NFPA 37's combustible-wall clearance behind a standby unit is 5 feet by default, not 3 — reducible to 18 inches only with specific tested/listed support; the 3-foot figure governs the front and sides, including vegetation. A portable generator's 20-foot clearance from the house is CPSC's separate rule, and a state noise ordinance — like Washington's 55 dBA daytime / 45 dBA nighttime residential limit — can force the unit further back than either code does.
NFPA 37's actual clearance numbers, and where they get conflated
A municipal summary of NFPA 37's standby-generator clearances lays out three separate measurements that are easy to blend into one number. Clearance from any operable wall opening — a window, door, vent or window well — is 5 feet, to keep carbon monoxide out of the house. Clearance from the back of the generator to a combustible stationary wall or building is also 5 feet by default; it can drop to 18 inches only if that specific unit is supported by Southwest Research Institute testing or the manufacturer's own listed installation instructions. Separately, clearance at the front and both ends of the unit — including trees, shrubs and bushes that could obstruct airflow or be damaged by exhaust heat — is 3 feet.
The 3-foot figure is real, but it applies to the front/side vegetation clearance, not the combustible-wall distance behind the unit. If your installer or your own planning is working from "3 feet from a combustible structure," confirm which of the two measurements is actually being met — getting them swapped understates the wall clearance by 2 feet unless your specific model carries the listing that allows 18 inches.
A portable generator's clearance is a different rule entirely
CPSC's portable-generator guidance is not interchangeable with NFPA 37's standby-unit clearances: it instructs owners to operate a portable generator outside only, at least 20 feet away from the house, with exhaust directed away from the home and other buildings. CPSC also reports that more than 80 consumers die each year from carbon monoxide poisoning caused by portable generators, and recommends battery-powered or battery-backed CO alarms outside every sleeping area and on every floor, tested monthly. If you're weighing a standby installation against keeping a portable unit for backup, don't apply one device's clearance rule to the other.
What a state noise ordinance actually specifies: Washington's numbers
Noise limits are set locally, and they vary — but seeing one state's actual numbers shows what "locally set" can mean in practice. Washington's statewide noise rule (WAC 173-60-040) sets a residential-source-to-residential-receiver daytime limit of 55 dBA, reduced by 10 dBA between 10 p.m. and 7 a.m. — a 45 dBA nighttime limit. The same table allows a residential source to reach 57 dBA daytime at a commercial receiving property and 60 dBA at an industrial one.
| Receiving property class | Daytime limit | Nighttime limit (10pm-7am) |
|---|---|---|
| Residential (Class A) | 55 dBA | 45 dBA |
| Commercial (Class B) | 57 dBA | 47 dBA |
| Industrial (Class C) | 60 dBA | 50 dBA |
Washington State Legislature, WAC 173-60-040 — one state's specific rule, not a national figure.
This is a single state's rule, narrower and more specific than the broader city-sourced ranges cited elsewhere on this page — treat it as one concrete example of the kind of ordinance you should look up for your own jurisdiction, not as a national number.
What to know before you sign for the installation
A standby generator installation commonly runs well past the dollar thresholds that trigger state consumer-protection rules for home-improvement contracts. In Texas, any homestead improvement contract must carry a mandatory lien-notice warning, and a job over $5,000 legally requires the contractor to deposit your payments into a dedicated construction account, which you can ask to verify in writing. Texas law also makes clear that if your contractor doesn't pay its subcontractors or suppliers, your property can still be subject to a mechanic's lien for the unpaid amount — even though you never contracted with that sub or supplier directly. That's a Texas-specific regime, but the underlying lien risk is worth asking any electrician or generator dealer about, wherever you live.
Standby generators are also commonly sold through in-home sales visits and demonstrations. If an estimate is pitched and signed at your house, the federal FTC Cooling-Off Rule (16 CFR Part 429) gives you the right to cancel for a full refund until midnight of the third business day after the sale, for purchases of $25 or more made at your home. It does not cover a contract you sign at the dealer's own permanent place of business, or one agreed entirely online, by mail or by phone.
Methodology
NFPA 37 clearance figures are a municipal government's (Topsham, ME) plain-English summary of the code, used because NFPA's own free-access viewer requires creating an account; treat it as a reliable secondary restatement, not a primary-text quote. CPSC's 20-foot portable-generator figure and Washington's noise limits are each that agency's/state's own published numbers and are not interchangeable with NFPA 37's standby clearances. Texas contractor-protection figures and the FTC cooling-off rule are cited as a state example and a federal rule respectively, not universal generator-industry standards. Price the equipment and installation with the standby generator cost calculator.
Sources
- Town of Topsham, Maine — Standby Generator Clearance: NFPA 37 Requirements — accessed 2026-09-21
- US CPSC — What to Know About Generators and Carbon Monoxide (Pub. 468, rev. 03/2022) — accessed 2026-09-21
- Washington State Legislature — WAC 173-60-040, Maximum permissible environmental noise levels — accessed 2026-09-21
- Office of the Attorney General of Texas — How to Avoid Home Improvement Scams — accessed 2026-09-21
- Federal Trade Commission — Buyers' Remorse? The FTC's Cooling-Off Rule May Help — accessed 2026-09-21