Interlock kit

How the plate actually works

The kit is a steel plate captive on the panel’s deadfront cover, cut so that it can occupy only one of two positions. In one position it covers the slot where the generator breaker’s handle would move, so that breaker cannot be switched on; in the other it covers the main breaker’s handle. Moving the plate from one to the other requires switching the main off first. There is no electronics, no sensing, and nothing to fail — the geometry of a piece of metal is the entire safety mechanism, which is why it is accepted as transfer equipment under NEC 702.5.

That also explains why the kit must be listed for your exact panel. The plate, the cover it rides on, the throw of the breaker handles, and the slot spacing all have to match to within a couple of millimetres. A generic plate on a drilled-out cover is not a listed assembly, will not pass inspection, and can leave enough travel for both breakers to be on together. Some panel families — certain obsolete Zinsco and Federal Pacific designs, and some split-bus panels — have no listed interlock available at all, in which case the answer is a transfer switch or a new panel, not improvisation.

On the electrical side, a standard two-pole breaker is installed in the panel and wired out to the exterior inlet; power flows backwards through it into the busbar. Breakers pass current in either direction, so this is fine, but a plug-on breaker that is back-fed must be held in place by an additional fastener — the hold-down kit sold for the panel — so it cannot be pulled off a live bus. That fastener is a standard inspection item and a standard omission on DIY installs.

The 120% rule does not apply here

A recurring confusion is whether the generator breaker plus the main breaker may exceed the panel’s busbar rating. The rule people are thinking of comes from NEC Article 705 and governs power sources that operate in parallel with the utility — rooftop solar, most commonly — where the busbar really can be fed from both ends at once and the 120% allowance limits how much.

An interlocked generator is not a parallel source. The plate guarantees that exactly one of the two breakers is closed at any moment, so the busbar never sees both. Article 705 does not apply, and the generator breaker is not constrained by the main breaker’s size. What does constrain it is everything downstream of the generator: its own output rating, the cord, the inlet, and the conductors between the inlet and the breaker.

Operating one: the manual load management you sign up for

The sequence matters and it is worth writing on a card taped inside the panel door. Switch the main off. Switch every branch breaker off, or at least all the large ones. Start the generator outside, at least twenty feet from the house with the exhaust pointed away, and let it warm up. Plug in the cord. Close the generator breaker, slide the interlock, and then bring branch circuits back one at a time, largest motor first. Reverse the order to shut down. Closing the generator breaker into a fully loaded panel means every load inrushes at once, which will stall or trip the generator.

You get no instrumentation. A manual transfer switch has wattmeters on both legs; an interlock gives you a panel that looks exactly as it always did. A clamp meter on one of the inlet conductors, or a plug-in energy monitor, or a whole-panel monitor is what turns this from guesswork into management. Know the generator’s running rating and aim to stay under roughly 80% of it continuously, leaving the rest as surge headroom.

Some circuits must simply stay off. An electric range, an electric dryer, an electric water heater, and a central air conditioner will each consume a portable generator on their own. Tape those breakers off or fit breaker lockouts, because the failure mode is somebody helpfully doing laundry. Remember too that on a 30 A inlet the entire panel is limited to 7,200 W at 240 V regardless of what the generator is capable of producing.

Interlock or transfer switch: choosing honestly

The interlock wins on cost and flexibility. It is typically a quarter to a third the installed price of a manual transfer switch, and because the generator feeds the whole busbar, any circuit in the panel is available — including ones you did not think of when the system was installed. If your needs change, you change which breakers you turn on, not which circuits are wired to a subpanel.

The transfer switch wins on discipline and acceptance. Its circuit selection is fixed, which is a limitation and also a guardrail: you cannot accidentally energise the range. It has built-in metering. It is accepted by every inspector, everywhere, which an interlock is not. And it is more forgiving of an operator who does not really understand the system.

The interlock is the wrong choice in four specific situations: when someone who will not remember the sequence may have to operate it alone, when the panel is full or has no free slot in the required position, when no listed kit exists for the panel brand, and when the local authority having jurisdiction will not approve one. It is also not a route to automatic operation — nothing about an interlock can be automated, and a hands-off system means a standby generator and an automatic transfer switch.