Portable Generators and Your House: What Is Safe

Why this matters

A portable generator is one of the most useful things you can own for a long outage, and it is also one of the few household machines that kills people every year when it is used the obvious way. The two ways it does that have nothing to do with the generator breaking. One is the exhaust: an engine makes carbon monoxide, and a generator running in a garage or under an open window can fill a house with it while everyone sleeps. The other is the connection: plug a generator into a wall outlet and it can push electricity back out of your house onto the utility's lines, where a line worker who believes the wire is dead is the one who gets hurt.

This card covers where the generator goes, what it can safely be plugged into, how to work out what it can run, and the point at which connecting it to the house becomes an electrician's job.

The two rules that are not negotiable

The generator runs outside, far from the house. Never in the garage, the basement, a crawlspace, a shed, a carport with walls, or a porch under a roof, and never there "with the door open." The U.S. Consumer Product Safety Commission and the CDC both advise placing it at least 20 feet from the house, with the exhaust pointed away from doors, windows and vents. Carbon monoxide (CO) is a gas you cannot see or smell, and an open garage door does not clear it fast enough: the gas pools and drifts into the house through the door to the kitchen, the attic and gaps you do not know about.

The generator never connects to a wall outlet. A cord with a plug on both ends, sometimes sold or made for this purpose, feeds power backward into your wiring. That power flows out through your service to the transformer on the pole or the pad, which steps it up to thousands of volts on the street line. It also bypasses the protection your breakers give, and when the utility restores power the grid and the generator meet head-on through your house, which can destroy the generator and start a fire. There is no careful way to do this. The safe ways to connect are below.

The setup that works

            YOUR HOUSE
 +--------------------------------+
 | window   door     CO alarm on  |
 | (closed) (closed) every level, |
 |                   battery      |
 |                   backed       |
 |                  [INLET BOX]---+-> transfer switch
 +---------------+----------------+   or interlock at
                 |                    the panel, put in
  20 ft or more  |                    by a licensed
  clear distance |                    electrician under
                 |                    a permit
            [GENERATOR]
          dry, level ground, open on all sides,
          exhaust end pointed away from house
                    |
                    +--> heavy-duty outdoor cords
                         -> fridge, freezer, lamps

There are two legitimate ways to get the power indoors, shown on the two sides of the drawing.

Cords straight to the things you want to run. Outdoor-rated extension cords, three-prong, run from the generator's own outlets to individual appliances: a refrigerator, a freezer, a lamp, a phone charger. Nothing connects to the house wiring at all. This is the setup a homeowner can do alone, and it is the right one for most short outages. Route the cords so you do not have to prop a door or a window open on the generator side of the house, because that opening is exactly where the exhaust gets in.

An inlet box and a transfer device. An electrician installs a weatherproof inlet on the outside wall and a transfer switch, or a listed interlock kit on your main panel, that makes it physically impossible for the generator and the utility to feed the house at the same time. Then one heavy cord runs from the generator to the inlet, and you can power hardwired circuits a cord cannot reach: the furnace blower, a well pump, the sump pump if it is on its own circuit. The National Electrical Code treats this as an optional standby system (NEC Article 702, as adopted by your jurisdiction) and requires transfer equipment that keeps the two sources apart. In most places it is permitted and inspected work, done by an electrician licensed where your state or locality licenses it, and it is not a do-it-yourself job even for a handy owner, because a mistake in that panel is the backfeed the device exists to prevent.

Working out what it can run

A generator is rated in watts, usually two numbers: running watts (what it can supply continuously) and starting watts (a short burst for the moment a motor kicks on). Your appliances say what they draw on a label, often in amps rather than watts. Watts are amps times volts, and ordinary household outlets are about 120 volts.

Here is a worked example. The figures are illustrative; read your own labels and your own generator's manual.

Say the generator's label reads 3,500 running watts and 4,500 starting watts, and you want to run these during an outage:

The total, 2,350 running watts, sits under the 3,500 rating with 3,500 - 2,350 = 1,150 W to spare. That spare matters, because many manuals advise against running a generator near its full rating for hours, and because the motors in the fridge, freezer and pump each draw a surge well above their running number at the instant they start. Start the largest motor first, let it settle, then plug in the next, rather than letting all three cycle on at once when the generator comes up. If the manual gives a starting-watt figure for typical appliances, use its number rather than a guess.

Check each cord, not just the generator. The fridge and freezer on one cord draw 720 + 600 = 1,320 W, and 1,320 / 120 = 11 A. The rating printed on that cord's jacket or tag has to be higher than 11 A. A thin indoor lamp cord can overheat at that load, and so can a very long cord, so use the shortest heavy-duty outdoor cord that reaches.

What changes the answer

Adding a space heater. A typical portable electric heater is labeled 1,500 W on high. Plugging one in on top of the list above gives 2,350 + 1,500 = 3,850 W, which is over the 3,500 running rating. The generator will either trip its own breaker or labor and overheat. Heating is the load that most often overloads a portable generator, and a gas furnace with a blower on an inlet and transfer switch usually keeps a house warmer for a fraction of the electricity.

Wanting the furnace, a well pump or anything hardwired. These do not have a plug, so no cord reaches them. The answer is the inlet and transfer device above, installed by an electrician. Do not remove a furnace's cord-and-plug or cut into its wiring to make it reach. If you wonder whether your furnace already has a plug, look but leave it as you found it.

A 240-volt appliance. A well pump, an electric dryer or a central air unit usually runs on 240 volts. Some generators have a 240-volt outlet, but feeding a 240-volt circuit in the house is always the inlet-and-transfer route, never an adapter.

Carbon monoxide: the alarm is not optional

Have a working CO alarm on every level of the house and outside each sleeping area, with battery power or a battery backup, before the generator ever starts. A plug-in alarm with no battery stops working in exactly the outage that brought out the generator. See related: What Your Carbon Monoxide Alarm Is Telling You.

Some newer portable generators sense CO building up around themselves and shut off. That feature helps, but it measures the air at the generator, not in your bedroom, and it does not make a garage a safe place to run one.

If the CO alarm sounds, or anyone feels a headache, dizziness, nausea or unusual sleepiness while the generator runs: get everyone, pets included, out into fresh air right away and call 911 from outside. Do not go back in to shut the generator off or open windows. Responders will tell you when the house is clear. See related: How to Respond When Your Carbon Monoxide Alarm Goes Off.

Fuel, weather and the other hazards

Refuel cold. Shut the generator off and let it cool before you add fuel. Gasoline spilled on a hot muffler or engine can ignite. Plan your refueling for daylight, and keep the fuel can closed and away from the running machine.

Store fuel outside the living space. Keep gasoline in an approved fuel container, in a detached shed or well away from the house, never in the basement or next to a water heater or furnace, whose flame can ignite vapor. Buy only what you expect to use; old gasoline gums up a carburetor and is a common reason a generator will not start when you need it.

Keep it dry, not enclosed. Water and electricity together can shock you. Run the generator on dry, level ground, and if it is raining, under an open canopy that leaves it open on all sides, never inside a structure. Touch it and plug cords in with dry hands, and never stand in water to do it.

Ground it the way the manual says. Some generators need a grounding connection and many do not, depending on how they are built. The manual states which; follow it rather than a neighbor's advice.

Keep children and pets away. A running generator is hot, loud and has moving parts and live outlets. Keep people clear of the exhaust side.

The usual mistakes

Running it in the garage "just for a minute" to get it started or out of the rain. Minutes are enough for CO to reach dangerous levels in an enclosed space, and the gas stays after the engine stops.

Putting it close to the house to keep it from being stolen. A generator beside the back door or under a bedroom window is a CO path. If theft worries you, chain it to something at the 20-foot mark.

Daisy-chaining cords or using indoor cords outdoors. Each joint and each undersized cord is a heat point and a shock point if it lies in wet grass.

Assuming the house is safe because the main breaker is off. Turning off the main breaker and backfeeding through an outlet is still an unprotected, uninspected connection, and it depends on someone remembering the main is off every time. The transfer device exists so nothing depends on memory.

When to stop and call

Call a licensed electrician to plan an inlet, a transfer switch or an interlock, to tell you whether your panel can take one, or to connect anything hardwired. The installation needs a permit in most places, and the inspection is part of what makes it safe. See related: What a Permit Means for You as the Homeowner.

Call 911 from outside for a sounding CO alarm or CO symptoms. Call the utility, from a safe distance, if you see a downed line, or if you smell gas or see sparking near the meter. See related: When to Stop and Call a Licensed Pro Around the House.

References

• U.S. Consumer Product Safety Commission, portable generator safety guidance (outdoor use, at least 20 feet from the house, exhaust away from openings)
• Centers for Disease Control and Prevention, carbon monoxide poisoning prevention during power outages
• NFPA 70, National Electrical Code, Article 702 (optional standby systems and transfer equipment), in the edition adopted by your jurisdiction
• The generator's own owner's manual for ratings, grounding and fueling
• See related: Your Home's Shutoffs: Water, Gas and Electric; How to Get Through a Power Outage Safely at Home; What Your Carbon Monoxide Alarm Is Telling You