Everything You Need to Know About Disconnect Switches

Up here in the coastal California mountains, most households have a standby generator to get through the frequent and prolonged power outages caused by increasingly extreme weather. The generator comes on automatically when the power goes out, and shuts off when power is restored.

Last winter, 11 atmospheric river events led to outages that necessitated more than a month of run time. That was too much for some generators, including ours, which burned out and had to be replaced.

A standby generator can’t be connected to the grid while it’s running, because that would endanger line workers. Portable or permanently installed standby generators can only be connected to the house via a special type of disconnect switch called a transfer switch. Whether manual or automatic, the transfer switch ensures the generator cannot back-feed electricity onto the power grid.

Our Generac generator came with an internal switch that disconnects it, but those that don’t need an external switch. It’s not just common sense, it’s a code requirement.

Disconnect switches are more common in industrial settings than residential ones, but there are still several places around the house where you’ll find one. The National Electrical Code (NEC) requires them for large electrical appliances, like air conditioners, heat pumps, furnaces and electric water heaters. This provides a way for technicians to shut off power so they can work safely.

The NEC previously didn’t mandate whole-house emergency disconnect switches, allowing the main service panel to serve as the main disconnect. But the 2020 NEC required every new one- and two-family dwelling unit have a readily accessible exterior emergency disconnect for first responders and firefighters.

The 2023 NEC expanded the code to say an exterior emergency disconnect must be installed when service equipment is replaced. So if you swap out your old 60-ampere electrical service with a 100- or 200-amp service, you’ll need an exterior emergency disconnect switch.

What Is a Disconnect Switch?

A disconnect switch isolates a building, appliance or circuit from power. That’s why it’s also known as an isolator, load-break or safety switch. When one is installed between a building’s electric meter and service panel — known as the “Main” service disconnect — it cuts all power to the building.

The switch include an enclosure, usually a metal 240-volt line and load contacts; optional fuse holders for cartridge or knife-blade fuses; and an actuator that projects from the enclosure and lets the user operate the switch without touching the circuit wires. The most common type of actuator is a handle, and it’s typically lockable to prevent the switch from being turned on again after it has been turned off.

Disconnect switches can also be locked in the ON position to prevent someone from tampering with the electrical system. It might seem counterintuitive, but the NEC does not prohibit the exterior emergency disconnect switch from being locked in the ON position. First responders have big tools for removing any small padlocks that get in their way.

When you install a disconnect switch for an appliance, like an air conditioner or heat pump, it must be readily accessible, within sight of the appliance and no more than 50 feet away. “Readily accessible” means it can be reached quickly for operation or inspection without requiring anyone to take actions, like using tools (other than keys), removing obstacles or climbing ladders.

In practice, it’s usually much closer — no more than a few feet away.

Types of Disconnect Switches

The most common type of disconnect switch, called an electrical disconnect switch, isolates an electric circuit from power. A heat pump has one of these switches. Although they don’t have much use around the house, you can also find battery disconnect switches that isolate batteries to prevent power leakage.

Disconnect switches are fusible or non-fusible. Besides a manual disconnect, a fusible switch also contains a fuse that blows during an overcurrent condition, like an overload, short-circuit or ground-fault. When the fuse blows, power is cut off and remains off until you replace the fuse.

Fused switches are most often used for large machinery in industrial and commercial settings. Master electrician John Williamson notes that in the past many home appliances had to have fusible switches, but this is no longer common.

If the label specifies the use of a fusible switch, however, that’s the kind you should install. Enclosed circuit breakers are also commonly used as disconnect switches.

Do Disconnect Switches Go Bad?

Yes. Like everything in the real world, disconnect switches deteriorate over time, and that means they can eventually fail.

Overheating is the most common cause of deterioration. That can happen because the contacts are loose or became oxidized. Switch enclosures are often favored by spiders, insects, snakes and even small rodents, and the mess they leave behind can corrode switch mechanisms.

A disconnect switch can also fail simply because the conductive parts rusted or corroded. That’s more likely to happen in a wet outdoor location or a humid basement than a place protected from moisture.

Can You Replace a Disconnect Switch?

It depends.

You can always replace an electrical disconnect switch as long as you have another way to turn off the power, and the circuit breaker in the main service panel that controls the circuit usually provides it.

You can do this job yourself. But if you do, it’s essential to check the line and load terminals with a voltage tester before you touch any wires, and proceed only if you get a zero reading. If you’re uneasy about doing the work, hire a licensed electrician.