Portable generator interlock and backfeed safety field guide

What a safe generator connection actually is

A safe portable generator connection is a wiring method that lets the generator power a building's circuits while making it physically impossible for that generator to feed power back onto the utility line. That isolation is the whole job. Everything else, the sizing, the cord, the grounding, hangs off the rule that the generator and the utility can never be connected to the building at the same time.

Two methods do this in a code-compliant way. A transfer switch breaks the utility connection before it lets the generator feed the load, and a listed interlock kit on the panel uses a mechanical slide that stops the main breaker and the generator breaker from being on together. Both force a clean break from the grid. Neither relies on you remembering to flip a breaker in the right order, which is exactly why they pass and a cord does not.

The deadly way is to plug a generator into a regular outlet with a cord that has a male plug on both ends, energizing the house wiring through the receptacle. That sends power backward through the panel, out the service, and onto the utility line, where it can kill a lineman who thinks the line is dead. It also fries the generator when utility power returns. The panel itself, its capacity and its breakers, is covered in the service panel upgrade guide, and the larger automatic standby approach is covered in the standby generator and transfer switch guide. This guide is the portable side: the interlock, the inlet, and the connection a homeowner or a small crew actually makes during an outage.

What is generator backfeed?

Backfeed is electricity flowing the wrong way through a building's wiring, out through the service, and onto the utility distribution line that is supposed to be dead. A generator connected without isolation does this. When you energize the house wiring while the main breaker is still closed to the utility, the generator pushes power back through the panel and the service drop to the transformer on the pole, which steps that voltage back up to the thousands of volts on the primary line.

That is what kills the lineman. During an outage, crews work the downed line believing it is de-energized, and a single house backfeeding through its transformer can re-energize a span of primary at lethal voltage with no warning to anyone on it. There are documented cases of a lineman dying after a homeowner started a backfed generator while restoration was underway. The person most at risk is often a worker a mile away who has no idea your generator is running.

Backfeed destroys equipment too. When the utility restores power, the generator is suddenly paralleled with the grid out of sync, and the resulting fault wrecks the generator, can start a fire at the panel, and can injure whoever is standing near it. The grid does not wait for you to disconnect. This is why the code does not treat isolation as optional and why every legitimate connection method exists to make backfeed physically impossible, not merely unlikely.

The suicide cord

A suicide cord is a cord with a male plug on both ends, used to feed a generator's output into a wall receptacle, usually a 30 amp dryer or range outlet, so it backfeeds the house. The trade name is not dramatic for effect. These cords kill people, and electricians react to them on sight.

Start with the obvious hazard. The moment the generator end is plugged in and the set is running, the prongs on the other end of the cord are live at 120 or 240 volts and completely exposed. Anyone who picks up that cord, or a child who finds it, contacts energized prongs with nothing between them and the conductor. Then there is the backfeed: a dryer outlet has no isolation from the rest of the panel, so feeding it sends the generator straight through the main breaker and onto the utility line.

These cords are effectively illegal. The NEC and OSHA both require that energized parts not be left exposed and that a generator be connected through a means that prevents simultaneous utility and generator connection, and a male-to-male cord into a receptacle violates both. There is no jobsite, no outage, and no shortcut where this is acceptable. If someone offers to wire your house this way to save the cost of an interlock, that is the moment to stop and call a licensed electrician.

The isolation rule

The rule behind every safe connection is one sentence: a generator tied to a building's wiring must be isolated from the utility by a method that makes simultaneous connection impossible. The NEC requires the connection to mechanically or electrically prevent the generator and the utility from feeding the panel at the same time. That is not a recommendation in an informational note. It is the requirement that drives which equipment is legal.

Mechanically means a physical barrier, like the sliding plate of an interlock kit, that cannot let both breakers be on. Electrically means a switch, like a transfer switch, whose contacts cannot connect the load to both sources. Either one satisfies the rule. A cord, a manual habit, or a promise to throw the main first does not, because the safety can be defeated by a single mistake, and the consequence of that mistake is a dead lineman.

Hold this rule above any clever workaround. The exact section and wording shift between code cycles and the jurisdiction adopts the edition, so confirm the citation with the AHJ, but the principle is stable across editions and it is the line that separates a legal install from a deadly one.

What is a generator interlock kit?

A generator interlock kit is a listed metal plate that mounts on the panel and physically prevents the main breaker and a back-fed generator breaker from being switched on at the same time. It is the affordable, code-compliant way to power an existing panel from a portable generator, and for many homes it is the method an electrician will reach for first.

The kit itself is inexpensive, often under fifty dollars for the plate, though the installed job with a generator breaker, an inlet box, and the cord typically runs several hundred to a few thousand dollars depending on the panel and the run. What you are buying is a mechanism that turns a back-fed breaker, which would otherwise be illegal, into a legal connection, because the interlock supplies the required prevention of simultaneous connection.

Use a kit listed for your specific panel. The plate is matched to the breaker layout and the panel make, and a generic or shop-fabricated plate is not a listed device and will not pass inspection. Manufacturers list kits for common panel lines, so confirm the kit is approved for the exact panel before ordering, and confirm the panel has room and a suitable breaker position. Most interlocks place the generator breaker at the top, opposite the main, so the slide can cover one or the other but never both.

How the interlock works

The mechanism is a sliding plate, and its simplicity is the point. With the plate in the normal position, the main breaker is on and the slide covers the generator breaker so it cannot be turned on. To switch to generator power, you turn the main breaker off, then slide the plate up, which uncovers the generator breaker and at the same time locks the main in the off position. Now the generator breaker can come on, and the main cannot.

Because the main is mechanically held off whenever the generator breaker is on, there is no path from the generator back to the utility. The slide enforces the order every time, with no electronics, no sensors, and no programming to fail. That is why electricians trust it: the failure modes of a complicated automatic system do not exist here, and the safety is a piece of steel that either lets a breaker move or does not.

The interlock feeds the whole panel through its bus, which is its strength and its catch. Every circuit in the panel is available, but the generator is now exposed to the entire panel load, so you have to manage what comes on by hand, turning off the large breakers before you start and leaving them off. The slide stops the backfeed. It does nothing to stop you from overloading the generator, which is the next section.

The manual transfer switch

A manual transfer switch is a listed device that moves the load between the utility and the generator, breaking the utility connection as it connects the generator. Where the interlock is a barrier added to a standard panel, the transfer switch is a purpose-built switch whose contacts physically cannot connect the load to both sources at once. It is the other code-compliant method, and on many jobs it is the cleaner one.

The common residential form is a small switch or a sub-panel wired ahead of selected circuits. You pick the circuits that matter during an outage, the furnace, the well pump, the refrigerator, a few lighting and receptacle circuits, and move them into the transfer switch. During an outage you flip each circuit, or a single main transfer handle, from utility to generator. When utility power returns, you flip back. The switch breaks the utility every time, so there is no way to backfeed.

Because the transfer switch only carries the circuits you moved into it, the generator can only ever be asked to power those circuits. That built-in limit is the reason a survey of residential electricians leans toward the transfer switch for whole-home readiness, while the interlock is the budget choice for an older panel where adding a sub-panel is impractical. The right answer depends on the panel and the loads, which is the next decision.

Transfer switch or interlock: which one?

Choose by what you want to power and what the panel allows. An interlock kit makes the whole existing panel available and costs less, but it puts the entire panel in front of the generator, so you manage the load by hand and you risk overloading the set if you forget to leave the big breakers off. A manual transfer switch limits the generator to a chosen set of circuits, usually six to ten, so it cannot be overloaded by accident, but it costs more and only powers what you wired into it.

The interlock wins when the panel is a listed match, has a spare breaker space opposite the main, and the owner wants access to any circuit on a larger generator. It loses when the panel make has no listed kit, when there is no room for the generator breaker, or when the owner is not the type to manage loads carefully. The transfer switch wins when you want a foolproof, fixed set of essential circuits and the owner wants to flip a handle and not think about it.

There is no universal best. A budget-minded homeowner with a compatible panel and a habit of turning things off is well served by the interlock. A family that wants a no-thought connection and a generator sized to a known list of loads is better served by the transfer switch. An electrician sizes the choice to the panel, the generator, and the person who will operate it. Both are legal. Neither tolerates a shortcut.

What about an automatic transfer switch?

An automatic transfer switch, an ATS, does the same isolation without a person. It senses the utility outage, signals the generator to start, and moves the load over on its own, then reverses when utility power returns. There is no manual step, which is exactly the point of a permanent standby system feeding a fixed generator.

That automatic approach belongs to the standby generator world, not the portable one. A portable generator is wheeled out, started by hand, and connected with a cord through an inlet, so the manual interlock or the manual transfer switch fits how it is actually used. An ATS pairs with a permanently installed standby set that is wired, fueled, and ready, and it carries its own commissioning, its own neutral and grounding decisions, and its own code articles.

If the goal is a system that rides through an outage with nobody home, that is the standby generator and transfer switch route, covered in its own guide. If the goal is to power a house from a portable generator you start during the storm, stay here with the interlock or the manual transfer switch. The two solve different problems, and trying to make a portable set behave like a standby system usually means you should have bought the standby system.

The power inlet box

The connection point for the generator cord is a power inlet box, a flanged inlet, not an outlet. A regular receptacle has slots and is designed to receive a male plug. The inlet has the prongs facing out behind a weatherproof flange, so the cord from the generator plugs into it and the only energized pins are inside the connector when it is mated. There is nothing exposed to touch, which is the whole reason it replaces the suicide cord.

The inlet mounts outdoors, on an exterior wall near where the generator will sit, and it is wired back to the generator breaker on the interlock or to the transfer switch. It carries a weatherproof, in-use rating because it lives outside and gets used in bad weather. The NEC also calls for a warning sign at the inlet indicating the type of derived system the wiring creates, so the next person knows how the neutral is handled.

Match the inlet to the generator's outlet and the cord. A common residential setup is a 30 amp or 50 amp inlet with a twist-lock configuration that matches the generator, such as an L14-30 for a 30 amp 120/240 volt set. For a portable generator rated 15 kW or less connected through a flanged inlet located outside the building, the code commonly does not require a separate disconnect at the inlet, but confirm the rating, the configuration, and the disconnect rule against the adopted edition and the AHJ before you buy the box.

What size generator and connection do I need?

Size the generator and the connection to the load you actually intend to run, not to the panel's total capacity. Add up the running watts of the circuits you need during an outage, the furnace blower, the well pump, the refrigerator, some lights and outlets, then account for the starting surge of anything with a motor, because a well pump or a refrigerator compressor pulls several times its running watts for the first instant of starting. The generator has to swallow that surge without sagging out.

The connection has its own rating. A 30 amp inlet and breaker support a 120/240 volt portable set up to roughly 7,500 running watts, and a 50 amp setup supports a larger one. The breaker that back-feeds the panel through the interlock, or the transfer switch rating, is matched to the generator's output, not to the panel's main. The conductors from the generator output to the first overcurrent device are sized per the generator's nameplate under the generator rules, commonly at not less than 115 percent of nameplate current, which is the manufacturer's and the code's number to confirm.

The honest version is that a portable generator will not run a whole house at once, and the sizing exercise is really about deciding which loads matter and whether the set can carry them with surge headroom. Confirm the wattage figures against the generator's data plate and the appliance ratings, and confirm the breaker and conductor sizing against the NEC and the AHJ. Undersize the set and it stalls or overheats. Oversize the connection and you have spent money the load never needed.

Managing the load during an outage

With an interlock, load management is a manual discipline, because the whole panel is live in front of the generator. Before you start the set, turn off the large breakers, the electric range, the dryer, the water heater, the central air, the EV charger, anything that draws heavy. Bring the generator up, throw the generator breaker, then turn on only the circuits you need, adding the larger ones one at a time and watching the generator hold its speed.

Sequence matters. Start the biggest motor load, the well pump or the air handler, when little else is on, so its surge has room. Then add the smaller steady loads. If the generator bogs down, lugs, or its overload trips, you have asked for more than it can give, so shed a load before you ask it to recover. A generator stalled under an overload that is left to struggle is a generator you are damaging.

A manual transfer switch does this management for you by only carrying the circuits you wired into it, so you cannot accidentally land the dryer on a 5,000 watt set. That is the trade: the interlock gives you every circuit and the responsibility to manage it, the transfer switch gives you a fixed safe set and no choices. Decide which one fits the person who will run it at two in the morning in a storm.

Does a portable generator need a bonded or floating neutral?

This is the part that catches good electricians, and getting it wrong leaves either a missing ground reference or extra current riding on the grounding system. A generator has either a bonded neutral, where the neutral is connected to the frame inside the set, or a floating neutral, where it is not. Which one you need depends entirely on whether the connection method switches the neutral, and the two have to match.

Most interlock kits and many manual transfer switches do not switch the neutral. The generator's neutral lands on the panel's neutral bus, where the neutral is already bonded to ground at the service. In that arrangement the system stays non-separately derived and the generator must have a floating neutral, so there is only one neutral-to-ground bond, the one at the service. If you connect a bonded-neutral generator to a panel through a non-switching method, you now have two bonds, the one at the service and the one in the generator, and neutral current splits and rides back on the equipment grounding conductor. That is objectionable current on the ground, and it is exactly what the code is written to prevent.

When a transfer switch does switch the neutral, a switched-neutral or four-pole arrangement, the generator becomes a separately derived system and needs its own neutral-to-ground bond at the set, made once, under the separately derived system rules commonly cited at NEC 250.30. One bond, in one place. The catch is that most portable generators ship with a bonded neutral so their on-board GFCI receptacles work for jobsite and OSHA use, which means connecting that set to a non-switching house method usually requires removing or lifting the bond, a modification the manufacturer's instructions and the AHJ have to bless. Confirm the generator's neutral configuration, the switch's neutral handling, and the bonding against the manufacturer, the NEC, and the AHJ before you energize. The inspector checks this one.

Grounding and bonding the set

Grounding follows from the neutral decision and the way the generator is used. For a portable generator feeding the building through an inlet and a transfer means, the equipment grounding conductor in the cord and the building wiring ties the generator frame to the system ground at the panel, and the neutral-to-ground bond lives in exactly one place, set by whether the system is separately derived. The point of all of it is one low-impedance path for fault current so a breaker can clear a fault.

A portable generator that only powers cord-and-plug equipment plugged into its own receptacles is treated differently than one wired into a building. The code, commonly at NEC 250.34, often does not require a separate grounding electrode, a driven rod, for a portable set whose frame, receptacle grounds, and equipment grounds are bonded together, because the frame serves as the grounding point. Whether a rod is required, and how the bond is made, changes once that set is connected to premises wiring, so the rod-or-no-rod question is answered by how the generator is connected and what the AHJ requires.

Do not guess here and do not copy a setup from a different generator or a different connection method. The frame bond, the neutral bond location, and any grounding electrode are one connected decision that depends on the set's neutral configuration and the transfer method. Confirm the grounding scheme against NEC Article 250, the generator rules in Article 445, the manufacturer's instructions, and the AHJ, and record how it was grounded so the next person can troubleshoot it safely.

The GFCI complication

Portable generators in the 15 kW and smaller range commonly have GFCI protection on their 125 volt receptacles, required for the cord-and-plug use the set is built for, commonly cited at NEC 445.20. That protection is the reason most of these generators ship with a bonded neutral: the set is built and listed to run as its own system with a single neutral-to-ground bond at the generator, the ground reference the onboard GFCI is designed around.

The complication shows up when you wire that bonded-neutral, GFCI-equipped generator into a house through a method that does not switch the neutral. The second bond at the service gives neutral current a path back on the grounding conductor, the GFCI sees an imbalance between the conductor it expects current on and the current actually flowing, and it trips, sometimes immediately, sometimes under load. The generator looks broken when it is wired wrong.

There are two clean answers, and which one applies depends on the equipment. Use a floating-neutral generator, or a generator whose bond can be lifted per the manufacturer, with a non-switching interlock or transfer switch, so there is only one bond. Or use a transfer switch that switches the neutral with a bonded-neutral generator, making it a separately derived system with its single bond at the set, an arrangement built to keep GFCI-equipped, bonded-neutral generators happy. Match the generator and the switch deliberately, and confirm the GFCI behavior and the listing against the manufacturer and the AHJ rather than disabling protection to make a mismatch stop tripping.

Where do you run a portable generator?

Outside, always, well away from the building, with the exhaust pointed away from it. Carbon monoxide is the number one way a portable generator kills, ahead of any electrical hazard. The CPSC attributes roughly 100 deaths a year in the United States to CO from portable generators, and portable generators account for a large share of all consumer-product CO deaths. One generator can put out as much carbon monoxide as hundreds of cars, and at that concentration it kills in minutes.

CO is colorless and odorless, so there is no warning. People die running a generator in an attached garage, in a basement, on a porch, or just outside an open window, and they die even with the garage door open, because the gas pools and seeps into the living space faster than the opening clears it. Never run a generator indoors, in a garage, in a crawlspace, or in any partly enclosed area, no matter how much ventilation you think you have.

The CPSC guidance is to run the set outdoors only, at least 20 feet from the house, with the exhaust directed away from doors, windows, and vents. Pair that with working CO alarms inside the home, because the alarm is the last line of defense if the wind shifts the exhaust toward an opening. This is the hazard that gets people who did the electrical side perfectly. The connection can be flawless and the family can still die in their sleep if the set is run in the wrong place.

Placement, weather, and the run

Beyond the carbon monoxide distance, the set needs a spot that keeps it dry, level, and reachable. Put it on a firm, level surface so it does not vibrate and so its oil and fuel read right. Keep it out of standing water and rain, under an open canopy or a listed generator cover made to run in weather, never inside a structure to keep it dry, because dry-but-indoors is how the CO deaths happen.

Distance from the inlet drives the cord. The generator sits at the CO-safe distance, and the inlet box is on the wall, so the cord spans that gap. Plan the location so a single properly rated cord reaches without strain and without crossing a walkway where it gets stepped on or driven over. Keep the connectors up off wet ground.

Mind the fuel and the heat. Refuel a hot generator and spilled gasoline on hot engine parts is a fire, so shut it down and let it cool before adding fuel. Keep fuel containers away from the running set and any ignition source. None of this is exotic, and all of it is the part people skip at hour 30 of an outage when they are tired and just want the lights back.

The generator cord

The cord between the generator and the inlet is a single, listed, properly rated generator cord, not a chain of household extension cords. It has the twist-lock connector that matches the generator outlet on one end and the plug that matches the inlet on the other, and it is sized for the full current the generator can deliver over the length of the run.

Size the conductor to the load and the length. A 30 amp connection commonly uses 10 gauge copper for a run up to around 50 feet, with the gauge going up for longer runs so the cord does not overheat or drop excessive voltage. Undersized cord runs hot, and a hot cord under a load for hours is a fire that starts at the weakest connector. Use a cord rated for the amperage and built for the connectors, not a repurposed range cord with the wrong ends.

Daisy-chaining light-duty extension cords is the failure here. People reach for the orange cords in the garage, link three of them, and run a heater through connectors never meant for that load. The connectors heat, the undersized wire heats, and the assembly that was supposed to be temporary becomes the ignition source. One correctly rated cord, the right length, the right ends, is the only acceptable run from the generator to the inlet.

The permit, the AHJ, and listed equipment

A permanent generator connection, an interlock kit, a transfer switch, or an inlet box wired into the panel, is electrical work that needs a permit and an inspection in most jurisdictions. The permit is not bureaucracy for its own sake. It puts a second set of trained eyes on the isolation method, the neutral and grounding, and the listing of the equipment, which is exactly where a deadly mistake hides.

The inspector confirms that the connection prevents simultaneous utility and generator connection, that the interlock kit or transfer switch is listed for the application, that the inlet is rated and located correctly with its warning sign, and that the neutral and grounding match the connection method. Use listed equipment throughout, the panel-specific interlock kit, a listed transfer switch, a listed inlet, because a shop-fabricated plate or an improvised connection is not listed and will not pass, and listing is the evidence that the device does what safety depends on it doing.

Hire a licensed electrician for this work and pull the permit. The exact requirements, the articles that apply, and what counts as a permitted modification all sit with the adopted code edition and the AHJ, and the AHJ has the final say on the job. The cost of doing it right is a fraction of the cost of a backfed line, a panel fire, or a death, and the permit is the cheapest part of doing it right.

The install in order

The install follows a fixed order regardless of method, and it starts with the panel dead. Verify the panel is de-energized with a meter you just proved on a known source, because the rest of the work happens inside an enclosure that is lethal when it is live. Then mount the hardware: the interlock kit on the panel and the generator back-feed breaker in its position, or the transfer switch ahead of the selected circuits.

Run the wiring from the connection point to the inlet box on the exterior wall, sized for the generator's rating, in the right method for the run. Land the conductors, get the neutral handled to match whether the method switches it, make the grounding and bonding to the single-bond rule for the system type, and install the inlet with its weatherproof rating and the required warning sign indicating the derived-system type.

Then label and prove it. Label the generator breaker, the inlet, and the operating sequence so the person using it in an outage sees the steps. Confirm the interlock physically prevents both breakers being on, or that the transfer switch cannot connect both sources, by operating it before the generator ever connects. Call the inspection. The mechanical proof that backfeed is impossible is the heart of the acceptance, so demonstrate it rather than assume it.

The safe operation sequence

Operating the connection is a fixed sequence, and the order is what keeps it safe. Start at the panel: turn the main breaker off, isolating the house from the utility. With an interlock, slide the plate so the main is locked off and the generator breaker is uncovered. Turn off the large branch breakers so you do not slam the generator with everything at once.

Now the generator. Place it outside at the CO-safe distance with the exhaust pointed away from the building, connect the cord from the generator to the inlet, then start the set and let it stabilize. Turn the generator breaker on to energize the panel, then bring on your branch circuits one at a time, largest motor load first while the set is lightly loaded, watching the generator hold.

Shutdown reverses it. Turn off the branch loads, then turn off the generator breaker, then shut the generator down and let it cool before refueling. With an interlock, slide the plate back, which uncovers and allows the main, then turn the main on to restore the utility. The interlock will not let you turn the main back on while the generator breaker is still on, which is the mechanical safety doing its job. Never reverse the order to skip a step, because every shortcut in this sequence points back at the backfeed you spent the whole install preventing.

Common mistakes

• Using a male-to-male suicide cord into a dryer or range outlet, exposing live prongs and backfeeding the utility line.
• Connecting a generator with no interlock kit or transfer switch, relying on remembering to throw the main, which the code forbids.
• Mismatching the neutral, a bonded-neutral generator on a non-switching interlock, so neutral current rides the grounding conductor and the GFCI trips.
• Overloading the generator by leaving the large breakers on, or by powering more than a transfer switch's chosen circuits, until the set stalls or overheats.
• Running the generator indoors, in a garage, or just outside a window, where carbon monoxide pools and kills even with doors open.
• Daisy-chaining undersized extension cords from the generator instead of one properly rated cord to a power inlet box.
• Skipping the permit and the inspection, so nobody confirms the isolation, the listed equipment, and the neutral and grounding before it is energized.
• Using a shop-made interlock plate or a kit not listed for the specific panel, which is not a listed device and will not pass.

What to document

Record the connection so the next electrician and the next homeowner know how the system is built and how to run it safely. The record answers the questions that matter in a hurry during the next outage and the questions an inspector or a future technician asks about the neutral and the grounding.

Capture the connection method, the generator's neutral configuration and where the single bond is made, the inlet rating and location, the generator and connection sizing, the cord rating, the operating sequence, and the permit and inspection. The table below is the minimum a defensible record carries, and every value that depends on the code or the equipment should be confirmed against the NEC, the manufacturer, and the AHJ rather than assumed.

Field checklist

Standards and references

The NEC, NFPA 70, is where the rules live. Optional standby systems, which is what most portable home connections are, fall under Article 702, and the power inlet and the warning-sign and disconnect provisions for a portable connection sit there, commonly at 702.7 and the related sections. The requirement that the connection prevent simultaneous utility and generator connection is the controlling principle behind every legal method. Confirm the exact section against the adopted edition, because the wording moves between cycles.

Generators have their own article. NEC Article 445 covers generator installation, the conductor ampacity at not less than 115 percent of nameplate, and the GFCI requirement for receptacles on 15 kW and smaller portable generators, commonly cited at 445.20. Grounding and bonding follow Article 250, with the separately derived system bond commonly at 250.30 and the portable-generator grounding allowances at 250.34, and which of those applies depends on the neutral configuration and the connection method. OSHA rules reinforce that energized parts cannot be left exposed, which is the other reason the suicide cord is prohibited.

Above the code sit the equipment listing and the manufacturer. The interlock kit, the transfer switch, and the inlet are listed devices, and a kit must be listed for the specific panel. The generator's instructions set the neutral configuration, whether the bond can be lifted, and the sizing. The CPSC carries the carbon monoxide guidance, run outdoors only, at least 20 feet from the house, exhaust away. The AHJ adopts the edition, takes the local amendments, and has the final say. When a standard, the listing, and the manufacturer disagree, the stricter controlling document wins, and the AHJ decides what is enforceable. Never backfeed: use a listed interlock or transfer switch, handle the neutral, grounding, and GFCI per code, and run the generator outside, because carbon monoxide kills.

Units and terms

The portable-generator vocabulary reads differently across a manufacturer sheet, a code book, and a hardware aisle, and the same idea carries more than one name. Generator output is in watts (W) and kilowatts (kW), split into running watts for steady load and starting or surge watts for the motor-starting moment. Connection current is in amps, and the common residential connections are 30 amp and 50 amp twist-lock configurations such as the L14-30.

The terms below travel across the whole connection. Read the generator's manual and the kit's listing for how a given product names its neutral configuration and its ratings, and confirm any code-driven value against the adopted NEC edition, the manufacturer, and the AHJ before relying on it.

Backfeed

Electricity flowing the wrong way out of the building onto the utility line, which can electrocute a lineman and destroy the generator

Suicide cord

A cord with a male plug on both ends, used to backfeed a house, with live exposed prongs; illegal and deadly

Interlock kit

A listed sliding plate on the panel that mechanically prevents the main and generator breakers from being on at the same time

Transfer switch (MTS / ATS)

A device that moves the load between utility and generator, breaking the utility; manual on a portable set, automatic on a standby set

Power inlet

A flanged inlet, not an outlet, mounted outdoors so the generator cord plugs in with no exposed energized parts

Separately derived system

A source, such as a generator behind a switched-neutral transfer switch, that needs its own single neutral-to-ground bond per NEC 250.30

Neutral bonding

The neutral-to-ground connection, made in exactly one place; a second bond puts neutral current on the grounding conductor

Floating neutral

A generator with no neutral-to-frame bond, used with non-switching connections so the only bond stays at the service

Related tools

Calculators and readiness checks for this work

Compare your options

FAQ

People also ask

• What is a service upgrade?
• What size standby generator do I need?
• What is the 4 to 1 ladder rule?
• What is a MEWP?
• What is a qualified rigger?