Transfer Switch Types: Manual, Automatic, and Interlock

What a transfer switch does

A transfer switch is the device that connects a generator to your home's electrical panel. Its job is to isolate the generator from the utility grid: when the generator is running, the switch makes sure your home circuits are fed by the generator and not by the grid, and it prevents the generator from sending power back out onto the utility lines. That backfeed can shock line workers and damage equipment.

The three main options are a manual transfer switch, an automatic transfer switch, and an interlock kit. All three isolate the generator from the grid, but they differ in how much work you do, how they respond to an outage, and how they are installed. If you are deciding between a portable and a standby generator, the connection type is part of the decision; see generator connection types for the bigger picture.

• Manual transfer switch: you operate it by hand.
• Automatic transfer switch: it operates itself when the grid fails.
• Interlock kit: a mechanical barrier inside the panel.

Manual transfer switches

A manual transfer switch is a separate enclosure, usually mounted next to your electrical panel, with a switch and a set of breakers for the circuits you want to power. When the utility is on, the switch feeds your panel from the grid. When the power goes out, you start the generator, then throw the switch to the generator position, and the selected circuits run from the generator.

Manual switches are the most common choice for portable generators. They require you to be home and to operate the switch yourself, which works well for outages that happen while you are around. You also decide which circuits to run, which lets you prioritize the refrigerator, furnace, and lights without loading the whole house.

Sizing matters: the switch's rating must match your generator's output and the circuits you plan to feed. Before you pick a switch, work out the wattage side of the question; how to size a transfer switch walks through the numbers, and how many watts do I need covers the load side.

Automatic transfer switches

An automatic transfer switch (ATS) continuously monitors the utility supply. When the grid fails, it starts the generator, waits for the generator to stabilize, and then switches the home's circuits over to generator power. When the grid returns, it switches back and shuts the generator down. All of this happens without you touching anything.

Automatic transfer switches are almost always paired with permanently installed standby generators. They are wired into the home and often connected to a fuel source such as natural gas or propane. Because they respond to an outage within seconds, they are a good fit for homes where an extended outage would disrupt medical equipment, heating, or security systems.

Standby generators are larger and require professional installation. If you are exploring that route, see the best home standby generators and best standby generators guides for models and considerations. You can also compare portable and standby approaches in generator connection types.

Interlock kits

An interlock kit is a metal bracket installed on your main electrical panel. It physically prevents the main breaker and a generator breaker from being in the ON position at the same time. When you want to run on generator power, you switch off the main breaker, slide the interlock into place, and then flip the generator breaker. This simple mechanical arrangement meets the same isolation goal as a separate transfer switch.

Interlock kits are inexpensive and work with many portable generators, but they have a few caveats. You must manually select which circuits to power, and you need to be comfortable working inside your panel. Some local electrical codes allow interlock kits, while others require a dedicated transfer switch. Check with a licensed electrician before choosing this route.

For a portable generator, an interlock kit or a manual switch are the two practical options. Both keep you in control and avoid the expense and complexity of an automatic unit. If you are still choosing a generator, the best portable generators guide covers a range of sizes, and the best 3000 to 5000 W generators and best 5001 to 9000 watt generators guides break down output classes.

How to choose

Decide first whether you want a portable or a standby generator. Portables are flexible and typically use a manual switch or interlock. Standby generators are fixed and almost always use an automatic transfer switch. Your budget, desired level of automation, and how often you lose power will guide that choice.

If you choose a portable generator, a manual transfer switch is the safest and most straightforward upgrade from extension cords. It lets you pre-select a few circuits and switch over in seconds. An interlock kit is a cheaper alternative that uses your existing panel, but it requires you to manage the load yourself and to follow the local electrical code.

If you choose a standby generator, an automatic transfer switch is built into the system. You do not need to be home for power to be restored. That convenience comes with a higher initial investment and a permanent installation, which is why standby units are often reserved for homes that need uninterrupted power.

Safety should shape your decision as much as convenience. The U.S. Consumer Product Safety Commission warns that portable generators must never be used indoors or in garages, basements, or sheds; they should be placed outside, far from windows, doors, and vents. The CDC adds that generators should be run more than 20 feet away from openings. A transfer switch does not change where you place the generator, but it lets you connect it safely to your home's wiring. Review the generator safety checklist for placement and alarms.

Frequently asked questions

Here are answers to common questions about manual and automatic transfer switches and interlock kits.

What to pick for your use

Questions

Can an interlock kit be used with any generator?

Interlock kits work with portable generators that have a dedicated breaker in the panel. The generator must be sized to handle the circuits you plan to power, and the panel must accept the interlock bracket. A licensed electrician should verify compatibility and local code.

Does an automatic transfer switch work with a portable generator?

Automatic transfer switches are designed for standby generators that are permanently installed and wired to the switch. Using one with a portable generator would require a manual start and manual connection, which defeats the purpose. Portable generators pair with manual switches or interlocks.

Is a transfer switch required by code?

Most electrical codes require a transfer switch or an interlock device to prevent backfeeding a generator into the utility grid. The exact requirement varies by local jurisdiction. Always consult a licensed electrician to ensure your installation meets code.

How far should a generator be from the house?

The CDC recommends operating portable generators outside, more than 20 feet from windows, doors, and vents. The CPSC also warns against using generators in garages, basements, or sheds. A transfer switch does not reduce those clearances.

Can I install a manual transfer switch myself?

Installation involves working inside your electrical panel and connecting the generator inlet. It is a job for a qualified electrician unless you have significant electrical experience. Improper wiring can create a shock hazard and violate local code.

What is backfeeding and why is it dangerous?

Backfeeding occurs when a generator sends power back through the utility transformer, energizing lines that utility workers expect to be dead. A transfer switch or interlock prevents that by physically isolating the generator from the grid. It is a serious safety risk and is illegal in many areas.