Off-grid (stand-alone) power systems
A stand-alone system is one not connected to the utility. Its supply may be smaller than the calculated load but must at least cover the largest single piece of equipment. Grid-tied batteries are not stand-alone, so their backup sizing follows 702.4.
What the code requires
Article 710 covers stand-alone systems, meaning power systems not connected to the utility, such as off-grid solar with batteries. Under 710.15, the building's premises wiring must meet the code just as if it were fed by a feeder or service. Wiring on the supply side also follows the code, except as modified by 710.15(A) through (F).
The supply may be smaller than the calculated load, but the total of all sources must be at least as large as the biggest single piece of utilization equipment. The calculated general lighting load does not count as one single load. Conductors from the source to the building disconnect are sized on the total of the sources' output ratings, and three-phase loads must be balanced to the supply.
A supply that delivers only 120 V may feed a 120/240 V, 3-wire panel only if there are no 240 V outlets and no multiwire branch circuits, and the total source output is less than the neutral bus rating. The panel then needs a warning label stating it has a single 120 V supply and that multiwire branch circuits must not be connected.
Three-phase, 3-wire or 4-wire systems are allowed. Energy storage or backup is not required, and voltage and frequency must be kept within the limits the loads can handle.
On the job
- A battery or solar system tied to the grid is not a stand-alone system, so its backup panel is not sized by the 710.15(A) largest-load rule; 702.4 applies instead.
- A 120/240 V panel fed by a single 120 V supply does not comply without the single-120 V warning label.
What changed in 2023
The 2023 NEC wording in 710.1 makes clear that a system connected to the utility is not stand-alone, so backup sizing for grid-tied batteries falls under the standby rules, such as 702.4, instead of 710.15(A). The 2023 edition also dropped the 2020 back-fed circuit breaker rule from 710.15.
Related sections
- NEC 710.1
- NEC 702.4: Sizing whole-home generators and backup batteries
- NEC 705.80
- NEC 110.21(B): Requirements for field-applied warning labels
- NEC 706.15: Disconnects and emergency shutdown for battery systems
Where this comes from
- NFPA: 2026 NEC (A2025) CMP-4 First Draft public input report, reprinting 2023 text
- Mike Holt: Summary of the 2023 Changes to the NEC
- Georgia DCA: code amendment form (Tesla), 4/12/2023, quoting 2023 NEC 702.4(A)(2)
- NFPA: 2026 NEC (A2025) CMP-13 First Draft public input report, reprinting 2023 text
- Read the official NEC free at NFPA
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