How far should a generator be from the house?

A standby generator can sit 18 in (457 mm) from the house wall at its back. It needs 3 ft (0.91 m) clear at the front and ends. And no window, door or vent may be within 5 ft (1.52 m) of any point of it. Those are the figures in the Generac and Briggs & Stratton manuals. A portable generator is a different machine with a different rule: run it at least 20 ft (6.1 m) from the house, with the exhaust pointed away. That is the advice of the US Consumer Product Safety Commission (CPSC) and the CDC. Kohler's print for one standby model asks 5 ft from a wall that can burn. Three makers are compared below. The rest of the yard is in outdoor room dimensions.

Standby or portable: two machines, two distances#

A standby generator is fixed in place outside the house. It sits in a metal enclosure on a pad of gravel or concrete. It runs on natural gas or propane from a fixed pipe. When the mains power fails, it starts on its own and a transfer switch moves the house onto it.

A portable generator is an engine and an alternator on an open frame or in a light case, often on wheels. It burns petrol, or sometimes propane from a tank. You wheel it out, start it by hand and plug things into it.

Both burn fuel, and both make carbon monoxide (CO). CO is a gas you cannot see or smell. CPSC calls it the "invisible killer", because it "can kill in minutes". The gap between a generator and a house does two jobs. It keeps a fire in the engine from reaching the wall. And it keeps the exhaust from getting inside.

Three more terms decide the answer:

• A combustible wall is one that can burn: timber framing, wood or vinyl siding.
• An opening is any way into the building. Generac's list runs from windows and doors to soffit vents, dryer vents, furnace intakes, crawl spaces and cracks in the walls. It includes windows that do not open.
• NFPA 37 is the US standard for installing fixed engines, from the National Fire Protection Association. Every standby manual here is built on it.

NFPA 37: 5 ft by default, 18 in after a fire test#

Section 4.1.4 of NFPA 37 covers engines placed outdoors. Briggs & Stratton's manual sums it up: engines and their housings "must remain at least 5 ft (1.5 m) from openings in walls and at least 5 ft (1.5 m) from structures with combustible walls."

The standard then lifts the gap in two cases. One is a wall with a fire-resistance rating of at least one hour, meaning a wall built and tested to hold back a fire for an hour. The other is "a fire test" which "demonstrates that a fire originating at the engine will not ignite combustible structures."

That second case is where 18 in comes from. Generac had its enclosures tested by the Southwest Research Institute (SwRI), an independent lab. The tests looked for "the worst case fire scenario" inside the generator. They found that a fire inside the metal enclosure "would not pose any ignition risk to nearby combustibles or structures." So Generac sets "18 in (457 mm) from the back side of the generator and 3 ft (0.91 m) from the front and ends" to a combustible wall.

Briggs & Stratton ran its own full-scale tests. Its manual says a fire in the enclosure "would not pose an ignitability risk to nearby combustibles or structures for at least one hour." Its result is the same 18 in at the back.

Here is the line most pages miss. The tested reduction is for the wall, not the openings. Generac says it in so many words: "This offset reduction does not apply to clearances from unprotected openings in the structure." A window stays at 5 ft. The fire test was about flame, and a window lets in exhaust.

Three makers: Generac, Briggs & Stratton and Kohler#

These are the figures as printed. Each is measured from the outside of the enclosure.

A screen fence must stay 3 ft (0.91 m) from the unit, whatever height a fence without a permit allows.

Three things stand out.

Generac and Briggs & Stratton agree almost line for line. The one wording gap is on openings. Briggs & Stratton says to "install the generator at least 5 ft (1.5 m) from all windows, doors, vents, window wells, or openings in the wall." Generac's current sheet defers to "locally adopted codes", then describes NFPA 37 as 5 ft from an opening. Read together, that is 5 ft.

Kohler's print keeps the NFPA default at the wall. The 20RESC drawing says "Combustible wall: 5 ft" and "Non-combustible wall: 18" min. 3 ft recommended." Its note gives the reason: NFPA 37 "states this distance should be at least 5 feet from a combustible material." On a timber or vinyl house, that puts this Kohler 42 in (1.07 m) further out than the other two. The print is dated 2020 and covers one model. Other Kohler models may print other figures, so read your own.

Kohler measures openings from the exhaust end. Its rule is that "the nearest window, vent, door or similar structure opening is at least 5 feet from the exhaust end." It adds that furnace and similar intakes sit "at least 10 feet from exhaust end of set". It also asks that the exhaust is "aimed away or parallel to structure" and "not directed at play areas, patios or other areas where people congregate." So place the unit before the seating in patio sizes and layouts, not after.

Worked through: 14 ft of blank wall for a 4 ft generator#

Take one side wall of a house clad in vinyl siding. It has a window near one end and a dryer vent near the other. Suppose the enclosure is 48 x 34 x 31 in (122 x 86 x 79 cm), length by width by height. That is the size Briggs & Stratton's brochure gives for its 15 to 20 kW units. Its long side runs along the wall.

Walk it through the clearances in the Briggs & Stratton 18 and 26 kW manual.

1. The back. It sits 18 in (457 mm) out from the siding. Vinyl siding burns, but the tested 18 in covers a combustible wall.
2. The ends. Each end needs 3 ft (0.91 m) clear of shrubs, fences and anything else.
3. The window. No point of the generator may be within 5 ft (1.52 m) of it. Measured along the wall, the nearest end of the enclosure sits 5 ft from the window's edge. That 5 ft already covers the 3 ft end gap.
4. The dryer vent. A dryer vent is an opening too, so the far end needs 5 ft from it as well.

Add those up along the wall: 60 in, then 48 in of generator, then 60 in. That is 168 in, or 14 ft (4.27 m) of wall with no opening in it. The 18 in at the back never decides anything here. The openings do.

Now swap in the Kohler 20RESC on the same wall. The back moves from 18 in to 5 ft (1.52 m) from the siding, because the wall can burn. Kohler measures the window and vent from the exhaust end, so point that end away from them. Any furnace intake must be 10 ft (3.05 m) from it.

Overhead: 5 ft above the top of the enclosure#

Generac and Briggs & Stratton use the same words: "Structures, overhangs, or projections from a wall above the engine generator or above the front, end, and rear clearances (A, B) must be at least 5 ft (1.52 m) vertical distance from the top of the engine generator."

Two things follow. First, the rule covers the clearance zones, not just the box. So an eave that reaches over the 18 in gap at the back counts, even if it stops short of the unit. Second, it is measured from the top of the enclosure, so a tall unit needs a higher overhang.

Take the 31 in (79 cm) tall Briggs & Stratton unit. Anything above it must be at least 31 + 60 = 91 in (2.31 m) over the pad. Suppose the soffit of a single-storey eave sits 8 ft (2.44 m) above the pad. That leaves 65 in (1.65 m), which passes by 5 in. The underside of a deck 7 ft (2.13 m) up does not pass.

The same overhead logic runs through the grill manuals, where no grill may stand under a combustible roof; see how far a grill should be from the house. Patio heaters print a figure above their tops too, in patio heater clearance. A raised deck above the spot has to clear 91 in at the bottom of its framing, not at its boards; the joist and beam depths that eat into that are in deck sizes and railing heights. Generac adds a rule for units set on a roof, platform or deck: the surface under the generator must be non-combustible for 12 in (30.5 cm) beyond it.

Portable generators: 20 ft, exhaust pointed away#

For a portable generator there is no tested enclosure and no maker's 18 in. The figure is 20 ft (6.1 m), and it comes from the federal safety agencies.

CPSC's guidance reads: "DO operate portable generators outside only, at least 20 feet away from the house, and direct the generator's exhaust away from the home and other buildings." The CDC says to use generators "outside, at least 20 feet away from doors, windows, and vents." Neither is a code distance. Both are the shortest gap the agency names.

CPSC's list of places where a portable must never run is just as specific: "inside a home, garage, basement, crawlspace, shed or on a porch." It adds "EVEN IF doors and windows are open." The garage case has been tested. CPSC's 2023 proposed rule cites tests in which two portables with a CO shutoff ran in an attached garage with the bay door fully open. Neither shut off. Both still raised CO in the living space to levels "that would have caused injuries to the home's occupants." The door between the garage and the house, set out in garage to house door and step dimensions, is not a seal against that. A garden building is no better, whatever its size in how big a shed can be without a permit.

The scale of the problem is in the same rule. CPSC counted an average of 85 generator CO deaths a year from 2017 to 2019. Of the 1,332 deaths in its records from 2004 to 2021, 79 were reported with the generator running outdoors. Outdoors lowers the risk. It does not remove it.

Why "3 feet" also circulates#

Portable generator manuals print a much smaller figure, and it is easy to read as the answer. Honda's EU22i manual says to "keep the generator at least 1 meter (3 feet) away from buildings and other equipment". Champion's model 42451 manual asks 5 ft (1.5 m) from combustible material and 3 ft (91.4 cm) on all sides "to ensure adequate cooling."

Those are fire and cooling gaps. Honda's sits under the heading "Fire and Burn Hazards". For the exhaust, both manuals give a rule, not a number: Champion says "away from doors, windows and other vents", and Honda says never "near open windows or doors". The 20 ft is the CO figure. The 3 to 5 ft is the fire figure. A portable needs both, and 20 ft covers both.

On a narrow plot this changes where the generator can go. A side yard 10 ft (3.05 m) wide cannot hold a portable 20 ft from the house. A standby unit fits there, 18 in from the wall. A portable has to go to the front or back garden, past the end of the house.

Which distance applies#

This table restates the rulings above. Each row points to the source behind it.

Where these figures stop applying#

The figures here are for the models named, from the documents linked. They are the makers' and agencies' figures, not a code that applies everywhere. Kohler's print says it directly: "The recommended distance from a structure is dependent on state and local codes." Your city, your gas utility or your inspector can ask for more. Generac also tells you to check with local utilities before choosing a spot. This page does not cover gas meter or electric meter spacing. It also does not cover a generator inside a building or on a roof, beyond the surface rule above, or any commercial unit.

Meeting every distance does not make the exhaust safe. Generac warns that a unit placed to NFPA 37, "including the offset reduction validated through testing by SwRI, may still allow CO within the structure". It may need to go further out. Corners, alcoves, courtyards and fences can collect exhaust, by Generac's own list. Briggs & Stratton says CO detectors "MUST be installed" indoors, Generac recommends one on each level, and CPSC says to fit CO alarms on every floor, whatever the generator. Generac also notes that common smoke alarms do not detect CO.

Other things on the same wall keep their own rules. A dryer vent's terminal must itself be 3 ft from openings, as dryer venting dimensions and clearances sets out. A hot tub measures from the water to the sockets and switches on the wall, in hot tub clearances and pad. For service space, Generac points to NEC Article 110.26, the article behind the working space in front of a panel in EV charger placement and heights. Metric figures are the documents' own where they print them, and conversions otherwise, as imperial and metric dimension systems explains; check your own with the unit converter. Terms such as combustible are defined with others in the glossary, and the codes this site uses are listed under standards and codes.