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Home / Code Guides / Generator Transfer Switch

Standby Generators & Transfer Switches — NEC 702 Requirements

The one absolute rule for backup power is that a generator can never be connected to house wiring in a way that could backfeed the utility — that’s how line workers get killed. Everything else in NEC Article 702 flows from that: listed transfer equipment, correct sizing, and proper grounding. Here’s what a compliant optional standby installation looks like.

RequirementModel Code StandardSection
Transfer equipment requiredAny generator connection to premises wiring requires transfer equipment that prevents simultaneous connection of the generator and the utility — a transfer switch or a listed mechanical interlock on the panelNEC 702.5
Sizing — manual transferWith manual transfer equipment, the system only has to be sized for the loads you intend to run, and the user is responsible for load management — you can legally run a small generator into a whole-house interlockNEC 702.4(B)(1)
Sizing — automatic transferAn automatic transfer switch requires the generator to carry the full calculated load that transfers, or the system must include automatic load management (load-shedding controls)NEC 702.4(B)(2)
Portable generator connectionCord-connected generators feed the house through a listed flanged power inlet; the cord and inlet must be rated for the load, and the inlet must be wired so its exposed pins are never energizedNEC 702.11, 406.9
Neutral bondingIf the transfer equipment does not switch the neutral (typical for residential), the generator is not a separately derived system — its neutral must not be re-bonded to ground at the generator, and many portable units need their bonded neutral addressed per the manufacturerNEC 250.30, 250.35
SignageA sign at the service entrance must indicate the type and location of the on-site standby power sourceNEC 702.7

Worth knowing

  • A “suicide cord” (male plug on both ends) into a dryer or range receptacle is illegal everywhere, defeats every safety layer in this article, and energizes utility lines — an interlock kit costs less than the fine.
  • Run portable generators outdoors only, at least 20 feet from the house with the exhaust aimed away — CO from a generator in a garage or near an open window kills faster than most people expect. Newer portables with CO shutoff sensors are worth the premium.
  • Permanently installed standby generators must also meet the manufacturer’s listed clearances to openings and property lines — commonly 18 inches from the wall and 5 feet from windows and doors, but the listing controls.
  • Recent NEC editions require GFCI protection on portable generator receptacle outlets (125V, 15/20/30A), which can nuisance-trip with some transfer setups — check compatibility before buying.
  • Optional standby systems fall under Article 702; if your generator serves life-safety loads (rare in homes), Articles 700/701 impose much stricter rules.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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