New U.S. data center backup diesel and natural gas generator installations are landing inside a three-layer compliance stack: federal EPA stationary-engine tiering, state permits that now reference 40 CFR Part 1039 directly, and a separate, increasingly contested debate over how many hours per year a Tier II unit is allowed to run outside a declared emergency [S1][S4].
Engine Technology Forum estimated in March 2026 that roughly 550 to 750 new U.S. data centers will be built over the next five years, each carrying multiple multi-megawatt gensets subject to federal, state, and local air permits covering emissions performance, testing, fuel storage, noise, and unit count [S2]. Environment America puts the existing fleet at about 55 GW of diesel generator capacity nationwide, with roughly 27 GW installed at data centers, equivalent to more than half of U.S. nuclear generating capacity [S5].
EPA tiering: what Tier 2 versus Tier 4 actually means for a data center genset
EPA stationary-engine designations set the NOx, CO, PM, and HC limits a generator must meet, and the tier required depends on how the unit will be operated, not just how big it is [S3]. Standby or "emergency" gensets, defined as those running only during power loss and routine maintenance testing, qualify for Tier 2 emission requirements, while "non-emergency" units that run in place of, or in addition to, grid power must meet Tier 4 [S3].
Tier 4 represents the most stringent nonroad standard for NOx and PM, and the jump from Tier 2 to Tier 4 cuts NOx by about 88% and PM by about 85% on a like-for-like basis [S5]. Tier 4 certified products also carry a self-protective requirement: the genset must shut down if the aftertreatment system senses a fault, which is a maintenance and dispatch issue as much as an emissions issue [S3]. For large data centers classified as major NOx sources, regulators commonly impose a 100 tons-per-year NOx ceiling, with air districts layering additional, tighter caps on top [S3].
Where 40 CFR Part 60 subparts IIII, JJJJ, and 63 ZZZZ actually bite
Emergency stationary engines at data centers typically fall under three federal rule sets: 40 CFR Part 60 subpart IIII (compression ignition, stationary), 40 CFR Part 60 subpart JJJJ (spark ignition, stationary), and 40 CFR Part 63 subpart ZZZZ, the NESHAP for stationary reciprocating internal combustion engines, which is the hazardous-air-pollutant rule [S1]. Each of these caps the hours an "emergency" engine can run in non-emergency service; the commonly cited baseline is 100 hours per year for testing and maintenance, with the exact clause living in 40 CFR 63.6640(f)(3)-(4), 60.4211(f)(3), and 60.4243(d)(3) [S1][S2].
Non-emergency use, including demand response, peaking, and prime power when a data center is not grid-connected, is treated differently: allowable runtimes and the required emissions certification tier are set by those uses, and the operator is no longer inside the emergency exception [S2]. Generators that lose their "emergency standby" classification under federal grandfathering rules are legally required to meet current Tier 4 standards, which is why reclassifying a unit, even temporarily, has permanent compliance consequences [S5].
DOE 202(c) emergency orders: the new off-the-clock run window
In spring and summer 2026, the Department of Energy, acting under Federal Power Act section 202(c), issued emergency orders directing RTOs and large-load customers, including data centers, to operate backup generation as a last resort before declaring an Energy Emergency Alert Level 3 during periods of extremely high demand [S1]. EPA's reading, posted on its Clean Air Act resources page for data centers, is that operation under those 202(c) orders counts as an emergency situation, so the hours run during those periods do not count against the 50-hour or 100-hour non-emergency caps described in the three subparts [S1].
That interpretation opens a path for grid-stress use of backup engines without tripping the 40 CFR Part 60 and Part 63 hour limits, but it does not change the underlying tier the engine is certified to: a Tier II unit dispatched under a 202(c) order still emits at Tier II levels [S1][S2]. For operators, the practical effect is a new category of "allowed" runtime that sits between pure emergency and a permitted non-emergency program, and that is reshaping how states are writing their own rules [S1].
Illinois CRGA: Tier 4 is now the floor for new diesel backup
Illinois' Clean and Reliable Grid Affordability Act (CRGA) is the cleanest example of a state pulling Tier 4 directly into data center siting law. For covered data centers, new or modified diesel backup gensets are expected to meet emissions standards at least as protective as EPA Tier 4, as codified in 40 CFR Part 1039, and natural gas backup gensets are expected to meet standards at least as protective as Tier 2 for nonroad large spark-ignition engines [S4].
The shift in Illinois matters because generator selection and permitting now have to be planned together: Tier-level expectations feed into equipment lead time, the emissions calculations that travel with the construction permit application, and which construction permit pathway the project can use [S4]. Operators who default to a Tier II diesel architecture to keep lead time short are now finding that the state permit pathway will not accept that configuration for a covered facility [S4].
Virginia: the bigger fight is hours, not just tier
Virginia's situation is the national flashpoint for run-hour policy, driven by the density of data centers in Northern Virginia's "Data Center Alley" and the political weight of community noise and diesel-odor complaints after a substation fire forced multi-day diesel operation in Loudoun County [S4]. A Sept. 30 memo from DEQ's Air and Renewable Energy Division director expands how "emergency" may be interpreted, including adding certain utility planned outages, with DEQ's example contemplating notice within 14 days or less, to the list of situations where Tier II generators can run [S4].
Virginia's framework treats the two tiers differently: Tier II units are generally limited to emergency use, while cleaner Tier IV units can operate outside emergencies but require more costly emissions controls, typically a selective catalytic reduction (SCR) stack [S2][S4]. On top of that, starting July 1, 2026, new air permit applications for diesel generators at Virginia data centers will generally be expected to meet Tier 4-equivalent emissions controls, or the applicant will need to justify an alternative approach to DEQ [S4]. Critics argue that expanding "emergency" through guidance rather than notice-and-comment rulemaking is exactly the kind of definitional drift the hour caps were written to prevent [S4][S5].
What this means for genset selection and aftermarket paths
For a new U.S. data center, the spec conversation is no longer just "MW and switchgear." It now starts with how the unit will be classified, emergency standby, limited-use, peaking, or prime, because that decision selects the tier, which selects the aftertreatment, which selects the lead time, the SCR balance-of-plant, the permit pathway, and the ceiling on annual non-emergency hours [S1][S2][S3]. Existing Tier II installations are not required to upgrade under federal grandfathering as long as they stay classified "emergency standby," which is a powerful incentive to keep that classification intact and resist re-rating [S5].
For older sites that need to cut emissions without replacing the genset, the aftermarket path is selective catalytic reduction (SCR) plus advanced engine controls, which can bring older units close to Tier 4 performance and is the same architecture used on factory Tier 4 products, and that retrofit is now being cited as a near-term compliance option for sites facing new state-level expectations [S2]. On the modeling and instrumentation side, accurate NOx reporting is increasingly tied to site-derated values rather than the nominal lab value the OEM publishes, so a function generator or data logger tied to a post-combustion probe is no longer optional for major-source sites [S3]. For cooling-loop NOx (boilers) and broader air permitting, see construction machinery and equipment and static var generator references that overlap with the same controlled-power envelope.
Non-emergency runtime, the 100-hour trap, and enforcement exposure
Under federal rules, Tier II diesel generators at data centers are permitted to run up to 100 hours per year for non-emergency uses, including testing and maintenance, with the exact wording anchored in the three subparts referenced above, and anything beyond that without a qualifying emergency can flip a unit out of compliance and force a Tier 4 reclassification [S1][S2][S5]. The synthetic-minor permit structure commonly used for large data centers makes that line enforceable on a per-unit, per-year basis, so a single over-budget genset can trigger state-level enforcement against the whole site [S7].
PEC's November 2025 writeup flagged a proposed increase to data center diesel generator non-emergency use, the kind of change that would test the 100-hour ceiling directly and that, if granted, would push more sites toward Tier 4 just to keep dispatch optionality, and that proposal is still working through the Virginia policy process in parallel with the July 1, 2026 Tier 4 permit expectation [S4][S6]. For an operator, the cleanest compliance posture right now is a Tier 4-capable architecture on the construction permit, an SCR-equipped genset ready to absorb 202(c) and demand-response hours, and a metering stack that can prove the per-unit hour count to the permit reviewer on demand [S1][S2][S4].
Two near-term signals worth tracking: the final form of Virginia DEQ's "emergency" guidance and whether the July 1, 2026 Tier 4 permit expectation holds in practice, and whether any additional state adopts a CRGA-style 40 CFR Part 1039 reference, which would set the next compliance baseline for new U.S. data center backup builds. The dry-type transformer and power-distribution side of the same data center build is covered separately in Dry-Type Transformer Market 2026: Data Center and Renewable Demand Reshapes Sourcing, and the e-waste trajectory of retired generator and UPS equipment is tracked in E-Waste Recycling 2026: Volumes, Metal Recovery Rates, and Process Choices.