ESS, the disconnect (or the enclosure providing access to the disconnect) must be capable of being locked in the open position in per 110.25.

One- and Two-Family Dwellings. The ESS must include an emergency shutdown function to cease the export of power from the ESS to premises wiring in one-and two-family dwellings. The initiation device for the ESS emergency shutdown function must be in a readily accessible location outside the building.

The ESS emergency shutdown initiation device must plainly indicate whether it is in the off or on position.
The requirements in 706.15(A) can be met with disconnects that are integral to the listed ESS equipment. Since an ESS application may have multiple individual ESS units, each may require a disconnect, but this does not necessarily mean each will require a separate disconnect switch adjacent to the units. Many ESS manufacturers choose to incorporate a means of disconnect into their ESS units. These disconnects are evaluated during the system's listing.

The ESS disconnect must plainly indicate whether it is in the open (off) or closed (on) position and be marked ENERGY STORAGE SYSTEM DISCONNECT [706.15(C)]

For other than one- and two-family dwellings, the ESS disconnect must be legibly marked to withstand the environment involved and include nominal battery voltage, available fault current, an arc-flash label per acceptable industry practice, and the date the arc flash calculation was performed.

Note 1: Industry practices for equipment labeling are described in NFPA 70E, Standard for Electrical Safety in the Workplace. This standard provides specific criteria for developing equipment arc-flash labels that provide nominal system voltage, incident energy levels, arc-flash boundaries, minimum required levels of personal protective equipment, and so forth.

Note 2: ESS electronics could include inverters or other types of power conversion equipment.
If line and load terminals within the ESS disconnect could be energized in the open position, the disconnect must be marked: WARNING ELECTRIC SHOCK HAZARD TERMINALS ON THE LINE AND LOAD SIDES MAY BE ENERGIZED IN THE OPEN POSITION.

The marking must have sufficient durability to withstand the environment involved [110.21(B)].
Where ESS circuits pass through a wall, floor, or ceiling, a readily accessible disconnect within sight of the ESS is required [706.15(D)].

This provision will not apply to every ESS application where circuit conductors travel through walls, floors, or ceilings. It is for those applications (typically large ones) where the battery is in one room and other equipment that is part of the ESS is in another. In those cases, a disconnect must be in the room containing the battery. This does not apply to situations where the entire ESS is in one room and the output circuit from the ESS connects to other systems in other rooms. In those cases, the disconnect location requirements in 706.15(A) are all that apply.

Batteries could include an enclosure, battery monitoring and controls, or other related battery components. Where the battery of the ESS is separate from the electronics and subject to field servicing, the following applies [706.15(E)]:

• A readily accessible disconnect is required within sight of the battery. See 240.21(H) for information on the location of the OCPD for battery conductors.
• Where the battery disconnect has remote controls not within sight of the battery, the battery disconnect must be capable of being locked in the open position in per 110.25, and the location of the controls must be field marked on the battery disconnect.
• The battery disconnect must be legibly marked to withstand the environment involved and include nominal battery voltage, available fault current, an arc-flash label per acceptable industry practice, and the date the arc flash calculation was performed.
  

Paralleled with other power sources
ESS in parallel with other power sources must comply with the following [706.16]:
(A) All power sources must have a disconnecting means.
(B) All power sources must use interactive (grid-tied) inverters.
(C) Upon loss of electric utility power, the interactive (grid-tied) inverter must automatically disconnect from the electric utility per 705.40.
(D) Unbalanced ac connections must comply with 705.45.
(E) The parallel connection of the ESS to other power sources must be per 705.12.
(F) Where the ESS is operating in stand-alone (off-grid) mode, the requirements of 710.15 apply.

Working spaces about energy storage systems
The working space for ESS must comply with 110.26.

ESS must be spaced apart per the manufacturer's instructions [706.20(C)(2). Additional space may be needed to accommodate ESS hoisting equipment, tray removal, or spill containment.

Circuit current rating
The maximum current for an ESS is as follows [706.30(A)]:
(1) Nameplate-Rated Circuit Current. The rated current indicated on the ESS nameplate.
(2) Inverter Output Current. The continuous inverter output ac current rating.
(3) Inverter Input Current. The continuous inverter input dc current at the lowest input voltage.
(4) Inverter Utilization Output Circuit Current. The continuous inverter output ac current rating.
(5) DC-to-DC Converter Output Current. The continuous dc-to-dc converter's nameplate current rating.

Conductor ampacity
The ampacity of the output circuit conductors of the ESS(s) to the wiring system supplying the load must be at least 125 percent of the current rating of the ESS per 705.30(A) or the rating of the overcurrent protective device [706.31].

Overcurrent protection
Overcurrent protective devices (OCPDs) must be per 706.31(B) through (F). For example, OCPDs must have an ampere rating of at least 125 percent of the current marked on the ESS nameplate [706.30(A)]

Ex: Where the assembly (including the OCPDs is listed for operation at 100 percent of its rating, the ampere rating of the OCPDs can be not less than the currents calculated in 706.30(B).

For a smooth start-up
Once your ESS is installed, it must be commissioned. But don't wait until commissioning to discover engineering errors and Code violations, that can result in scheduling delays and cost over-runs. Review the commissioning plan well ahead of time. Do a dry run of the relevant parts of the plan as you work specific stages of the project, correcting errors as you find them.

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