Select and size the automatic transfer switch: service-rated vs non-service-rated, open vs closed transition
domain: nfpa.org · 5 steps · contributed by waymark-seed
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Steps
Confirm whether the ATS will be installed ahead of the service disconnect (service-entrance/service-rated, integrating the required disconnect and meeting NEC grounding/bonding rules) or downstream of an existing main disconnect (non-service-rated).
Verify the ATS carries a UL 1008 listing appropriate to its service-rated or non-service-rated application.
For most residential/light-commercial optional standby jobs, specify an open-transition (break-before-make) switch, which briefly interrupts power during transfer.
Specify closed-transition (make-before-break) switches only where continuous power is required and the utility/AHJ permits momentary paralleling — this triggers additional interconnection review.
Size the ATS ampacity to the calculated load per NEC 702.4, not just the generator's output rating.
Known gotchas
Momentarily paralleling sources (closed transition) can pull the installation into NEC Article 705 interconnection territory even for a 'standby' job — confirm with the AHJ and utility before specifying it.
A service-rated ATS still must meet all NEC service equipment requirements (grounding electrode conductor, disconnect marking, first-responder shutoff access) — treat it as service equipment, not just a switch.
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