Understanding Automatic Transfer Switches: What Every Facilities Manager Needs to Know

An automatic transfer switch (ATS) is the critical link between your commercial standby generator and your facility's electrical system. Without a properly sized and installed transfer switch, even the most powerful generator is useless — it cannot safely connect to your building's electrical circuits. Understanding how transfer switches work and how to specify them is essential knowledge for any facilities manager overseeing a backup power installation.

What Does a Transfer Switch Do?

A transfer switch serves two primary functions: it monitors utility power and automatically switches your facility's electrical load between the utility and your generator. When utility power fails, the ATS:

Detects the power outage (typically within 1-2 seconds)
Sends a start signal to the generator
Waits for the generator to reach stable operating voltage and frequency (typically 10-30 seconds)
Disconnects your facility from the utility (open transition) or synchronises momentarily (closed transition)
Connects your facility to the generator

When utility power returns and stabilises, the process reverses — the ATS transfers the load back to utility power and sends a cool-down/shutdown signal to the generator.

Transfer Switch Types

Open Transition (Break-Before-Make): The most common type for commercial installations. The switch disconnects from utility power before connecting to generator power, resulting in a brief (sub-second) interruption. Suitable for most commercial loads. Code-compliant without utility coordination.

Closed Transition (Make-Before-Break): Momentarily parallels utility and generator power during transfer, eliminating any interruption. Requires synchronisation between utility and generator. Typically required for sensitive IT loads, data centres and medical facilities. More expensive and requires utility approval in many jurisdictions.

Soft Loading: Gradually transfers load from one source to another over several seconds, reducing mechanical stress on the generator and electrical transients. Used in larger commercial and industrial applications.

Sizing Your Transfer Switch

Transfer switches are rated in amperes and must be sized to handle the maximum current that will flow through them. Key sizing considerations:

Ampere rating: Must equal or exceed the ampere rating of the main breaker or the sum of all circuit breakers feeding through the transfer switch
Voltage rating: Must match your electrical system (120/240V single phase, 120/208V three phase, 277/480V three phase)
Number of poles: Must match your system — 2-pole for single phase, 3-pole or 4-pole for three phase
Withstand and close-on rating: Must handle the maximum fault current available at the installation point

For most commercial facilities, the transfer switch amperage matches the generator's output amperage. A 100kW generator on a 120/208V three-phase system produces approximately 277 amps, requiring a minimum 400-amp transfer switch.

Installation Considerations

Transfer switches must be installed by licensed electricians in compliance with the National Electrical Code (NEC), particularly Article 700 (Emergency Systems), Article 701 (Legally Required Standby Systems) and Article 702 (Optional Standby Systems). Local codes may impose additional requirements.

The transfer switch is typically installed near your main electrical panel. For facilities with multiple electrical panels or buildings, multiple transfer switches may be required, each protecting specific load circuits.

Maintenance

Transfer switches require regular maintenance including: monthly exercise tests (automated in most modern ATS units), annual inspection of contacts and connections, testing of voltage and frequency sensing circuits, and verification of transfer timing and generator start signals. Most manufacturers recommend annual professional inspection to ensure reliable operation when the next outage occurs.