Standby Generators & Transfer Switches
A standby generator is a permanently installed generator that starts by itself when utility power fails. It’s safer than a portable generator when installed correctly, but only if a transfer switch keeps its power from flowing back onto the utility lines, and only if exhaust stays out of the building.
- Electrical
- Homeowners
- Facilities
What a standby system includes
- The generator, usually run on natural gas or propane (sometimes diesel), on a pad outside.
- An automatic transfer switch (ATS) that senses an outage, disconnects the building from the utility, starts the generator and connects the building to it. When the utility returns, it switches back.
- Wiring and controls that tie the generator, the switch, the main panel and the fuel supply together.
- A manual transfer switch or interlock is used with some portable generators, but the principle is the same: the building and the utility must never be connected at the same time.
Backfeed: the deadly mistake
- If a generator is connected to a building’s wiring without a proper transfer switch, its power can flow backward onto the utility lines. A line that the utility thinks is dead becomes live.
- Line workers, neighbors and anyone near a downed line can be electrocuted. The transformer also steps the voltage up to a dangerous level.
- Never plug a generator into a wall outlet with a “suicide cord” (a cord with two male ends). Always use a transfer switch or interlock installed by a licensed electrician and approved under local code. See Downed Power Lines.
- A transfer switch also protects the generator and your appliances from damage when power returns.
Installation
- Have a licensed electrician and a licensed gas fitter or contractor install it, with permits and inspections as local rules require. See Building Permits & Inspections.
- Wiring and installation follow the National Electrical Code, including the rules for optional standby systems, and the manufacturer’s instructions. Emergency or legally required systems for buildings follow stricter codes.
- Placement: outdoors, on a firm, level pad, with the clearances from the building, windows, doors, vents, fuel meters and property lines that the manufacturer, local code and fuel supplier require. Many manufacturers specify at least several feet from the building and from openings, so check the manual and local rules.
- Keep the air intake and exhaust clear of snow, leaves, mulch and plants. Don’t enclose it unless it’s designed for that.
- The fuel line needs a licensed installer. Check for leaks and sizing. See Gas Leaks.
Carbon monoxide and fire
- Generator exhaust has carbon monoxide (CO), a colorless, odorless gas that kills. Keep exhaust away from windows, doors and vents.
- Install CO alarms in the home or building, and test them. See Smoke & CO Alarms and Carbon Monoxide.
- The unit gets hot. Keep combustibles away and keep children and pets back.
- Know where the gas shutoff is. See Shutting Off Utilities.
Safe use and upkeep
- Let the system run its weekly self-test (many units exercise themselves) and check the controller for alarms.
- Follow the manufacturer’s service schedule: oil and filter changes, spark plugs, battery tests, coolant, and inspection of transfer switch contacts. Have a qualified technician do annual service.
- Keep the battery charged and replace it on schedule.
- Know which circuits it covers. Large loads like central air or an electric range may not be on it, or may need load management.
- Keep the area around the unit clear and dry.
- Don’t open an energized transfer switch or controller. Only qualified persons should work on them. See Qualified vs. Unqualified.
- Don’t refuel a diesel or gasoline-fired unit while it’s running.
- Don’t disable safety devices or alarms.
Working on a building with a standby system
Electricians and other trades should know a generator can start automatically at any time. Before working on circuits, lock out both the utility and the generator, and disable the automatic start. See Lockout/Tagout and Testing for Absence of Voltage.
Compare with portable generators
Portable generators belong outdoors, far from the home, never in a garage or basement, with GFCI-protected cords. See Power Outages & Generators.
Standby system checklist
- Installed by licensed professionals, permitted and inspected
- Transfer switch in place, so there is no backfeed
- Exhaust and air intake clear of openings and obstructions
- CO alarms working inside the building
- Annual service done and weekly test checked
- Lockout plan includes the generator for any electrical work
Standards & references
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