Diesel
Stored on site; EPA calls it the most common choice. FEMA notes it degrades over time, so test and polish fuel and avoid oversized tanks. Downsides: deliveries, spill risk, environmental rules.
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Guide
Commercial standby generators for public agencies
A commercial standby generator is a permanently installed engine-generator set that powers selected loads when utility power fails, usually switched in by an automatic transfer switch. For a public agency, the right unit follows from four decisions: how the electrical code classifies the loads, the NFPA 110 level and type, a fuel you can count on in a disaster, and an engineered load calculation.
Standby, prime and continuous ratings
ISO 8528-1 defines four ratings for engine-driven generator sets. The Defense Department's design criteria, UFC 3-540-01, repeat them and warn that manufacturers' own definitions may differ, so ask which one a quote uses.
| Rating (ISO 8528-1) | Load | Hours per year |
|---|---|---|
| Emergency standby power (ESP) | Variable, during a utility outage or a test | Up to 200 |
| Limited-time running power (LTP) | Under agreed operating conditions | Up to 500 |
| Prime power (PRP) | Variable, averaging no more than 70 percent of the rating (UFC 3-540-01) | Unlimited |
| Continuous power (COP) | Constant | Unlimited |
Nameplates usually show both standby and prime kW. An NFPA 110 annex note says a 100 kW standby set is normally considered an 80 kW set for prime power. For outages lasting days, size against the prime rating.
Which code drives the requirement
The National Electrical Code (NEC, NFPA 70) has three articles for backup power. NFPA 110, the standard for emergency and standby power systems, lines up with the first two.
| NEC article | Covers | Power within | NFPA 110 |
|---|---|---|---|
| 700, emergency | Systems a government authority classes as emergency, for life safety | 10 seconds | Level 1 |
| 701, legally required standby | Other code-required loads, such as heating, sewage disposal and lighting | 60 seconds | Level 2 |
| 702, optional standby | Loads that life safety does not depend on | No set time; may be manual | Outside its scope |
Level 1 applies where failure could cost lives or cause serious injury, Level 2 where it is less critical. NFPA 110 also sets Type, the most seconds the load may go without power (Type 10 means 10 seconds), and Class, the hours it must run at rated load without refueling, from Class 0.083 (five minutes) to Class X. NEC Article 708 adds rules for sites that must run continuously for public safety. Your local code authority, the authority having jurisdiction (AHJ), decides what applies.
What drives the requirement, by facility type
Code-required power covers short-term needs such as evacuation, FEMA notes, not long outages. The mission sets the rest.
| Facility | What usually drives it | Loads to list first |
|---|---|---|
| Water and wastewater | State design standards. The 2018 Recommended Standards for Water Works, used in several Great Lakes and upper Mississippi states, call for dedicated standby power to treat or pump water to meet average day demand. | Treatment processes and key pumping stations |
| Schools and shelters | Life safety code loads, plus the shelter mission. FEMA says the part of a school used as a shelter should be classified Risk Category IV. | Life safety loads, then heating and food preparation for multi-day shelters |
| 911 and public safety | Continuity. FEMA notes EOCs (emergency operations centers) and 911 dispatch are in greatest demand right after an event. | Dispatch and communications. FEMA says EOCs often power the whole facility, with fuel for a week or more. |
| Municipal buildings | Life safety code loads; the rest is a continuity choice under NEC Article 702. | Life safety, then what staff need. FEMA says climate and air conditioning affect size most. |
Fuel choices and trade-offs
Natural gas
No tank to refill and, per EPA, quieter and cleaner. FEMA warns pipeline supply can fail in wind, flood or earthquake or be shut off before storms, and advises against it for critical facilities unless service is assured.
Bi-fuel and dual-fuel
EPA describes units that use natural gas and diesel, so diesel takes over if gas is cut. CISA says to test fuel switching rigorously and suggests two smaller generators on different fuels instead.
Propane
Stored on site and, per FEMA, keeps indefinitely. EPA notes it suits rural areas but needs deliveries, limits generator size, and leased tanks can usually be filled only by the leasing supplier.
For Level 1 systems where off-site fuel is likely to be interrupted, NFPA 110 requires an alternate fuel stored on site, with automatic transfer.
Sizing basics
Start with a load list. UFC 3-540-01 asks designers to find peak demand, the largest step load, motor starting and nonlinear loads such as drives and UPS units. Check both capacity figures: kilowatts (kW), the real power that does work, and kilovolt-amperes (kVA), voltage times current. kW equals kVA times the power factor.
Motors set the peak. UFC 3-540-01 puts inrush at up to six times running current. In a FEMA example, starting a 10 hp motor with NEC code letter K on top of 35 kVA of other load needs about 125 kVA; with code letter D, about 80 kVA. Staggered starts, soft starters and variable frequency drives shrink the generator but cut starting torque.
Do not overshoot. FEMA advises against a diesel set more than double the load, since light loading causes wet stacking: unburned fuel and soot in the exhaust.
Transfer switches, enclosures and fuel tanks
An automatic transfer switch (ATS) senses the outage, starts the generator, moves the load once voltage and frequency are stable, then transfers back when utility power returns. NEC emergency and legally required standby systems need one; optional standby may be manual. Since its 2017 edition, the NEC has also required emergency systems that rely on one generator to have a permanent way to connect a temporary source during repairs, with exceptions; the emergency generator rental guide covers these connection points.
EPA recommends weatherproof enclosures with room for service and placement above flood level, commonly three feet above the 100-year floodplain. For diesel tanks, it cites an Army Corps of Engineers rule of thumb: 0.07 gallons per hour × kW × 24 = gallons per day. More than 1,320 gallons above ground, counting containers of 55 gallons or more, can bring a site under EPA's Spill Prevention, Control, and Countermeasure (SPCC) rule.
EPA rules for stationary emergency engines
New stationary diesel engines fall under EPA's New Source Performance Standards, 40 CFR part 60, Subpart IIII; natural gas and propane engines fall under Subpart JJJJ, with the same operating rules. To keep emergency status under 40 CFR 60.4211(f):
- Emergency use has no time limit.
- Maintenance checks and readiness testing may use up to 100 hours per calendar year.
- Up to 50 of those hours may be non-emergency use, but not peak shaving, non-emergency demand response or selling power, apart from narrow grid-reliability dispatch.
- Running outside these limits means meeting every requirement for non-emergency engines.
EPA notes that in most cases a state or local air permit is also needed.
Testing and maintenance
During Hurricane Sandy, EPA notes, many generators failed after a day or two because they had not been exercised and maintained. NFPA 110 requires diesel sets in an emergency power supply system to run monthly for at least 30 minutes, at a load that holds the manufacturer's minimum exhaust temperature or reaches 30 percent of nameplate kW. A load bank, a portable resistive load, can make up the difference.
Sets that cannot reach that load also need a periodic test with added load; the required steps depend on the NFPA 110 edition your AHJ (authority having jurisdiction) enforces, so confirm it. Transfer switches are operated monthly. Log every test hour, since they count toward EPA's 100.
How to buy on Sourcewell Contract #062326-BIDR
Sourcewell, a State of Minnesota local government unit and service cooperative, competitively solicits contracts for public agencies. Its solicitation #11338 lists BIDR Technologies under Sourcewell Contract #062326-BIDR, whose scope runs from standby, prime and mobile systems through transfer switches and fuel tanks to installation, maintenance and rentals. The contract page has the full list, and the Sourcewell generator contract guide explains it.
- Confirm eligibility: governmental, education, nonprofit, tribal and other public agencies in the US and Canada.
- Register with Sourcewell (see cooperative contracts). It is free for government and education entities, with no fees and no obligation to buy.
- Request a quote from BIDR Technologies through the power generation page or BIDR Power.
- Finalize with the contract number and your Sourcewell account number, after your own due diligence.
Questions buyers ask
What size generator does a school or city building need?
No reliable rule of thumb exists. Size comes from a load list: running load, the largest motor or group starting at once, allowable voltage dip and nonlinear loads. In FEMA's example, one motor's code letter moves the peak from about 80 kVA to about 125 kVA. Have an engineer run the numbers.
Is a standby-rated generator enough for a long outage?
ISO 8528-1 limits emergency standby power to 200 hours a year, while prime power allows unlimited hours at a variable load. EPA notes standby units are designed for short-term use. For outages measured in days, ask for the prime rating and size against it.
Can an emergency generator do peak shaving or demand response?
Not if it is to stay an emergency engine under EPA's Subpart IIII. The 50 non-emergency hours allowed each year cannot be used for peak shaving, non-emergency demand response or selling power, apart from narrow grid-reliability dispatch. Outside those limits, all non-emergency engine requirements apply.
Diesel or natural gas for a water treatment plant?
Start from the worst event you plan for. Diesel is stored on site but needs deliveries and fuel care. Natural gas needs no tank, but FEMA warns supply can fail in wind, flood or earthquake. Bi-fuel units, or two generators on different fuels, hedge that risk. Check your state's design standards.
Plan your standby generator on Sourcewell Contract #062326-BIDR
Review the scope, visit BIDR Power, or send BIDR Technologies your load list for a quote.
Sources
- UFC 3-540-01, Engine-Driven Generator Systems for Prime and Standby Power Applications (Change 3, 2023), U.S. Department of Defense, via WBDG
- FEMA P-1019, Emergency Power Systems for Critical Facilities: A Best Practices Approach to Improving Reliability, FEMA, via WBDG
- Power Resilience Guide for Water and Wastewater Utilities (2023), U.S. EPA
- Resilient Power Best Practices for Critical Facilities and Sites (November 2022), CISA
- NFPA 110 First Draft Report, First Revisions (incl. 8.4.2, 5.1.1, A.4.4.1, A.4.4.2), NFPA
- The role of NFPA 110 and its interaction with other codes, Consulting-Specifying Engineer
- EPSS classifications, Consulting-Specifying Engineer
- Emergency and Standby Power for Commercial Occupancies, IAEI Magazine
- Maintenance and Testing of Emergency Power Supply Systems, EC&M
- 2017 NEC Significant Code Changes Part 5 (700.3(F)), Ezekiel Enterprises (EZ-PDH)
- 40 CFR 60.4211, emergency stationary CI engine operating requirements, eCFR
- 40 CFR 60.4243, emergency stationary SI engine operating requirements, eCFR
- 40 CFR 112.1, SPCC rule applicability, eCFR
- Recommended Standards for Water Works, 2018 Edition (section 2.6, Standby Power), Great Lakes-Upper Mississippi River Board, via New York State Department of Health
- Applied Mathematics for the Petroleum and Other Industries, page 33 (power factor), PETEX, The University of Texas at Austin
- Solicitation 11338: Electrical Energy Power Generation Equipment with Related Parts, Supplies, and Services, Sourcewell
- How it works, Sourcewell