When utility power fails, a backup generator is only half the solution. The other half is the device that moves your load from the grid to the generator without a crew member flipping breakers in the dark. That device is the automatic transfer switch. Pairing a diesel generator with automatic transfer switch equipment gives industrial sites, hospitals, and data centers a hands-off way to restore power within seconds of an outage.
This guide explains what an ATS does, how the transfer sequence works with a diesel generator, the transition types available, and how to size and set up an ATS on a rental project.
What Is an Automatic Transfer Switch (ATS)?
An automatic transfer switch (ATS) is electrical equipment that connects a facility’s loads to one of two power sources: the primary source (normally the utility) and a secondary source (the backup generator). The ATS monitors the utility line continuously. When it detects a failure, it sends a start signal to the generator and shifts the connected load circuit to generator power once the unit is ready.
An ATS is fuel-agnostic. It does the same job whether the backup generator burns diesel, natural gas, or propane. Stag Power Rentals runs a diesel-only fleet, so every ATS setup we support is built around a diesel generator. Our fleet does not include natural gas or dual fuel units.
How a Diesel Generator With ATS Restores Power
The ATS monitors utility voltage, frequency, and phase on every leg of the service. A typical automatic transfer sequence looks like this:
- Monitoring utility power. ATS monitors read voltage and frequency from the primary power source in real time.
- Detecting the outage. A blackout, brownout, or phase loss outside programmed limits starts a Time Delay Engine Start (TDES) timer, typically a few seconds, that filters out momentary blips and avoids nuisance starts.
- Sending the start signal. The ATS control logic signals the diesel generator to crank.
- Waiting for stable output. The switch holds until the generator reaches rated speed, voltage, and frequency.
- Transferring the load. The ATS breaks the utility connection and connects critical loads to generator power.
- Retransfer and cooldown. When utility power is stable again, a programmed delay passes, the load returns to the grid, and the generator runs unloaded for a cooldown period before shutting down.
On a properly configured system, the full transfer typically takes about 10 seconds. NFPA 110 Type 10 systems serving life safety loads must restore power within that window.
Automatic Transfer Switch Transition Types
Transfer switches differ in how they move the load between two power sources. The right transition type depends on how sensitive your connected equipment is to interruption.
Open Transition (Break-Before-Make)
The most common design. The switch breaks the connection to one source before making the connection to the other, creating a brief interruption. It is simple, reliable, and suitable for most commercial and industrial loads.
Delayed Transition (Neutral Pause)
The switch pauses in a center-off neutral position for a set delay. That pause lets large motors and transformers lose residual voltage before reconnecting, which protects motors and reduces heat and mechanical stress on electrical equipment.
Closed Transition (Make-Before-Break)
The ATS briefly runs both sources in parallel, typically for less than 100 milliseconds, so the load never sees an interruption during a planned retransfer. Closed transition requires utility approval and synchronization controls, and it is common in data centers and healthcare.
Why Pair a Diesel Generator With Automatic Transfer Switch Equipment
Fast, Hands-Off Response
Manual transfer switches need someone on site to move the load. An automatic transfer switch reacts to power outages at 2 a.m. on a holiday as quickly as at noon on a weekday, with no interruption waiting for staff to arrive.
Safety and Code Compliance
A transfer switch prevents back-feeding generator power into the utility line, which protects line workers and your equipment. The National Electrical Code requires a transfer switch whenever a generator connects to a building’s electrical system, and NEC Articles 700, 701, and 702 set rules for emergency, legally required, and optional standby systems. NFPA 110 requires automatic transfer for emergency power supply systems. Installation must also comply with local building codes and safety regulations, including proper clearance and placement of the generator and switch.
Continuous Delivery to Critical Loads
Diesel generators operate effectively under heavy motor loads and run for extended periods on on-site fuel, and diesel fuel does not degrade as quickly as gasoline, which suits standby duty. Like any engine, they need regular maintenance to stay ready. Combined with an ATS, they keep electrical power flowing to the circuits that matter most: life safety lighting, refrigeration, process controls, and servers that cannot tolerate data loss.
Applications for a Diesel Generator With ATS
- Healthcare facilities: Hospitals and medical centers rely on ATS to keep life support, surgical suites, imaging, and ICU climate control running.
- Data centers: Data centers pair UPS systems with a backup generator and ATS to prevent server shutdowns, cooling failures, and SLA breaches.
- Manufacturing: Industrial operations protect production lines, sensitive controls, and safety systems.
- Commercial buildings: Offices and retail keep lights, HVAC, elevators, and point-of-sale systems online for customers and tenants.
- Financial institutions: Banks maintain ATM uptime, security monitoring, and processing centers.
- Construction and shutdowns: Temporary power during service entrance upgrades or switchgear maintenance keeps a facility supporting its normal operation.
How to Size and Select an ATS for a Diesel Generator Rental
Proper sizing starts with your load. Use this checklist when planning an ATS with a rental generator:
- Amperage: Match the ATS ampere rating to the service or the circuits it will carry. A whole-service transfer on a 400-amp panel needs a 400-amp switch. Size continuous loads at 125% per the NEC and base the total on an NEC load calculation.
- Starting and running load: The diesel generator must handle both starting and running wattage, since motors can draw several times their running current at startup.
- Voltage and phase: Match the switch to both utility and generator voltage, and confirm compatibility with single-phase or three-phase systems.
- Pole configuration: The choice between 3-pole and 4-pole depends on the site’s grounding. A 4-pole ATS switches the neutral conductor and is used on separately derived systems; a 3-pole ATS leaves the neutral solidly connected.
- Service entrance rating: If the ATS installs at the service entrance, it must carry a service-entrance rating.
- Enclosure: Outdoor installations typically need a NEMA 3R enclosure; harsh or washdown areas may need NEMA 4.
- Controls: Decide whether you need remote monitoring, exercise timers, load shedding, or event logs.
- Bypass-isolation: For critical facilities, a bypass-isolation ATS allows maintenance without interrupting power.
A common rule of thumb for generator sizing is the 80% rule: plan for continuous load at no more than 80% of the generator’s rated capacity to leave headroom for motor starts and future load. Oversizing causes its own problem: a diesel engine that runs lightly loaded for long periods can suffer wet stacking, so choosing the right generator size matters in both directions.
Diesel Generator Considerations for ATS Setups
Diesel units bring a few practical factors that the ATS and site plan should account for:
| Factor | Requirement | ATS / Site Feature |
| Fuel supply | Adequate on-board diesel and a refueling plan | Fuel level monitoring and low-fuel alarms |
| Engine warm-up and cooldown | Time to reach rated speed; unloaded cooldown after retransfer | Programmable start and cooldown timers |
| Environment | Outdoor, dust, rain, or heat exposure | NEMA-rated enclosure |
| Voltage stability | Large motor loads can cause voltage dips | Delayed transition and load sequencing |
| Noise | Diesel units are louder than small gasoline units | Sound-attenuated enclosure and careful placement |
| Exercising | Regular testing under load | Built-in exercise scheduler |
To keep noise down on long-running jobs, see our guide on ensuring quiet operation with industrial generator rentals.
Can Portable Generators Use an Automatic Transfer Switch?
Yes, if the generator has a remote two-wire start. Many towable and trailer-mounted diesel units accept a start signal from an ATS, so portable generators can provide automatic standby power on a temporary basis. With the ATS wired in, the generator is always ready to start. Smaller portable units without remote start need a manual transfer switch instead.
For temporary hookups, a generator docking station with heavy-duty inlet connectors lets a rental unit plug into the building quickly. Plan a dedicated receptacle for the generator’s battery charger so the starting battery stays charged while the unit waits on standby.
Frequently Asked Questions
What does an ATS do for a generator?
It monitors the primary power source, starts the backup generator when utility power fails, transfers the load once generator power is stable, and returns the load to the utility when the grid comes back.
Do you need an ATS for a generator?
You need a transfer switch, manual or automatic, any time a generator connects to a building’s wiring. An ATS is required when the system must respond without a person present, such as emergency and legally required standby systems.
What is ATS in a diesel generator set?
In a diesel generator set, the ATS is the switch and controller that link the genset to the facility’s distribution. It decides when to start the engine and when to move the load.
How many types of ATS are there?
By transition type there are three: open transition, delayed transition with a neutral pause, and closed transition. ATS units also vary by pole count, enclosure, and whether they include bypass-isolation.
Is an ATS the same as switchgear?
No. Switchgear is a broader assembly of breakers and controls that distributes power. An ATS is a specific device that selects between two power sources, and it can stand alone or be integrated into switchgear.
What are the disadvantages of automatic transfer switches?
An ATS adds cost and is a potential single point of failure if it is not maintained. Regular exercising and inspection keep it reliable.
Is it cheaper to buy or rent a generator with ATS?
For short-term projects, planned shutdowns, hurricane season, or bridging power during permanent equipment failure, renting usually costs less than buying and avoids maintenance and storage.
Rent a Diesel Generator and Transfer Switch From Stag Power Rentals
Stag Power Rentals supplies industrial diesel generator rentals from 20kW to 2,000kW, load-bank tested before dispatch from our Dickinson, Texas yard. Serving Houston and the Texas Bay Area, we offer:
- A diesel-only fleet with transfer switch rental available for automatic or manual setups;
- Flexible rental terms including single, double, and triple shift billing;
- Fast deployment for emergency power needs;
- Complete power distribution equipment, including cables and panels;
- Support from our team throughout your rental period.
Call Stag Power Rentals at 1-844-STAG-PWR or request a quote today to plan a diesel generator with automatic transfer switch for your facility.
