How to Install a Bypass for a Generator Automatic Transfer Switch

What is an ATS bypass and why add one?

An automatic transfer switch (ATS) monitors utility power and switches your home load to a backup generator when the grid fails. When the ATS itself needs repair or replacement, you cannot simply turn it off without cutting power. A bypass provides an alternate, manually controlled path for electricity to reach your panel. It lets you isolate the ATS for safe servicing while the house remains powered from either the utility or the generator.

This is especially important for whole-house standby systems, where downtime during an outage is unacceptable. Adding a bypass is a common upgrade for homeowners who want to perform routine maintenance, firmware updates, or hardware swaps without scheduling an outage. For a quick review of how transfer switches work, see our generator transfer switch types article, or compare manual vs automatic transfer switches.

Planning and safety considerations

Adding a bypass is not a do-it-yourself job for most homeowners. It requires work on service-entrance conductors and a thorough understanding of interlock mechanisms. A licensed electrician should design and install the circuit according to local codes. You must ensure the bypass switch has the same voltage and ampere rating as the current service.

Before any work, de-energize the system by opening the utility disconnect and shutting down the generator. Use lockout/tagout procedures to prevent accidental re-energization. Also, remember that running generators produce carbon monoxide. The U.S. Consumer Product Safety Commission (CPSC) states that portable generators should never be used inside homes, garages, basements, or sheds; they must be placed outdoors, far from windows, doors, and vents. The Centers for Disease Control and Prevention (CDC) recommends operating generators more than 20 feet from any window, door, or vent, and having a battery-operated or battery-backup CO alarm in the home. Follow these rules if you need to run the generator for testing during the installation.

Tools and materials

You will need a manual transfer switch or a dedicated bypass switch rated for your service. Common choices include a two-pole, double-throw switch that can be locked in position. You also need correctly sized wire, a junction box, conduit, wire nuts or terminals, and a torque screwdriver. A multimeter is essential for verifying continuity and voltage absence. Use only components listed for the purpose by a recognized testing laboratory, as advised by the U.S. Environmental Protection Agency (EPA).

If you are adding the bypass to an existing ATS, you may need a mechanical interlock accessory designed for your specific switch model. This prevents the bypass and the ATS from feeding power at the same time, which would create a backfeed hazard. Secure all connections with proper torque and double-check the wiring diagram supplied with the switch.

• Manual bypass switch or two-pole double-throw switch
• Wire sized to match your service amperage
• Junction boxes, conduit, and fittings
• Wire nuts or compression terminals
• Multimeter for voltage checks
• Lockout/tagout equipment
• Personal protective equipment

Installation steps

The exact steps depend on your existing equipment and local code, but the general sequence is as follows. Always have a qualified professional inspect the installation before restoring power.

Step 1: Turn off the main utility breaker and the generator's disconnect. Lock the disconnects in the OFF position. Verify with a multimeter that no voltage is present on the lines you will touch.

Step 2: Mount the bypass switch next to the ATS, inside an approved enclosure. The enclosure must be accessible and properly bonded to the grounding system.

Step 3: Route the load side conductors from the main panel to the bypass switch. These conductors carry power to your circuits and must be connected to the load terminal of the bypass.

Step 4: Connect the utility feed and the generator feed to the two supply terminals on the bypass switch. The ATS is installed between these sources and the load. With the bypass, you can temporarily connect one source directly to the load.

Step 5: Wire the neutral and ground according to code. The switch must not create a parallel path with the ATS unless the design explicitly allows it.

Step 6: Install the interlock mechanism so that the bypass and ATS cannot both be closed. Follow the manufacturer's instructions carefully, as an incorrect interlock can cause a dangerous backfeed.

Step 7: After wiring, verify phase rotation and voltage at each side with a multimeter before closing the switch. Test the bypass by operating it under no load first, then with a small load.

• De-energize and lock out all sources
• Mount and bond the bypass enclosure
• Connect load conductors
• Connect utility and generator feeds
• Wire neutral and ground per code
• Install and verify the interlock
• Test under no load and low load

Testing and maintenance

Once installed, test the bypass periodically by switching between utility and generator with the ATS isolated. Check for loose connections, heat marks, or unusual noise. Exercise the mechanical parts at least once a year to keep them from seizing.

If you are performing routine maintenance on the ATS, follow the recommended generator exercise schedule to keep the generator in good shape. The bypass itself should be inspected whenever you service the transfer switch, and you should replace any worn handles or indicators according to the manufacturer's guidance.

When to use the bypass

The bypass is most useful when you need to inspect, calibrate, or replace the ATS. It also allows you to clean the enclosure, tighten terminals, or upgrade the switch without losing power. If you know an outage is coming and the ATS is unreliable, you can manually set the bypass to the generator feed and skip the ATS entirely.

In a natural disaster or prolonged outage, the bypass gives you a simple way to keep the generator connected even if the ATS electronics fail. Just remember that the bypass is a manual switch, so you must be present to change sources. For a simpler alternative, consider a portable generator with a manual transfer switch; see our guide to portable generators.

Related buying guides

If you are planning a standby system, a bypass is a useful feature to discuss with your installer. Review our guides to home standby generators and standby generators to understand what is available. For smaller installations or portable setups, a manual transfer switch may be sufficient; see the generator transfer switch types article for a comparison.

What to pick for your use

Questions

Is a bypass required by code?

Not for residential systems, but some commercial installations require maintenance bypasses. Always check with your local building department and a licensed electrician.

Can I install a bypass myself?

Unless you are a licensed electrician, no. The work involves service-entrance wiring and can be deadly if done incorrectly. Hire a professional who understands transfer switch interlocking.

What size bypass switch do I need?

The bypass must match the ampere rating of your main service, which is often 200 amps for modern homes. Your electrician will confirm the exact rating based on your panel and local code.

Can I use the bypass to manually switch between generator and utility?

Yes, but the interlock prevents both from being closed at once, so you must choose one source. This is the same principle as a manual transfer switch, and it ensures the generator never feeds back into the utility line.