Virginia Regulatory Town Hall
Agency
Department of Environmental Quality
 
Board
Air Pollution Control Board
 
Guidance Document Change: DEQ Guidance Memo APG-578 addresses the use of emergency generators in the case of “sudden and reasonably unforeseeable events” as the result of a planned electric outage.
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12/1/25  5:55 pm
Commenter: Keith Edward Snyder

TIER 4 REQUIRED AIR QUALITY STANDARDS
 

Tier 4 Standards Are Required As I Am Reading this.  

 
Tier 4 standards for emergency generators primarily involve meeting the EPA's stringent emissions requirements for new, large stationary diesel engines, which include reducing oxides of nitrogen (NOx) and particulate matter (PM) to the lowest possible levels. Generators must use technologies like selective catalytic reduction (SCR) and diesel particulate filters (DPF), which often require diesel exhaust fluid (DEF), to achieve these standards. While standard emergency use has a 100-hour annual limit for non-emergency operation, facilities can use Tier 4-certified generators for more extensive use, such as storm avoidance, without limits. 
 
Key standards and requirements 
  • Emissions control: Tier 4 standards focus on reducing harmful emissions, specifically 
    NOx
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     and 
    PM
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    , to very low levels.
  • Technological requirements: To meet these standards, generators must incorporate advanced emission control systems, such as:
  • Application and use:
    • Standard emergency generators: Are allowed to run for a maximum of 100 hours per year for non-emergency situations like testing and maintenance.
    • Tier 4 generators: Can be used for applications beyond standard emergency use, including storm avoidance and microgrids, with no limits on hours for such use. This makes them a better option for facilities needing more extensive backup power.
  • Local and state requirements: Facilities in areas with local non-attainment zones may have even more stringent clean air standards that necessitate Tier 4 certification for their emergency power systems.
  • NEC requirements: Beyond federal EPA standards, generators must also comply with the National Electrical Code (NEC) for proper installation, which includes requirements for listed equipment, overcurrent protection, and conductor sizing. 
 
Summary of compliance 
  • For standard emergency use: Tier 4 is not required by the EPA for the 100-hour rule, but it is the highest standard for clean air technology.
  • For non-standard use: Tier 4 is the necessary standard for generators that will operate outside of standard emergency use, such as for peak shaving or storm avoidance.
CommentID: 238196