What the ATS actually does
The generator gets the attention; the transfer switch does the work. A standby system without an ATS is a machine waiting for someone to show up with a flashlight. The automatic sequence:
- Sense. The ATS monitors utility voltage continuously. When it sags or fails past set thresholds, a short timer runs (so a one-second flicker doesn't start a diesel).
- Start. The ATS signals the generator to crank. The engine starts, comes to speed, and its voltage and frequency stabilize.
- Transfer. The switch mechanically moves the load from the utility contacts to the generator contacts. For life-safety systems, NFPA 110 Type 10 expects this whole chain — sense to power-on-load — inside 10 seconds.
- Retransfer. When utility power returns and holds stable through a timer, the switch moves the load back.
- Cooldown. The engine runs unloaded for a few minutes to cool, then shuts down and re-arms.
Every step is automatic — that is the point. The building staff's job is to test it, not to operate it.
Open, closed, or delayed transition
The transition type is how the switch behaves in the instant of transfer:
| Type | How it transfers | Interruption | Typical use |
|---|---|---|---|
| Open transition (break-before-make) | Disconnects one source, then connects the other | A momentary blink (well under a second) | The standard choice for most buildings |
| Delayed transition (center-off) | Opens, pauses deliberately in a neutral position, then closes | An intentional pause of seconds | Buildings with large motors/UPS — lets residual motor voltage decay |
| Closed transition (make-before-break) | Parallels generator and utility for an instant, then releases the utility | None — seamless | Facilities that can't blink; requires utility approval and protection |
Open transition is simpler, cheaper and fine for the vast majority of commercial buildings — refrigeration and computers ride through the blink or are on a UPS. Delayed transition earns its cost where large motor loads would fight a fast reconnection. Closed transition is the premium path — no interruption when retransferring or testing under load — but paralleling with the grid, even for milliseconds, means your utility must approve it and protective relaying comes along.
Service-entrance vs downstream
Where the switch sits in the electrical system matters as much as its type:
- Service-entrance-rated ATS replaces (or incorporates) the main disconnect — the switch itself becomes the first piece of equipment after the meter. It carries utility overcurrent protection and the required neutral bonding. Common on smaller buildings where one switch covers the whole service, and it saves panel space.
- Downstream (standard) ATS sits after the main breaker, feeding a selected panel or set of loads. This is how you back up only critical circuits — or how codes force you to separate systems (life-safety loads on their own switch, everything else on another).
Larger facilities commonly run multiple switches: one for code-mandated loads, one for optional loads, sometimes one per building wing. That layout is a code and cost decision made during design — see how it shows up in the project budget.
Sizing: match the service, not the generator
The single most misunderstood point. The ATS carries utility power to your building every day of its life — the generator only feeds it during outages. So its ampere rating must match the service or feeder it sits in, not the generator's output:
A building with a 400 A, 208 V service and a 50 kW generator needs a 400 A transfer switch. A 200 A switch behind a 400 A service is a code violation and a fire risk — regardless of generator size.
Transfer switches come in standard ratings; the common commercial ladder runs 30, 40, 70, 80, 100, 150, 225, 260, 400, 600, 800, 1,000, 1,200, 1,600, 2,000, 3,000 and 4,000 A. You select the first standard size at or above the circuit's requirement. When the generator side drives the choice (a dedicated critical-loads panel, for instance), practice is to compute the generator's full-load amps, apply a 125% continuous-load factor, and round up the ladder — our kW ↔ amps converter turns any kW rating into amps at your voltage and phase in seconds.
Also on the spec sheet: the withstand and closing rating (WCR) — the fault current the switch can survive — must coordinate with the upstream breaker or fuse. That's an engineer's line item, but if a bid is silent on it, ask.
NEC 700, 701, 702 — in plain words
The National Electrical Code (NFPA 70) treats backup power as three different animals, and the ATS requirements follow:
- Article 700 — Emergency systems. Loads where power loss threatens life safety: egress lighting, exit signs, fire alarm, some ventilation. Fastest restoration expectations (this is where NFPA 110 Type 10's 10 seconds lives), dedicated wiring separated from everything else, and switches listed for emergency use.
- Article 701 — Legally required standby. Loads a code requires but that aren't immediately life-threatening — certain smoke control, communications, elevators. Restoration expectations are looser (commonly within 60 seconds), separation rules are lighter.
- Article 702 — Optional standby. Everything you back up because outages cost you money: refrigeration, POS, production, HVAC. No mandated restoration time — but the equipment still must be properly rated and installed.
The practical consequence: a facility with all three classes of load generally cannot lump them onto one switch — emergency loads get their own transfer equipment. This is why bids for the same building can show one ATS or three. Classification calls belong to your electrical engineer and your AHJ — verify locally before design.
Failure points, and the testing that catches them
Transfer switches are electromechanical and spend years doing nothing. What actually fails:
- Control boards and voltage sensors — the electronics that decide to transfer; utility surges age them.
- Coils, solenoids and linkages — the muscle of the transfer mechanism; a switch never exercised is a switch that sticks.
- Contacts — pitting and oxidation from years of carrying load (and from utility sags causing chatter) raise resistance and heat.
- The start-signal path — a corroded terminal or broken wire between ATS and generator means a perfect engine that never gets asked.
The countermeasure is boring and effective: the monthly NFPA 110 exercise runs the generator under load through the transfer switch — proving the sense-start-transfer-retransfer chain end to end, not just the engine. Add an annual maintenance visit with a thermal scan of the contacts and an inspection of the mechanism, and log everything: for code-covered systems the paperwork is part of compliance.
Frequently asked questions
Do I need an automatic transfer switch with a standby generator?
Yes — an ATS is what makes a standby generator "standby." A manual transfer switch is legal for many optional-load setups but requires a person on site during the outage. Life-safety systems require automatic transfer.
What size transfer switch do I need for a 200-amp service?
A 200 A service-entrance ATS if it replaces the main, or a switch matched to whichever feeder it sits in. Never size the switch to the generator — size it to the circuit it carries. Convert generator kW to amps with the kW ↔ amps tool.
What's the difference between open and closed transition?
Open transition breaks the utility connection before making the generator connection — the building blinks. Closed transition overlaps both sources momentarily for a seamless transfer, but needs utility approval and added protection. Most commercial buildings run open transition.
How often should a transfer switch be tested?
Exercise it monthly under load as part of the generator test — NFPA 110 expects the monthly test to include operation of the transfer switch — plus annual professional maintenance. Confirm the schedule for your occupancy with your AHJ.
Put the whole system together
The ATS is one decision inside a sizing-fuel-code chain. Size the generator with the commercial method or the selector wizard, convert ratings with the kW ↔ amps tool, and see the NFPA 110 testing guide for the maintenance life you're signing up for.