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

An expensive 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 always an interruption - a brief switching break, plus the generator start time after a utility failure.

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.

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 zero-interruption transfer when both sources are available; after a utility failure it transfers open.

"A closed transition ATS ensures that loads are switched without interrupting power - provided both sources are available. 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.

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.

Normal and emergency sources, the transfer switch and the transition modes

A typical emergency power arrangement, not a specific building. Shown on emergency.

Detail A: inside the transfer switch

One line for all poles; shown on emergency

Electrical points

1. 1

   Normal source: the utility supply; the ATS senses voltage on both sources continuously.

2. 2

   Main switchboard: the ATS continuous rating must equal or exceed the overcurrent device it is connected to (e.g. a 400 A ATS for a 400 A main breaker).

3. 3

   Emergency source: a generator conforming to CSA C282 or batteries; automatic transfer equipment actuates on normal supply failure. Rules 46-202, 46-206

4. 4

   Start signal: the ATS signals the generator to start on utility failure; an exerciser and test mode keep it ready.

5. 5

   Transfer switch: senses, signals, transfers and retransfers after a retransfer delay. Its withstand and closing rating (WCR) must exceed the available fault current: a 10 kA WCR where 22 kA is available can fail.

6. 6

   Life-safety distribution: emergency wiring kept separate from other wiring, and overcurrent protection coordinated with the life-safety feeders and branch circuits. Rules 46-108, 46-208

7. 7

   Transition mode: open for general loads, delayed for large motors and transformers, closed (utility approval) for hospitals and data centres, soft load for peak shaving.

IT loads ride through on a UPS; closed transition only avoids the break on retransfer, and needs in-phase monitoring and the distributor’s approval to parallel.

• Normal (utility) source
• Emergency (generator) source
• Load side
• Start signal

Figure 1. Normal and emergency sources, the transfer switch and the transition modes

Transition Mode Selection Guide

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 46-206 - emergency power supplies are controlled by automatic transfer equipment that actuates upon normal supply failure
• CEC 46-108 - life-safety conductors require approved emergency wiring methods and separation from non-emergency wiring except where permitted
• CEC 46-208 - emergency supply overcurrent protection must be coordinated with life-safety feeders and branch circuits
• CEC 46-202 - emergency power may be supplied by rechargeable batteries or a generator; capacity and duration are tied to building-code requirements
• OBC/NBC + CSA C282 - life-safety transfer timing, generator start performance, fuel supply, and testing must be confirmed for the specific building classification

ATS Sizing: Getting the Ratings Right

An ATS must be rated for three critical parameters:

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

The 7 Most Common ATS Design Mistakes

• Wrong transition mode - relying on the ATS instead of a UPS for IT 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 loads must be coordinated carefully so Section 46 wiring, transfer, and overcurrent requirements are not compromised

Frequently Asked Questions