Quick answer
A generator interlock kit is a metal plate or slide fitted to the front of a main electrical panel that physically blocks the main breaker and a generator back-feed breaker from being switched on at the same time.
With the main off, the plate lets the generator breaker turn on, so a portable generator can feed the panel through an outdoor inlet. Its single job is to stop backfeed onto utility lines.
Quick facts
The key facts about Generator Interlock Kit, with sources:
- Purpose
- Prevent the utility and generator from being connected at the same time 1
- Evaluated under
- UL 67 (panelboards), not UL 1008 (transfer switches) 1
- Marking to look for
- Panelboard marked for Article 702 use, naming the interlock kit catalog number 1
- Paired with
- A portable generator, inlet box and cord 2
- Who installs
- A qualified electrician, per OSHA generator guidance 2
Key takeaways
- An interlock is a mechanical block: main breaker on means generator breaker off, and the reverse.
- It lets a portable generator feed the whole panel while you choose which breakers stay on.
- UL treats a panel interlock as transfer equipment when it is part of a listed panelboard and the panel is marked for it.
- A generic or wrong-model plate is not the same thing; inspectors look for the matching catalog number.
- It is electrician work with a permit, and it does nothing about carbon monoxide.
What an interlock kit is
Inside a main panel, the main breaker brings in utility power. When you add a generator, an electrician installs a second breaker, the generator breaker, that brings power in from an outdoor inlet. If both were on at once, the generator would feed the utility lines. That is backfeed.
An interlock kit is a small plate that bolts to the panel cover. It slides so that only one of the two breakers can be switched on. It has no wires and no electronics. Its safety comes entirely from its shape and fit.
IAEI’s UL column explains that panelboard circuit breaker interlock kits are considered transfer equipment under NEC 702.5 and can be used in optional standby systems, including those fed by portable generators 1.
Backfeed kills line workers
OSHA warns that attaching a generator to a building’s wiring without a properly installed transfer switch can energize wiring for great distances and electrocute utility workers 2. Never connect a generator to a panel through an outlet or a “double male” cord. Only a qualified electrician should install an interlock and inlet.
How the interlock works in an outage
- Normal: main breaker ON → interlock plate blocks the generator breaker OFF
- Outage: owner turns the main breaker OFF → plate slides → generator breaker can turn ON
- Portable generator outdoors → cord → inlet box → generator breaker → panel bus
- Owner turns off big loads, then turns on the breakers the generator can carry
- Grid returns: generator breaker OFF → plate slides back → main breaker ON
- Start generator outdoors20 ft from the home 3
- Main breaker offPlate now frees the generator breaker
- Plug in at the inletCord from generator to the outside inlet
- Generator breaker onPanel is now on generator
- Add loads slowlyLargest motor first, stay under rating
Types of interlocks
| Type | How it works | Notes |
|---|---|---|
| Maker’s own kit for that panel | Sliding plate made by the panel maker for a named panel line | Panel marking can name the kit’s catalog number 1 |
| Third-party kit listed for that panel | Plate made by another firm for specific panel models | May carry a UL Recognized Component Mark for field install 1 |
| Factory-installed interlock | Fitted at the factory on a generator-ready panel | Comes with the panel |
| Key interlock | A single key that must be moved between two locks | More common on larger commercial gear |
Example: choosing breakers to run on a portable generator
An interlock example shows the owner’s part after an electrician has installed an interlock. Values are illustrative; read your generator and appliance labels. The rule is to keep the total under the generator’s running rating.
| Breaker | On or off during outage | Reason |
|---|---|---|
| Main breaker | Off | Interlock requires it before the generator breaker can turn on |
| Refrigerator | On | Food safety; a closed fridge stays cold about four hours without power 4 |
| Furnace | On in winter | Heat; blower and controls are modest loads |
| Central air conditioner | Off | Too large for most portable generators |
| Electric water heater | Off | Large heating load |
| Lighting circuits | A few on | Turn on only rooms in use |
Our sizing calculator checks whether your chosen breakers fit the generator before the next storm.
Where you will see it
You see the plate on the front of the main panel, near the top, by the main breaker. Outside, there is a power inlet box where the generator cord plugs in. On a quote, it appears as interlock kit, generator breaker, inlet box, cord and permit.
On the panel label, look for the marking IAEI describes: “Suitable for use in accordance with Article 702,” and for kit-form panels, the interlock kit catalog number 1.
Benefits and drawbacks
What it does well
- Prevents backfeed by design 1.
- Lets you run any circuit in the panel, one breaker at a time.
- Fewer parts than a separate transfer switch box.
- Works with a portable generator you already own.
Where it falls short
- Manual: someone must be home to switch over.
- Every breaker is on the generator side, so it is easy to overload the generator.
- Only valid when made or listed for your exact panel.
- Needs a free spot for the generator breaker near the main.
Limits and fine print
- Panel-specific. A plate that fits “most panels” is not proof of listing; the panel marking must match 1.
- Code reading varies. Your local inspector decides whether an interlock is accepted and which code edition applies.
- No load control. Unlike some transfer switches, nothing stops you from turning on too much.
- No CO protection. The generator still runs outdoors, 20 ft from the home 3.
What drives the cost
No dollar figures here. These factors move the quote. See the project cost tool.
- Kit availability. A maker’s kit for your panel may cost more than a third-party one.
- Breaker space. Moving circuits to free the top slot adds labor.
- Inlet box and cord. Rated to match the generator’s outlet.
- Distance. From the inlet location to the panel.
- Permit and inspection. Set by your town.
- Panel age. Older panels may not have a listed kit; a new panel may be needed.
How installation works
A licensed electrician does this. Panel covers stay on for you; this is not owner work.
- The electrician confirms a listed kit exists for your panel make and model.
- A permit is pulled where required.
- A generator breaker, the interlock plate and an outdoor inlet are installed.
- Breakers and the inlet are labeled.
- An inspector checks the kit, marking and wiring; you learn the switch-over steps.
Upkeep
- Practice the switch-over once a year in daylight.
- Make sure the plate slides freely and its screws are present.
- Keep the inlet cover closed and the cord dry and undamaged 2.
- Never remove the plate to “make it easier.”
Warning signs and when to call
Call an electrician if the plate is missing, bent or loose, if the generator breaker can be turned on with the main on, or if breakers feel hot. Any of these defeats the safety purpose.
If you smell burning, see smoke, or the generator stalls repeatedly, turn the generator off and call for help. Our electrical safety guide lists the signs that need a pro.
Carbon monoxide still applies
An interlock makes the wiring safe, not the exhaust. CPSC reports about 85 deaths a year from CO from gasoline-powered portable generators, an average for 2017 to 2019 3. Run the generator outdoors, 20 ft away, with CO alarms on every level.
Rules and standards
Generator interlock kits are covered by the NEC transfer-equipment rule and by UL listing standards.
| Authority | Rule | What it says |
|---|---|---|
| NFPA 70 (NEC) | 2017 ed. as cited by UL, Section 702.5 | Transfer equipment must prevent inadvertent interconnection of sources 1 |
| UL | UL 67 (panelboards) | Interlock kits that are part of a panelboard are evaluated here 1 |
| UL | UL 1008 (transfer switches) | Covers separate transfer switches, not panel interlocks 1 |
| OSHA | Portable generator fact sheet (2005; does not mention interlocks) | Never attach a generator to a building’s wiring unless a qualified electrician has installed it with a transfer switch 2 |
| Local inspector | Adopted NEC edition and amendments | Final say on acceptance |
Interlock vs transfer switch
| Feature | Interlock kit | Manual transfer switch | Automatic transfer switch |
|---|---|---|---|
| Keeps sources apart | Yes, by blocking breakers | Yes | Yes |
| Standard | UL 67 panelboard 1 | UL 1008 1 | UL 1008 1 |
| Circuits available | Whole panel | Only the ones wired in | Whole home or a subpanel |
| Operated by | Owner | Owner | Controller |
| Paired generator | Portable | Portable | Standby |
Myths
- Myth Any interlock plate is fine.
- Reality It must be made or listed for that panel, with the panel marked accordingly 1.
- Myth With an interlock I can backfeed through a dryer outlet.
- Reality No. OSHA warns against attaching a generator to a building’s wiring without a properly installed transfer switch 2. An interlock setup uses a dedicated inlet and generator breaker installed by an electrician, never an outlet.
- Myth Interlocks are never allowed by code.
- Reality UL treats listed panel interlocks as transfer equipment under 702.5; acceptance rests with your inspector 1.
State and local differences
States adopt NEC editions on their own schedules, and local inspectors interpret transfer equipment rules. Outage exposure also differs: EIA reported nearly 53 hours without power for South Carolina customers in 2024 versus 11 nationally 5.
Is an interlock right for you?
An interlock suits an owner with a portable generator, a panel that has a listed kit, and the discipline to manage loads by hand. If you want the generator to start on its own, look at a standby generator and an automatic switch instead.
Ask an electrician whether a listed kit exists for your panel, and compare it with a transfer switch quote. Our hiring guide covers what to ask.
- If Listed kit exists for your panel
- Then Interlock is an option
- If No listed kit
- Then Manual transfer switch
- If You want load limits built in
- Then Transfer switch with fewer circuits
- If You want hands-free backup
- Then Standby generator with an ATS
Questions about Generator Interlock Kit
What does a generator interlock kit do?
It stops the main breaker and the generator breaker from being on at the same time.
With the main off, the plate lets the generator breaker turn on so a portable generator can feed the panel through an outdoor inlet.
That keeps generator power from flowing back onto the utility lines, where it could shock line workers.
Is a generator interlock kit legal?
A listed interlock made for your panel is generally treated as transfer equipment. IAEI’s UL column says panelboard interlock kits are considered transfer equipment under NEC 702.5 and are evaluated under UL 67. Acceptance still rests with your local inspector and the code edition your jurisdiction enforces, so have an electrician confirm it.
Is an interlock kit as safe as a transfer switch?
For backfeed, a listed and correctly installed interlock does the same job: only one source can feed the panel. The difference is load control. An interlock puts every breaker on the generator side, so it is easier to overload the generator. A circuit-selector transfer switch limits you to the circuits wired into it.
Can I install a generator interlock kit myself?
No. Installing the generator breaker, the plate and the outdoor inlet means working inside the main panel, which is electrician work and usually needs a permit. OSHA advises that generators connect to buildings only through equipment a qualified electrician installs. Your part is learning the switch-over steps once it passes inspection.
Do interlock kits fit every panel?
No. Kits are made for specific panel lines, and the panel must have room for the generator breaker in the right spot. IAEI notes that kit-form panelboards can be marked with the interlock kit’s catalog number. If no listed kit exists for your panel, a transfer switch is the usual route.
What size generator can I use with an interlock?
The generator’s outlet, the inlet, the cord and the generator breaker must all match in amperage. Beyond that, the generator only needs to carry the breakers you turn on. Use a sizing calculator to list loads and keep the running total under the generator’s running watt rating, allowing for motor start-up surges.
Do I need a permit for a generator interlock?
In most towns, yes. Adding a breaker, an inlet and a second power source is permanent wiring that inspectors want to see. The inspector checks that the kit matches the panel and its marking and that the inlet and breaker are rated correctly. Your electrician will know the local rule.
What is the order of steps when using an interlock?
Follow the written steps your electrician leaves at the panel, because the order of starting the generator and plugging in the cord differs by maker.
One rule never changes: the main breaker goes off before the generator breaker goes on, and the generator breaker goes off before the main goes back on.
Add loads one breaker at a time so the generator does not stall.
Does an interlock kit protect me from carbon monoxide?
No. It only controls which breaker can be on. Carbon monoxide comes from the generator exhaust, and CPSC reports about 85 deaths a year from gasoline-powered portable generator CO (2017 to 2019 average). Keep the generator outdoors at least 20 feet from the home, exhaust away, and keep CO alarms on every level.
Sources
- IAEI Magazine, UL Question Corner: panelboard interlock kits as transfer equipment (NEC 702.5, UL 67, UL 1008), 2017 NEC, May/June 2017, retrieved .
- OSHA, Fact Sheet: Using Portable Generators Safely (FS-3286, September 2005), retrieved .
- CPSC, New Report on Carbon Monoxide Fatalities, Generator Safety (2022), retrieved .
- Ready.gov, Power Outages, retrieved .
- U.S. EIA, Today in Energy: hurricanes in 2024 led to the most hours without power in 10 years, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Checked against IAEI’s UL column on interlock kits, OSHA and CPSC generator guidance, Ready.gov and EIA outage data on 2026-10-10. Held for review by a qualified electrician before indexing.
Suggest a correction. We fix errors and say what changed.