Automatic Transfer Switch (ATS) Design: Transition Modes, Sizing & the Mistakes That Leave Buildings in the Dark

A $500,000 generator is worthless if the transfer switch doesn't work. The ATS is the single most critical link in any emergency power system " it decides when, how fast, and how cleanly load moves between utility and backup power. Get the transition mode wrong, and you'll either trip breakers, crash servers, or violate utility interconnection rules. Here's everything you need to know.

What is an Automatic Transfer Switch?

An Automatic Transfer Switch (ATS) is an electrically operated switching device that automatically transfers load between a Normal (utility) source and an Emergency (generator) source when it detects a power failure. It monitors both sources continuously and executes the transfer sequence without human intervention.

The ATS performs four critical functions:

• Sensing " monitors voltage on both Normal and Emergency sources
• Signaling " sends start signal to the generator upon utility failure
• Transferring " switches load from one source to another
• Retransferring " returns load to Normal once utility is restored and stable

The 4 Transition Modes Explained

The transition mode defines how load is transferred between sources. Choosing the wrong mode is the most common " and most expensive " ATS design error.

1. Open Transition (Break-Before-Make)

The simplest and most common mode. The ATS disconnects from the current source before connecting to the alternate source. There is a momentary interruption " typically 60"100 milliseconds.

Parameter	Value
Transfer time	60"100 ms
Power interruption	Yes
Sources paralleled	No
Best for	General commercial, HVAC, lighting
Utility coordination	Not required

Use when: Load equipment can tolerate a brief outage (motors, heaters, general lighting, most HVAC systems).

2. Delayed Transition

Similar to open transition, but with a programmed delay between disconnecting from one source and connecting to another. The delay allows residual voltage on motor loads to decay before re-energizing " preventing damaging out-of-phase reconnection.

Parameter	Value
Transfer time	1"30 seconds (adjustable)
Power interruption	Yes (intentionally longer)
Sources paralleled	No
Best for	Large motor loads, transformers
Utility coordination	Not required

Use when: Downstream loads include large motors or transformers that could create dangerous inrush currents if reconnected before residual voltage decays.

3. Closed Transition (Make-Before-Break)

Both sources are momentarily paralleled " the ATS connects to the new source before disconnecting from the original. The entire parallel window is less than 100 milliseconds. This provides a seamless, zero-interruption transfer.

Parameter	Value
Transfer time	< 100 ms (parallel window)
Power interruption	No
Sources paralleled	Yes (momentarily)
Best for	Hospitals, data centers, critical digital equipment
Utility coordination	Required

"A closed transition ATS ensures that loads are switched without interrupting power " provided at least one source is operational. But it requires utility coordination because you're momentarily paralleling two live sources."

Critical requirements for closed transition:

• In-phase monitoring " both sources must be synchronized before paralleling
• Utility approval " the local distribution company must permit source paralleling
• Fault withstand rating " ATS must handle fault currents from both sources simultaneously

4. Soft Load Transition

Like closed transition, both sources are paralleled " but for a user-selectable extended period. During this window, the ATS controller gradually increases generator output while reducing utility demand, creating a smooth, transient-free load transfer.

Parameter	Value
Transfer time	User-selectable (minutes)
Power interruption	No
Sources paralleled	Yes (extended)
Best for	Peak shaving, planned maintenance, utility demand response
Utility coordination	Required (more stringent)

Advanced applications: Soft load ATSs can import/export power, perform peak-shaving to reduce utility costs, and maintain parallel operation of both sources for maximum reliability.

Transition Mode Selection Guide

Application	Recommended Mode	Reason
General commercial / retail	Open Transition	Cost-effective, simple, no utility coordination
Large motor loads (chillers, pumps)	Delayed Transition	Prevents out-of-phase reconnection damage
Hospitals / healthcare	Closed Transition	CEC 24-302 + CSA Z32 require zero-interruption
Data centers (Tier III+)	Closed Transition	Digital equipment sensitive to > 8ms interruption
Peak shaving / demand response	Soft Load	Gradual load migration, utility cost reduction
High-rise residential	Open Transition	Elevator pre-transfer, fire pump standard

CEC Section 46: Emergency Power Requirements

The Canadian Electrical Code Section 46 governs emergency power systems, and the ATS is at the heart of compliance:

• CEC 46-302 " ATS must automatically transfer life safety loads within 10 seconds of utility failure
• CEC 46-304 " Emergency distribution must be separate from normal distribution
• CEC 46-108 " ATS must be listed/certified (CSA C22.2 No. 178 or UL 1008)
• CEC 46-202 " Generator must reach rated voltage and frequency within 10 seconds
• OBC 3.2.7 " Buildings requiring emergency power must have automatic transfer " manual transfer is not acceptable for life safety

ATS Sizing: Getting the Ratings Right

An ATS must be rated for three critical parameters:

Rating	What It Means	How to Determine
Continuous current	Max sustained load current	Must equal or exceed the overcurrent device it's connected to (e.g., 400A ATS for 400A main breaker)
Voltage rating	System voltage class	Match system: 120/208V, 347/600V, etc.
Withstand & closing rating (WCR)	Max fault current the ATS can close into and withstand	Must exceed the Available Short Circuit Current (ASCC) at the ATS location

Common sizing mistake: Specifying an ATS based only on load current without verifying the withstand rating. A 400A ATS with a 10 kAIC withstand rating installed where the ASCC is 22 kA will fail catastrophically during a fault.

The 7 Most Common ATS Design Mistakes

• Wrong transition mode " using open transition for data center loads (causes server crashes)
• Inadequate withstand rating " ATS WCR lower than available fault current
• No step loading sequence " connecting all loads through a single ATS simultaneously, overloading the generator on startup
• Missing retransfer delay " snapping back to utility the instant it returns, causing double-transfer if utility is unstable
• Ignoring utility coordination " specifying closed transition without local distribution company approval
• No exerciser/test mode " ATS sits idle for years, then fails during an actual outage because contacts have corroded
• Single ATS for mixed loads " life safety and optional standby on the same ATS violates CEC 46-304 separation requirements

Frequently Asked Questions