Automatic Transfer Switches Explained: The Brain of Your Generator System

An automatic transfer switch (ATS) is arguably the most critical component of your standby generator system. It's the "brain" that monitors utility power, starts your generator when needed, and safely switches your home to backup power—all automatically, even when you're not home. Here's everything you need to know.

What Is an Automatic Transfer Switch?

An ATS is an electrical device that automatically transfers your home's power source from the utility to your generator when the utility fails, and back again when utility power is restored. It prevents dangerous backfeed that could harm utility workers and damage your equipment.

Key Functions

• Utility monitoring: Continuously checks for proper voltage and frequency
• Generator start signal: Automatically starts the generator when utility fails
• Transfer switching: Safely disconnects utility and connects generator
• Retransfer: Switches back to utility when stable power returns
• Generator shutdown: Signals generator to cool down and stop

How the Transfer Process Works

Outage Detection (0-3 seconds)

The ATS constantly monitors utility power. When it detects a loss or significant deviation from normal voltage/frequency, it waits a brief programmed delay (typically 2-5 seconds) to confirm the outage isn't just a momentary fluctuation.

Generator Start (3-8 seconds)

Once outage is confirmed, the ATS sends a start signal to the generator. The generator cranks and starts, reaching stable operating speed within seconds.

Transfer (8-15 seconds)

After the generator is running and producing proper voltage, the ATS opens the utility connection and closes the generator connection. This "break before make" design ensures the two sources are never connected simultaneously.

Operation

Your home now runs on generator power. The ATS continues monitoring both the generator output and utility lines.

Utility Restoration

When utility power returns, the ATS waits a programmed delay (typically 5-10 minutes) to ensure stability before transferring back. It then reconnects utility, signals the generator to cool down, and shuts it off.

Types of Transfer Switches

Service Entrance Rated ATS

The most common type for whole-house generators. It's installed between the utility meter and main panel, transferring all circuits to generator power. Benefits:

• Simplest installation
• All circuits available on generator
• No circuit prioritization needed
• Best value for whole-house coverage

Load Center ATS (Critical Loads Panel)

For smaller generators that can't power everything. A subpanel is installed with selected critical circuits that transfer to generator power:

• Lower cost option
• Works with smaller generators
• Requires deciding which circuits to include
• May require re-running wires to the critical loads panel

Smart Load Management ATS

Advanced systems that actively manage generator load by cycling certain circuits:

• Allows smaller generators to effectively power more
• Prioritizes essential loads automatically
• Sheds loads to prevent generator overload
• Higher equipment cost, but may allow smaller generator

ATS Features to Consider

Amperage Rating

Your ATS must be rated for at least your home's electrical service:

• 100-amp service: Common in older homes
• 200-amp service: Standard in modern homes
• 400-amp service: Large homes, may need special ATS

Open vs Closed Transition

• Open transition: Brief power interruption during transfer (standard)
• Closed transition: No interruption—momentarily parallels sources (premium feature)

Most residential applications use open transition. The 100-200 millisecond interruption is imperceptible for most loads.

Voltage Sensing

Look for adjustable voltage sensing that initiates transfer:

• Undervoltage threshold (typically 80-85% of nominal)
• Overvoltage protection
• Single-phase and three-phase monitoring

Exercise Schedule Programming

Many ATS units can schedule automatic generator exercise cycles (weekly tests) to ensure the generator stays ready.

Installation Requirements

Location

The ATS is typically installed:

• Adjacent to or integrated with main electrical panel
• Outdoors in weatherproof enclosure (for outdoor generators)
• Indoors in utility room or garage

Code Requirements

National Electrical Code (NEC) and Virginia codes require:

• Listed and approved transfer equipment
• Proper neutral-ground bonding
• Utility notification in some jurisdictions
• Permit and inspection

Common ATS Problems and Solutions

ATS Won't Transfer

• Check control power/battery backup
• Verify generator is starting and producing power
• Check voltage sensing adjustment
• Inspect transfer mechanism for obstructions

ATS Transfers Unnecessarily

• Voltage sensing may be set too sensitive
• Utility power quality issues (sags, brownouts)
• Adjust delay timers if nuisance transfers are common

Generator Starts But ATS Doesn't Transfer

• Generator output voltage may be out of spec
• ATS waiting for generator warm-up period
• Transfer mechanism may need service

Leading ATS Manufacturers

• Generac: Integrated with their generators, excellent app connectivity
• Kohler: Heavy-duty construction, matched with Kohler generators
• Eaton: Wide range of commercial and residential options
• ASCO: Industry leader in commercial transfer switches
• Briggs & Stratton: Good value residential options

Professional Installation Is Essential

ATS installation involves working with your main electrical service—this is not a DIY project. Improper installation can result in:

• Dangerous backfeed to utility lines
• Damage to generator or home electrical system
• Code violations and permit issues
• Voided warranties
• Insurance problems

At AJ Long Electric, we're factory-trained on Generac, Kohler, and other major brands. We properly size, install, and program your ATS for reliable, code-compliant operation. Contact us to discuss your generator and transfer switch needs.