Transfer switch vs interlock kit: safe ways to power your panel
Updated · Reviewed against the current BackupAtlas dataset
Short answer: both a transfer switch and an interlock kit exist to do one thing safely — connect a generator to house wiring without ever letting generator and utility power meet on the same circuit at the same time. A transfer switch is a dedicated panel that isolates a chosen set of circuits; an interlock is a mechanical plate that makes it physically impossible to close both the main and generator breakers together. Either way, this is a licensed electrician's job and normally requires a permit — never a "backfeed" cord plugged into a wall outlet.
Why you can't just plug a generator into an outlet
Backfeeding — running a cord from a generator into a dryer outlet to energize the whole house — is illegal under the National Electrical Code and dangerous for two reasons. It can send power out through the utility transformer onto the line a lineworker believes is dead, and when utility power returns while the generator is still connected, the two sources can collide and destroy the generator, the panel, or both. A transfer switch or interlock exists precisely to make that collision impossible.
Manual transfer switch
A manual transfer switch is a small sub-panel installed next to (or replacing part of) the main panel. An electrician moves a defined set of circuits — typically 6–10 "essential" circuits: refrigerator, furnace blower, well pump, some lighting and outlets — onto the transfer switch. A double-pole, double-throw mechanism (or a physically separate breaker set) ensures each circuit is fed by either the utility or the generator, never both. You plug the generator into an outdoor inlet box, start it, and flip the switch(es) manually for the circuits you want powered.
Interlock kit
An interlock kit is a sliding metal plate mounted on the main breaker panel that makes it mechanically impossible to switch on both the main utility breaker and a dedicated generator breaker at the same time. Unlike a transfer switch, the interlock doesn't pre-select circuits — the generator connects to the whole panel through its own breaker, and you manage the load by switching individual circuit breakers on or off until the total draw fits inside the generator's capacity. This gives more flexibility (any circuit can be powered, not just a pre-wired subset) at a materially lower installed cost than a transfer switch.
An interlock kit must be listed for the exact panel model — generic or universal kits are not code-compliant, and some jurisdictions require a transfer switch regardless. See the glossary entries for interlock kit and transfer switch for the full definitions.
Automatic transfer switch (standby systems)
Home standby generators use an automatic transfer switch (ATS), which monitors utility voltage continuously and performs the whole sequence — detect outage, start the generator, wait for stable output, switch the load, and switch back when utility power returns — without anyone present. ATS units are rated by amperage (100 A or 200 A for whole-service-entrance installs) and often include load-shedding modules that automatically drop lower-priority circuits (see load shedding) so a smaller generator can protect a larger panel.
Code and permit caveats
- Requirements vary by jurisdiction and utility — some areas mandate a transfer switch over an interlock, require a specific inlet type, or add setback rules for standby units. Local amendments to the NEC are common; the electrician pulling the permit will know the local rules.
- Standby systems typically need both an electrical permit (for the ATS and service connection) and a gas or plumbing permit (for the fuel line), with separate inspections.
- Insurance and resale: a permitted, inspected installation is documented; a DIY backfeed setup is not just unsafe but can void a homeowner's policy and complicate a home sale.
Why this is an electrician's job, not a DIY one
Sizing the transfer switch or interlock correctly requires a load calculation against your actual panel — see the sizing guide for the load side of that math — plus knowledge of local code amendments, correct inlet and breaker ratings, and safe grounding and neutral bonding (see floating vs bonded neutral). A licensed electrician is the right call for every part of this connection; this guide explains the concepts so you can have an informed conversation, not a substitute for that assessment.