20.2 Interconnected Power Sources, Energy Storage & Power Control Systems (Articles 705, 706, 480 & 130)
Key Takeaways
Under the 120% rule, 125% of the power source output current plus the rating of the OCPD protecting a busbar may not exceed 120% of the busbar ampacity, with the source breaker at the opposite end of the bus from the main supply (705.12(B)(3)).
A 200 A panelboard busbar with a 200 A main breaker can accept a back-fed breaker of up to 40 A under the 120% rule, enough for an inverter with an output current of 32 A.
Upon loss of the primary utility source, an interconnected power production source must automatically disconnect from the utility conductors and stay off until the utility is restored (705.40).
A permanent plaque or directory at each service and at each power source disconnect location must identify all power sources supplying the premises (705.10 and, in 2026, 230.70(D)).
The 2026 NEC 120.7(B) limits a power control system's control setting to 80% of the rating of the OCPD for the circuit it monitors when used in load calculations.
Interconnected electric power production sources (Article 705)
Article 705 covers sources that operate in parallel with the utility or another primary source: utility-interactive PV inverters, wind turbines, generators that parallel, fuel cells, and energy storage that exports.
General rules
| Rule | Requirement |
|---|---|
| Listing (705.6) | Interactive inverters, generators, and ESS must be listed or field labeled for interconnection |
| Installation (705.8) | By qualified persons |
| Identification (705.10) | A permanent plaque or directory at each service equipment location, and at each power production source disconnect location, showing all sources and their disconnect locations |
| Loss of primary source (705.40) | On loss of the utility, the source must automatically disconnect from the utility conductors and not reconnect until the utility is restored (anti-islanding) |
| Disconnecting means (705.20) | Simultaneously disconnect all ungrounded conductors of the source; readily accessible; externally operable without exposing live parts; indicate open or closed; rated for the available fault current |
Supply-side connections (705.11)
A source may connect on the supply side of the service disconnect, between the meter and the main. The connection must use listed means that do not compromise the listing of the service equipment, such as connectors marked "SVC" or suitable for use on the line side of service equipment (230.46). The 2026 NEC limits the length of the source's tap conductors to 16.5 ft, or 66 ft where cable limiters are installed, before they reach their OCPD, and clarifies that the connection is made to existing service equipment.
Load-side connections (705.12)
A source connected on the load side of the service disconnect must have a dedicated OCPD and disconnect, and it must not overload the busbars or feeders it shares. For a busbar, one of these methods must be met:
| Method | Rule |
|---|---|
| 100% rule | 125% of the power source output current + the rating of the OCPD protecting the busbar ≤ 100% of the busbar ampacity |
| 120% rule | 125% of the power source output current + the rating of the OCPD protecting the busbar ≤ 120% of the busbar ampacity, with the source breaker at the opposite end of the busbar from the main supply, and a label: "WARNING: POWER SOURCE OUTPUT CONNECTION — DO NOT RELOCATE THIS OVERCURRENT DEVICE" |
| Sum of breakers | The sum of the ratings of all OCPDs on the busbar, excluding the main, ≤ the busbar ampacity, with a label prohibiting additional loads |
| Center-fed dwelling panels | A modified 120% rule for center-fed panelboards in dwellings, with the source breaker at either end |
| Engineering supervision | Connections designed under engineering supervision for other configurations |
Example: a 200 A busbar with a 200 A main breaker. The 120% limit is A, so the back-fed breaker may be up to A. A 40 A breaker supports an inverter output current of A. A larger inverter needs a smaller main breaker (main breaker derating), a supply-side connection, or a power control system.
Energy storage systems (Article 706)
An energy storage system (ESS) is one or more components assembled together to store energy for later use, such as a home battery with its inverter.
| Rule | Requirement |
|---|---|
| Listing (706.5) | ESS must be listed |
| Disconnecting means (706.15) | A means to disconnect the ESS from all wiring systems, readily accessible, located within the ESS or within sight and within 10 ft of it (or remotely controlled as permitted), and marked |
| Emergency shutdown | For one- and two-family dwellings, an ESS emergency shutdown function with its initiating device at a readily accessible location outside the building |
| Working space | Per 110.26, with ventilation and spacing as required by the listing |
| Identification | Where the ESS disconnect is not at the service, a plaque or directory at the service identifies its location (230.70(D)) |
Stationary batteries (Article 480)
The 2026 NEC deleted "standby" from the scope, so Article 480 applies to all stationary battery installations that are not part of a listed ESS. New 480.14 requires overcharge control for all stationary batteries to prevent fires, thermal runaway, and equipment failure. Batteries also need suitable racks, ventilation for vented cells, working space, and disconnecting means.
Energy management and power control systems (Article 130)
The 2026 NEC relocated energy management system (EMS) rules from Article 750 to a new Article 130:
- Part I applies to all EMS, which monitor and control loads, such as load shedding to save energy.
- Part II applies to power control systems (PCS): EMS that also control loads, sources, or storage to prevent overloading of conductors and equipment. A PCS may control generation, energy storage, loads, and circuit controllers.
- Listing: EMS are evaluated under UL 916; a new standard, UL 3141, is being developed for PCS.
- Settings: PCS settings must be protected from unauthorized changes and marked.
Load calculations (120.7): a listed PCS may be used to determine service and feeder size. Its control setting may not exceed 80% of the rating of the OCPD for the circuit it monitors. If the PCS monitors only controlled loads, its setting replaces those loads in the calculation; if it monitors both controlled and noncontrolled loads, the minimum operating current of the controlled loads is used. Annex D Examples D14(a) through D14(d) show the method.
EVSE and standby uses: a PCS can let several EV chargers share a feeder, and the 2026 NEC 702.4 recognizes PCS for managing standby generator loads.
A 225 A panelboard busbar is protected by a 200 A main breaker. Under the 120% rule, what is the largest back-fed breaker for a PV inverter at the opposite end of the bus?
20 A
40 A
70 A
90 A
What must a utility-interactive inverter do when utility power is lost under 705.40?
Continue supplying the utility lines to support the grid
Disconnect automatically and stay off until utility power returns
Switch the whole house to inverter power after 10 seconds
Trip the service main breaker through a shunt trip
Under the 2026 NEC 120.7(B), what is the maximum control setting of a power control system used in a load calculation?
80% of the monitored circuit's OCPD rating
100% of the monitored OCPD rating
125% of the calculated circuit load
60% of the service disconnect rating
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