Reliability of fuel supply tends to be of great concern for authorities having jurisdiction
(AHJs). On-site fuel (most often diesel) is typically required for life-safety applications, and
many mission-critical applications like 911 call centers specify it because it is perceived
to be more reliable. Nonetheless, maintenance issues and delivery concerns of diesel
fuel in an emergency, combined with the reliability and cost-effectiveness of natural gas,
must be considered in a standby power system. NFPA and NEC offer provisions for the
use of natural gas in standby power applications that had previously been the clear
domain of diesel-fueled systems. There are also many ways to work with the AHJ to clear
the way for the use of gaseous fuel in a standby power system.
nAtUrAl gAs fUel
Historically, choosing between diesel-fueled and natural gas-fueled
gensets has been relatively clear-cut. Power density, large-kW
application cost advantages, and perceptions of reliability have made
diesel the primary fuel of choice for large commercial and industrial
However, natural gas-fueled gensets are becoming more powerful and
more cost effective. Fuel storage, environmental, and diesel delivery
issues are making system designers, electrical contractors, and
electrical engineers as well as owners and operators seek alternatives
to diesel-fueled gensets.
When selecting or specifying either gaseous- or diesel-fueled gensets,
however, one must consider the reliability of the fuel infrastructure,
regulatory issues, and how local authorities perceive and regulate these
trAditionAl ApproAches to fUel
Reliability of fuel supply tends to be of great concern for authorities
having jurisdiction (AHJs). Typically, AHJs require the use of on-site fuel
for facilities that are categorized as NFPA 110, Level 1 loads; these
are defined as backup power systems installed “where failure of the
equipment to perform could result in loss of human life or serious
injuries.” NFPA 110 Level 1 loads are typically associated with NFPA
70: National Electrical Code (NEC), Article 700: Emergency Systems.
Mission-critical applications like these, which include hospitals and
911 call centers, rely on diesel-fueled generators because on-site fuel
storage is perceived to be more reliable.
While NEC requires an on-site fuel supply for mission-critical and
legally-required standby systems, it allows the AHJ to permit the use
of other than on-site fuels in applications that have a low probability
of a simultaneous failure of both the off-site fuel delivery system and
the power from the outside electrical utility company. However, storing
diesel fuel on-site is seen as increasing the reliability of backup power
in non-code-required applications, and makes it possible to provide
backup power in remote areas that do not have access to a gaseous-
Traditions are typically hard to break, which could explain why many
AHJs continue to require diesel-fueled gensets. Diesel has been the
traditional fuel of choice for backup and standby gensets. Historically,
the use of diesel as a fuel for gensets has been a long-standing, proven
technology. Because of this, the perception within the market is that
diesel engines are the most reliable prime movers for backup power
diesel fUel concerns
While many users may prefer diesel as a genset fuel, several issues
associated with diesel should be considered. Annex A of NFPA 110
identifies several considerations associated with diesel fuel. For
example, temperature variations can accelerate fuel degradation.
Prolonged exposure to ambient air can cause diesel fuel to oxidize.
Ambient temperature changes can lead to condensation within the fuel
tanks. The water from this condensation can lead to microbiological
contamination and growth, which in turn can cause corrosion of
steel tanks and components, filter plugging, operational issues, or a
hydrocarbon release to the environment.
For these reasons, facility managers must be diligent in maintaining
their generators’ diesel fuel supply. Even in ideal cases, diesel fuel has
a shelf life is less than 12 months, at which time the fuel must be
polished (filtration and water removal) or replaced. Ultra-low-sulfur fuel
is particularly prone to fuel breakdown, because the extra refining steps
required to remove sulfur also remove some of the elements that keep
the fuel stable.
Despite their widespread use, diesel-fueled gensets are vulnerable
to events that could cripple transportation infrastructures, making
refueling problematic—or perhaps even impossible. Hurricanes and
severe storms can close roads. Grid failures can make it impossible
for suppliers to pump fuel into delivery trucks. For example, Hurricane
Sandy, which slammed into the Northeastern U.S. in late October of
2012, caused significant damage to fuel distribution infrastructure. Fuel
distribution networks were paralyzed. Critical terminals for petroleum
and petroleum products were badly damaged. Many service stations
lost power and couldn’t pump gas or diesel fuel. Whereas diesel gensets
can operate reliably during extended outages, users can’t always count
on the availability of diesel fuel and deliveries.
reliAbility of the nAtUrAl gAs
While weather, human-caused disasters, and even labor issues such
as strikes can impact diesel fuel delivery and availability, the natural
gas utility infrastructure is generally reliable. Typically, factors that tend
3to impact the electrical infrastructure do not impact the natural gas
infrastructure. In most cases, natural gas is delivered by underground
pipelines that are usually not affected by severe weather that can cause
electrical power outages.
According to Platts, the natural gas supply throughout the Northeast
U.S. was not interrupted during Hurricane Irene, which hit the U.S. in late
August, 2011. A check of natural gas pipelines throughout the Northeast
area showed no notices of service interruptions or other issues.
An analysis of the Northeast snowstorm that occurred in late October,
2011 by the Federal Energy Regulatory Commission (FERC) and the
North American Electric Reliability Corporation (NERC) showed that more
than 3 million customers lost electric power. However, the analysis made
no mention of any natural gas infrastructure interruptions.
After Hurricane Sandy, New Jersey Natural Gas reported that it was
shutting off service to about 28,000 customers because of storm
damage. However, the number affected by this outage was only a
fraction of the nearly 2 million New Jersey customers without electrical
Using natural-gas-fueled gensets instead of diesel-fueled gensets
depends on the intended application. What this means is that, unless the
AHJ allows otherwise, mission-critical applications require on-site fuel
storage, or diesel. However, for non-code-required applications, natural
gas can be a viable option. Storage is not an issue because natural
gas is supplied continuously by the local municipal infrastructure. This
means that this continuous supply extends genset run time.
Numerous NFPA codes and standards apply to standby generators.
NFPA 110 and the NEC are the most frequently referenced standards for
ensuring safe, reliable, and compliant genset specification, installation,
commissioning, and operation. Most of the requirements applicable to
genset fuel refer to diesel-powered units. However, there are provisions
and exceptions associated with natural gas-powered units.
According to NFPA 110, Article 5.1.1(3), natural gas is an acceptable fuel
for an emergency power system. However, the exception that immediately
follows states: “For Level 1 installations in locations where the probability
of interruption of off-site fuel supplies is high, on-site storage of an
alternate energy source sufficient to allow full output of the EPSS to be
delivered for the class specified shall be required.”
While most of NFPA 110 applies to diesel fuel delivery, storage, and quality
requirements, Article 7.9 includes specific requirements for natural gas:
7.9.7—Where the gas supply is connected to the building gas supply
system, it shall be connected on the supply side of the main gas shutoff
valve and marked as supplying an emergency generator.
7.9.8—The building’s main gas shutoff valve shall be marked or tagged
to indicate the existence of the separate EPS shutoff valve.
7.9.9—The fuel supply for gas-fueled and liquid-fueled prime movers
shall be designed to meet the demands of the prime mover for all of the
(1) Sizing of fuel lines
(2) Valves, including manual shutoff
(3) Battery-powered fuel solenoids
(4) Gas regulators
(5) Regulator vent piping
(6) Flexible fuel line section
(7) Fuel line filters
(8) Fuel vaporizers (LP-Gas)
(9) Ambient temperature effect of fuel tank vaporization rates of
LP-Gas where applicable.
While NFPA 110 has plenty to say about genset fuel systems, the NEC
has very little. NEC, Article 700.12(B)(3) states: “Prime movers shall not
be solely dependent on a public utility gas system for their fuel supply.”
However, the exception that immediately follows states: “Where acceptable
to the authority having jurisdiction, the use of other than on-site fuels shall
be permitted where there is a low probability of a simultaneous failure of
both the off-site fuel delivery system and power from the outside electrical
4Addressing nAtUrAl gAs reliAbility
with the AhJ
The AHJ has the final say regarding genset applications. Therefore,
consulting the AHJ regarding the reliability of the genset’s fuel supply
is a must.
Because of its availability, many AHJs prefer on-site fuel storage,
meaning diesel fuel. It is generally required by the NEC, Article 700 in
many municipalities. However, convincing the AHJ to consider approving
the use of natural gas-fueled gensets may be quite challenging. While
most AHJs do not have to be convinced that diesel fuel is reliable,
providing proof that the natural gas fuel source is reliable may open
eyes and change minds.
One way to convince the AHJ to consider natural gas is to get the
utility to certify reliability. The reliability of a natural gas fuel source
can be proven with a letter from the natural gas vendor that contains:
• A statement that the fuel source is reasonably reliable
• A description supporting the assertion that the source is
• A statement of the low likelihood of an interruption
• A description supporting the assertion that the likelihood of
interruption is low
• A signature from technical personnel
Working with both the local gas utility and the AHJ can help engineers,
owners, and operators understand the reliability of the natural gas
supply as it compares to on-site diesel fuel. Ensuring that the facility
is not subject to curtailment policies can help prevent natural gas fuel
Another way to alleviate discussions about the reliability of on-site
vs. utility-supplied natural gas is to consider a system that uses both
fuels—individually or simultaneously. Bi-fuel gensets, for example, run
on both diesel fuel and natural gas and leverage the benefits of each
while minimizing their respective drawbacks.
Bi-fuel gensets combine the power density and capital cost benefits of
diesel engines with the extended run time of natural gas. These units
start up on diesel fuel, which helps gensets meet the10-second startup
requirements for mission-critical facilities. As loads are added, natural
gas is introduced and increased while diesel fuel is decreased. Typically,
bi-fuel gensets operate on up to 75% natural gas, 25% diesel with no
reduction in power under typical load conditions. However, bi-fuel units
will not run on 100% natural gas.
Because less diesel fuel is consumed, run times are significantly extended,
which could be a significant issue during emergencies that challenge
municipal infrastructures, thereby affecting diesel fuel deliveries. With
natural gas as the predominant fuel, diesel tanks can be smaller, which
reduces the risk of fuel contamination as well as the cost of maintaining
Bi-fuel gensets are only slightly more expensive than diesel-only units.
Also, fuel redundancy is built in. In the unlikely event that the natural gas
supply is interrupted, or if there is a fault in the bi-fuel delivery system,
the genset controls automatically revert to all-diesel operation without
For decades, diesel has been the traditional fuel of choice for commercial
and industrial backup and standby gensets. However, today, diesel is not
the only option. Now, engineers, owners, and operators have genset fuel
choices, each of which offers unique benefits.
Take time to learn how these genset fuel choices can be applied to
ensure that the best solutions are selected. Also, consult the local AHJ
to understand the policies on using particular fuels in given applications.
Bulletin 0200930SBY Printed in USA 11.13
©2013 Generac Power Systems, Inc. All rights reserved.
Generac Power Systems, Inc.
S45 W29290 Hwy. 59
Waukesha, WI 53189