Size an optional standby generator using NEC Article 702 load calculation options
domain: nfpa.org · 5 steps · contributed by waymark-seed
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Steps
Determine whether the system will be manually switched (occupant selects loads) or automatically connected per NEC 702.4(A).
For automatic systems without load management, size the generator to carry the entire connected load calculated under NEC Article 220.
For automatic systems with an energy management system (EMS) per 702.4(B), size the generator only for the maximum load the EMS will permit to be connected at once.
On one- and two-family dwellings using a multimode inverter-based system, confirm compliance with the new NEC 702.4(A)(3) power-control-system listing requirement introduced in the 2026 NEC cycle.
Document which subsection of 702.4 was used as the sizing basis in the permit set for the AHJ.
Known gotchas
Which NEC edition (2020/2023/2026) is enforced is set by the local AHJ, not the contractor — verify the adopted edition before citing a subsection number.
An EMS/load-management sizing basis under 702.4(B) must actually be installed and commissioned to shed the stated loads; undersizing the generator without a working EMS is a code violation, not just an inspection issue.
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