Interlock kit vs transfer switch: what is actually legal

People hear two stories. One is that an interlock kit is a cheap metal plate that makes a portable generator legal. The other is that only a transfer switch is code, and the plate is a hack. Both are half right, which is why quotes argue past each other.

The job is the same either way: the house can be on the utility, or on the generator, never both. If both are on, you can push power onto the street and shock a lineman who thinks the line is dead. You can also toast the generator when utility power comes back. NEC Article 702 (optional standby systems) is the rule that says transfer equipment has to prevent that interconnection. An interlock and a transfer switch are two machines that answer the same rule.

This is not a DIY wiring class. A licensed electrician pulls the permit, matches the kit to your panel, and stands the inspection. You are here to know which product belongs on the bid, and which Amazon plate to refuse.

The automatic vs manual transfer switch page is the “what happens when the lights go out” version. This page is the “will the inspector sign this” version.

What do these two jobs cost?

HomeGuide’s generator cost guide puts a transfer switch or interlock, unit plus labor, at $400 to $2,500 for the switch family, and an interlock at $400 to $850 installed. Broken out on their transfer-switch page:

• Manual transfer switch: $400 to $1,300 installed ($200 to $800 for the switch itself)
• Automatic transfer switch: $600 to $2,500 installed ($400 to $2,000 for the switch itself)
• Interlock kit: $400 to $850 installed

HomeGuide: generator cost

HomeGuide: generator transfer switch installation cost

The hardware on an OEM interlock is often $60 to $150. You are paying for panel work, a generator breaker, a retainer so that breaker cannot fall out, an inlet on the wall, and the permit. A six-circuit manual transfer switch is a different box: selected circuits move over, the rest of the house stays dark. An automatic transfer switch is usually paired with a standby generator that starts itself. Do not compare a $500 interlock on a portable you already own against an ATS buried inside an $8,000 standby quote. Different jobs.

Is an interlock legal?

Yes, in a lot of places, when it is a listed kit for that panel, installed so the main breaker and the generator breaker cannot be on at the same time, with a legal inlet, and with a permit the inspector will sign.

LegalClarity’s 2026 summary is the useful one-liner: generator interlock kits are legal throughout the United States when they are properly listed by a recognized testing laboratory and installed according to the NEC and local rules. NEC 702.5 (the numbering moves a bit by edition) requires transfer equipment that prevents the utility and the generator from being tied together, and that equipment has to be listed.

LegalClarity: are generator interlock kits legal

“Listed” is the word that kills the $15 universal plate. Square D, Eaton, Siemens, and GE sell interlock kits that are accessories for specific load-center series. The inspector is looking for that part number on a panel they recognize. A laser-cut slider that “fits QO or Homeline 150 or 200 amp” from a no-name listing may physically slide. It may not be the accessory that was evaluated with that panelboard. Some jurisdictions will pass a third-party kit that carries its own listing. Plenty will not. Ask your electrician what this AHJ (the local inspector) has been accepting, not what a forum in another state accepted in 2019.

Wikipedia’s caveat is worth keeping in your pocket: even kits tested to a UL standard do not always carry a UL Listing mark, and some inspectors refuse those. If your town is in that camp, the cheap plate was never the plan. Budget the manual transfer switch.

Removing the panel cover can, on some designs, let someone turn both breakers on. That is one reason inspectors care about the kit, the retainer clip on a back-fed breaker (NEC 408.36(D) in recent editions), and whether the cover still interlocks when it is on. This is panel work. It is not a Saturday with a drill and a YouTube overlay.

When is the interlock the right product?

It fits a portable generator, a homeowner who will be there to start it, and a panel that has a listed kit and a spare two-pole slot for the generator breaker.

You turn the main off, slide the plate, turn the generator breaker on. The whole bus is now on the generator, so you still have to shed load with your hands: leave the range, dryer, and the 40-amp car charger off, or you will stall a 7,500-watt machine. That is the trade. You can pick any circuit. You also have to remember which ones to leave off.

You still need a legal inlet box, not a cord poked into a dryer outlet. The dryer outlet trick is the backfeed. The inlet and the interlock are the pair that make a portable setup inspectable.

An interlock is a bad plan if nobody in the house can do the sequence in the dark, if you travel through storm season, or if the generator is a standby that is supposed to start itself. Standby wants an automatic transfer switch. That is a different product.

When does the inspector want a transfer switch?

When there is no listed kit for your panel. Federal Pacific, old Zinsco, a packed 100-amp box with no spare slot, a meter-main combo the OEM never made an interlock for. The electrician will say so on the first visit if they are honest. Believe them and price a manual transfer switch or a small generator subpanel.

When the generator is a standby unit. Automatic transfer is the point of paying for that engine. An interlock cannot start the generator for you.

When the utility, the insurer, or the AHJ has a local rule that names a transfer switch. NEC is the floor. Your town can be stricter. The permit conversation is how you find out, not a product page.

When you want a limited set of circuits and you do not want to stand at the panel choosing. A six- or ten-circuit manual transfer switch is that box. You lose the “any circuit” flexibility of an interlock. You gain a labeled set of loads that already match the generator.

What kit is on Amazon, and what is not?

The part has to match the panel brand, the series, indoor vs outdoor cover, and the main-breaker frame. A Homeline 200-amp indoor cover uses a different kit than a Homeline 100-amp outdoor, and a QO panel is not a Homeline panel even though both say Square D.

Square D’s HOMCGK2C is the indoor Homeline kit for 150- to 225-amp main-breaker load centers. It interlocks the QOM1 or QOM2 main with a two-pole branch breaker back-fed from the generator. If that sentence does not describe the label inside your door, this is not your kit.

Outdoor Homeline 100- to 125-amp covers use a different number (HOMCRBGK1C is the common one). Outdoor 150- to 225-amp uses HOMRBGK2C. Eaton, Siemens, and GE have their own lists. Take a photo of the panel label, including the series letter, and send it before anyone orders steel.

Do not buy the $12 “compatible with HOMCGK2C” clone because it is cheaper than the OEM accessory. If the inspector wants the listed kit, the clone is scrap. If your electrician has a third-party kit they have passed in this town, they will name it. Let them.

The kit is not the whole job. You still need the two-pole generator breaker, the retainer, the inlet, the cord that matches that inlet, and load discipline. HomeGuide’s $400 to $850 installed is the realistic pile, not the $90 Amazon line.

What goes wrong?

A cord in the dryer outlet. That is backfeeding. It is illegal, it can kill, and no interlock on the panel fixes a plug in the wrong receptacle. The inlet on the wall is the legal connection.

The wrong kit on the right panel. Spacing between the main and the generator breaker is not universal. A 1-3/8-inch slider on a panel that needs 3/4-inch either binds or does not block. OEM kits exist because the geometry is specific.

No spare slot. An interlock needs a two-pole breaker in a position the plate can actually block. A full panel needs tandem breakers, a subpanel, or a transfer switch instead. That is a different quote.

No permit. The cheap install that skips inspection is the one your insurer may argue about after a fire. The legal story is listed equipment plus a signed inspection, not a forum post.

Treating the interlock like an automatic switch. You still start the generator, you still throw the main, you still slide the plate. If that sequence is too much for the people who live here, pay for automatic.

How do I decide this week?

1. Photograph the label inside your panel door: brand, series, amp rating, indoor or outdoor.
2. Ask a licensed electrician whether this town accepts a listed interlock on that panel, or wants a transfer switch.
3. If they say interlock, have them order the OEM kit for that series, not a universal plate, and include the inlet and the generator breaker in the price.
4. If they say transfer switch, decide whether you want a small manual switch for selected circuits or an automatic switch for a standby generator. HomeGuide’s $400 to $1,300 vs $600 to $2,500 is the split.
5. Write down who will start the generator in the dark. If the answer is “nobody, we travel,” stop shopping interlocks.

A legal portable hookup is an inlet, a listed transfer method, and a person who will run it. The $15 plate is only the middle piece, and often the wrong one.