How to Lay Out a Standby Generator Site for Clearances
Why this matters
A standby generator is an engine with an exhaust outlet, parked a few feet from a house. Where the pad goes decides whether exhaust can reach a bedroom window, whether the unit can be serviced without unbolting it, whether the gas utility will set the meter, and whether the inspector passes the job or orders the pad moved after the concrete has cured. Moving a set pad is the most expensive mistake in a standby install, and it is decided in the half hour with a tape measure before anyone digs. This card owns siting distances for the shelf. Noise and property-line rules are 'Sound Attenuation and Setback Standard'; exhaust and CO practice is 'Exhaust Clearance and CO Safety Standard'.
Before you start
Bring: the maker's installation manual for the exact model (the clearance page and its drawing), a 25 ft tape and a long tape or measuring wheel, marking paint and flags, a camera, and the site-walk notes showing where the gas meter, electric meter and transfer switch are.
Check first: that the one-call utility locate has been requested and marked before any stake, rod or shovel goes into the ground. If a buried gas line is struck during pad work, everyone moves away from it, nobody starts an engine or uses a phone near it, and the gas utility and 911 are called from a distance; a struck electric line means stay clear of anything touching it and call the utility. Layout itself only paints the grass; the locate protects whoever digs next.
Know the source of every number. Clearances come from the maker's manual for that model and from the codes as adopted locally, primarily NFPA 37 (stationary engine installation), NFPA 70 (electrical working space), the utility's meter rules, and zoning. Where they differ, the most restrictive governs. The figures in the worked example are this example's; read yours off the documents.
Step 1: Read the clearance page and write each value down
Open the manual to its clearance drawing and copy every distance onto your layout sheet with its condition: from what to what, measured how. Typical entries are a minimum distance from the house wall, a minimum distance from any window, door, opening or air intake measured in any direction, clear space in front and on service sides, clear space overhead, and distances from combustible materials.
Why: makers obtain their reduced clearances by testing a specific enclosure. The listing that lets a unit sit close to a wall belongs to that model; the neighbor's unit from another maker may need more.
The code baseline behind it. NFPA 37, in the edition your jurisdiction has adopted, in substance requires an outdoor engine to keep a minimum distance from openings in walls and from structures with combustible walls, commonly 5 ft, with reductions allowed where the walls are fire-rated or the enclosure has been tested and approved for less. Check the current edition and chapter 4 for the exact conditions. That is why many residential manuals keep 5 ft from openings even where they allow the unit itself much closer to the wall.
What breaks if you skip it: you lay out from memory of a different model, and the inspector measures against the manual in the unit's door.
Step 2: Fix the points that cannot move
Mark the gas meter, the electric meter, the transfer switch location, and the route the gas line and conduit will take. These set the practical zone: short runs to both meters, and a unit position that does not block either.
Why: the utility owns the meter set and its rules. Many gas utilities require a stated distance between the meter or its regulator vent and an ignition source, and a running engine is one. The electric meter and the transfer switch need clear working space in front of them; NEC 110.26, as adopted by your jurisdiction, in substance requires working space in front of electrical equipment likely to be examined or serviced while energized, commonly 3 ft deep at residential voltages. The generator's own breaker and controls need the same.
Step 3: Mark a trial position and measure every opening
Paint the four corners of the unit's footprint at the trial position, the wall distance first. Then measure from the nearest point of the unit to every opening within reach of the manual's distance, in any direction: windows (including basement windows at grade), doors, soffit and gable vents above, dryer and bath vents, the furnace or water heater combustion-air intake, a crawlspace vent.
Why "any direction": exhaust rises and drifts. An opening 3 ft to the side and 4 ft up is 5 ft away in a straight line, and a soffit vent directly overhead is the most commonly missed opening of all.
Why measure, not estimate: the margin on a typical house wall is a foot or two. Pacing it off is how a window ends up at 4 ft 6 in.
Step 4: Check service access and overhead
Confirm the clear space the manual requires on the front and service sides. A service tech needs to open the lid or panels, reach the oil drain, the filter, the battery and the controller, and pull the unit's top or side off for repair. A unit squeezed against a fence or an AC condenser meets the letter of a wall clearance and cannot be serviced.
Look up. No overhang, deck, soffit vent or tree limb over the unit unless the manual allows it, and no air conditioner condenser blowing into its intake. Note drainage, flood level and snow drift: a unit that sits in a puddle or a drift fails in exactly the weather it exists for.
Step 5: Check the property line, zoning and sound rules
Measure from the unit to the property line and compare it with the local zoning setback for mechanical equipment, if there is one. Ask the building department or the shop's permit file. Some localities also set a sound limit at the property line, and a homeowners' association may have its own rules. 'Sound Attenuation and Setback Standard' covers the noise side.
Why last: it is the check most likely to be different in the next town, and the one an office can confirm before the install crew arrives.
Step 6: Mark the pad and photograph it
When every distance passes, re-paint the corners, flag them, and photograph the layout with the tape stretched to each opening. The photos go in the job file with the measured distance table.
Worked: one wall, one move
The back wall of a two-story house on natural gas. Say the manual gives: 18 in from the house wall, 5 ft from any window, door, opening or air intake in any direction, 3 ft clear in front and on each service end, open sky overhead. Say the gas utility requires 3 ft from the meter, the electric side keeps 3 ft of working space in front of the meter and transfer switch, and the town sets a 10 ft side-yard setback for mechanical equipment.
First trial. The unit was painted between the gas meter and the back door, 18 in off the wall. Measured from the nearest edge of the unit along the wall: gas meter 6 ft west, window W1 4 ft west, door D1 11 ft east, electric meter 13 ft east. W1 at 4 ft fails the 5 ft rule by 1 ft.
The move. Sliding the unit 2 ft east fixes W1 without reaching D1. Every distance changes, so every distance is measured again rather than adjusted on paper:
PLAN VIEW, back wall (north is up)
........ PROPERTY LINE ......................
| 14 ft to line
+---------+---------+
3 ft | GENERATOR | 3 ft
clear | exhaust faces N | clear
+---------+---------+
| 18 in to wall
==+=====+=============================+=====+== WALL
| | | |
GAS W1 D1 ELEC
METER window door METER
8 ft 6 ft 9 ft 11 ft
(unit edge to item, along the wall)
No eave or vent over this stretch
Re-measured. Gas meter 6 plus 2 is 8 ft, W1 4 plus 2 is 6 ft, D1 11 minus 2 is 9 ft, electric meter 13 minus 2 is 11 ft, and the tape confirmed each. The second-story windows above W1 are 9 ft up and 6 ft over, more than 5 ft in a straight line. The unit faces the yard with its exhaust pointed north, away from the wall, and the property line is 14 ft away.
| Item | Required | Measured | Result |
|---|---|---|---|
| House wall | 18 in | 18 in | Pass, at the minimum |
| Window W1 | 5 ft | 6 ft | Pass by 1 ft |
| Door D1 | 5 ft | 9 ft | Pass by 4 ft |
| Overhead | Open | Open | Pass |
| Gas meter | 3 ft | 8 ft | Pass by 5 ft |
| Electric meter working space | 3 ft clear in front | Unit 11 ft away | Pass |
| Service ends | 3 ft each | 3 ft each | Pass, at the minimum |
| Property line | 10 ft | 14 ft | Pass by 4 ft |
Two items sit exactly at their minimum. That is a pass, and it is also where a pad poured 2 inches off the paint fails. The crew is told which two lines have no slack.
What would change it. A propane tank instead of natural gas brings a third party: the propane supplier sets the tank location under NFPA 58 as adopted, including its distance from the generator as an ignition source, and that can move the unit. A neighbor's bedroom window 6 ft past the property line is not a code clearance, but exhaust does not stop at the line; point the exhaust away from it and confirm with the CO survey at commissioning.
Finish test: is the layout right?
- Every value on the manual's clearance page has a measured line in the table, with its source.
- Every opening within the manual's distance was measured in a straight line, including above and around corners, and is on the photo.
- No item fails, and the at-minimum items are flagged for the pad crew.
- The meters and the transfer switch keep their working space.
- Zoning and sound were confirmed with the building department or the permit file, not assumed.
The layout proves the geometry. The run proves the air: at commissioning, with the unit running, CO is read at each opening per 'How to Survey Carbon Monoxide at Openings During a Generator Run', and the readings go on 'Standby Generator Commissioning Record'.
References
- NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, Chapter 4, in the edition your jurisdiction has adopted; check the current edition.
- NFPA 70, National Electrical Code, 110.26 (working space), as adopted by your jurisdiction; NFPA 58 for propane tank siting where it applies.
- Generator maker's installation manual clearance page, and the gas utility's meter installation rules.
- See related: Sound Attenuation and Setback Standard; Exhaust Clearance and CO Safety Standard; How to Survey Carbon Monoxide at Openings During a Generator Run.