Transfer switch vs interlock: connecting portable power to your panel

There is a gap in most plug-in backup plans that only shows up after the box arrives. The unit publishes 6,000 or 7,200 watts, and the extension cord in your hand reaches a refrigerator. The loads that actually make a house habitable in an outage, the well pump, the furnace, the air handler, the water heater, have no plug at all, and on several of these units the headline wattage is only available through a 240V-capable port that nothing in the house plugs into. Closing that gap takes one of two listed devices and a licensed electrician. This guide explains what each device is and what the published specifications say. We have not tested any of these products, and the Amazon links above are affiliate links, disclosed at the top of this page.

What a manual transfer switch is

A manual transfer switch is a small separate panel mounted beside your main one. During installation, a fixed set of circuits is moved out of the main panel and into the switch, and each of those circuits gets a toggle with two positions: utility, or generator. An inlet box mounted outside the house gives you somewhere to plug the power station in, wired back to the switch.

Reliance Controls is the reference product in this category, and it publishes real specifications for its Pro/Tran 2 kits:

• Circuits. The ten-circuit kit is configured as one 2-pole 20 A circuit, two 1-pole 20 A circuits and six 1-pole 15 A circuits. A six-circuit version exists alongside it.
• Rating. 30 A at 125/250 V, with a NEMA L14-30 power inlet.
• Source size. Reliance rates these kits for sources up to 7,500 running watts.
• What is in the box. The indoor transfer switch, an outdoor-rated power inlet box, a 10-foot outdoor generator cord with molded ends, and wire connectors. A non-metallic inlet box version exists for coastal installations where salt air is a corrosion problem.
• Listing and warranty. cULus 1008 Listed, assembled in the USA, with a published 5-year warranty.
• Enclosure. Metal, roughly 11.75 by 11.25 by 4.75 inches on the ten-circuit unit.

Reliance describes the function in its own words as back-feeding protection: the switch eliminates the danger associated with back-feeding during a power outage. That is the entire point of the device, and it is why a switch costs more than a cord.

One naming note worth knowing before you shop. Reliance publishes a Pro/Tran to Pro/Tran 2 cross reference in its own catalog, and it lists the older 31410CRK as the predecessor of the current 310CRK ten-circuit kit, with 31406CRK mapping to 306CRK on the six-circuit side. Both model numbers refer to the same ten-circuit, 30 A configuration.

What an interlock kit is

An interlock kit takes the opposite approach. Instead of relocating circuits, it leaves your panel alone and adds a metal plate over the breaker faces that makes two positions mechanically exclusive: the main breaker on, or a back-fed generator breaker on, never both. You feed the whole panel from the alternate source and manage the load yourself by switching individual breakers off.

The trade is straightforward. An interlock gives you access to every circuit in the house instead of a fixed six or ten, and it makes you responsible for not exceeding the source, because nothing in the arrangement stops you turning on the dryer. A transfer switch makes the decision once, at install time, and then cannot be got wrong at 2 a.m.

The constraint that catches people is that an interlock must be listed for the specific panel. Square D's QOCGK2C is described in its own product listing as a QO generator interlock kit for 150 A to 225 A indoor load centers, two pole, used with the QOM2 main breaker. That is three simultaneous matches: panel family (QO, not Homeline), amp range, and main breaker type. Square D publishes separate kits for Homeline panels and for other amp ratings. A plate that lines up with the breaker handles but is not listed for that panel is not a compliant installation, whatever it looks like when fitted.

Why the third option is not an option

The arrangement people reach for instead, a cord with a male plug on both ends running from the generator or power station into a dryer or range receptacle, has a name in the trade that tells you most of what you need to know. It is not a shortcut version of the two devices above. It is a different thing that removes the protection both of them exist to provide.

Volusia County, Florida publishes a residential generator installation handout that states the requirements directly, under NEC Article 702. Permanently installed equipment for connecting a portable generator must comply with the transfer equipment section. The manual transfer equipment "will require an interlock device suitable for the intended use," and that device "must be installed to prevent the inadvertent interconnection of the normal and the optional system supply." The connection device "must be a UL Standard 231 Listed Power Outlet." And, in one sentence: "The back-feeding of electrical outlets within the home is not permitted." The same handout reproduces NEC 702.8(A) on signage: "A sign shall be placed at the service-entrance equipment that indicates the type and location of onsite optional standby power sources." Source: Volusia County, Residential Generator Installations, read August 6, 2026. Two caveats: that document is written against the 2008 NEC, and it is one county's guidance rather than a national rule.

Three concrete problems sit behind that language. A double-male cord has live exposed pins on one end whenever the other end is energized. Nothing prevents the main breaker being closed at the same time, which puts your source in parallel with the utility. And when the utility is out, that path can energize the service conductors leaving your house. Federal guidance published on Ready.gov (read August 6, 2026) keeps generator connection deliberately simple: always connect the generator to appliances with heavy-duty extension cords. Anything beyond that is transfer equipment territory.

Your own authority having jurisdiction publishes the requirements that apply to you, and code editions differ by state and county. The consistent element across all of them is that transfer equipment is required and back-feeding is not permitted.

Choosing between them

• You know exactly which circuits matter and want it to be foolproof: a manual transfer switch. The circuit list is decided once by someone competent, and there is no way to overload the source by accident. This is the better answer if anyone other than you may operate it.
• You want access to every circuit and accept managing load yourself: an interlock kit, provided one is listed for your exact panel. This is also the smaller physical install, since nothing is relocated.
• Your source publishes more than a 30 A inlet can carry: neither of the consumer kits above is sufficient on its own, and the conversation becomes a backup panel with an electrician's load calculation. The EcoFlow DELTA Pro Ultra at 7,200 W continuous is already at the edge of the 7,500 running watt rating Reliance publishes for its 30 A kits.
• You want the battery and the hookup matched: the Jackery Explorer 5000 Plus listing bundles Jackery's own Smart Transfer Switch, which removes the sourcing question even though it does not remove the electrician.

What this does and does not solve

Transfer equipment solves reach. It does not add capacity, and it does not change what your battery can start. A 30 A switch happily passes a load your power station cannot support, and the surge arithmetic for motor loads is unchanged by anything in this guide: our well pump guide works through what published locked rotor figures do to that plan, and our 120V vs 240V guide covers why the headline watts live where they do.

It also does not make a plug-in unit into an installed system. Switching is manual on both devices here, which means someone has to be home and awake. Installed home batteries handle transfer automatically with published switchover times, and that path involves permits and a utility interconnection rather than an afternoon: our installation guide covers that process, and the critical loads panel guide covers the same circuit-selection exercise on the installed side.

Both devices are licensed electrician work. We publish no wiring instructions for either, and the manufacturer's installation literature plus your local permit office are the authorities on how the job is done.

Every figure above is a manufacturer publication or an authority's published guidance recorded with its stated basis: transfer switch specifications and the Pro/Tran cross reference from Reliance Controls' own product literature, interlock kit compatibility from Square D's own catalog listing, code requirements from a county building department's published guidance on NEC Article 702, and generator connection guidance from Ready.gov. None of it is a measurement we made, and none of it substitutes for the requirements your own inspector applies.