Published: 27 Jan, 2025

Cummins Natural Gas Generator Specifications: The Complete GTA Lean Burn Rating Guide

A gas genset that looks right on a datasheet can still fall short on site. The ratings published for spark ignited engines assume a reference fuel, a reference altitude, and a reference ambient temperature, and every one of those assumptions moves when the machine reaches a mine in Mali, a bottling plant in Texas, or a wellhead in Nigeria. Cummins Natural Gas Generator Specifications are published across four separate rating columns for exactly this reason, and reading the wrong column is one of the most common and most expensive specification errors in distributed generation.

This guide sets out the published standby and prime ratings for the Cummins GTA lean burn natural gas range at both 50 Hz and 60 Hz, explains how rating basis, methane number, and site conditions change the usable output, and gives the checks to run before committing to a unit. USP&E Global has spent 25 years engineering, building, and operating power stations across 35+ countries with 350+ engineers and technicians, holding ISO 9001:2015 and ISO 45001:2018 certification across more than 150 delivered projects. Your plant has to carry a real load in real conditions. Our role is to make sure the numbers behind it hold up.

Three definitions first, because everything below turns on them. Standby rating is output available for the duration of a utility outage, at varying load, with no overload capability. Prime rating is output available for unlimited hours at varying load. kVA is apparent power, and it exceeds kW by the inverse of the power factor, conventionally 0.8 for three phase industrial sets.

What Cummins Natural Gas Generator Specifications Cover: The GTA Lean Burn Range

The Cummins natural gas generator line is built on the GTA series of spark ignited, turbocharged, lean burn engines, sold both as complete generator sets and as G-Drive engines to packagers. Lean burn means the engine runs with excess air relative to the fuel, which lowers combustion temperature and therefore lowers oxides of nitrogen at the exhaust.

The series is named by displacement. Understanding that naming is the first step to reading any Cummins gas spec sheet correctly.

Cummins GTA Engine	Displacement	Typical Genset Range (60 Hz Standby)	Cummins Product Family
GTA8.3	8.3 litres	125 kW to 150 kW	Lean burn gas
GTA855	14 litres	185 kW to 280 kW	Lean burn gas
GTA19	19 litres	325 kW to 350 kW	Lean burn gas
GTA28	28 litres	500 kW	Lean burn gas
GTA38	38 litres	575 kW to 690 kW	Lean burn gas
GTA50	50 litres	750 kW to 1,000 kW	Lean burn gas

One naming trap is worth flagging. The GTA855 is a 14 litre engine, 855 referring to displacement in cubic inches. Cummins describes the GTA855 Gas Series as supplying two power nodes and gas types within 14 litre spark ignited engines. Older packager literature sometimes writes the same engine as GTA14, so a spec sheet listing both GTA855 and GTA14 is describing one engine under two conventions, not two different machines.

The GTA50 is the largest platform in the range. Cummins describes it as its largest engine platform, providing gas solutions at a displacement of 50 litres, with power nodes in the standby application and continuous rated models offered in two frequencies.

Fuel Flexibility Across the GTA Range

Not every GTA model tolerates the same gas. The GTA855 Gas Series is documented as capable of running down to 800 BTU per cubic foot, with the ability to run on fuels between 33 methane number and pipeline natural gas, including wellhead and non-standard fuels. The GTA38 Gas Series runs pipeline natural gas and also offers a propane option. This matters enormously for associated gas, landfill gas, and wellhead applications where the fuel is not pipeline quality and never will be.

Cummins Natural Gas Generator Specifications by Model: 50 Hz and 60 Hz Ratings

The table below gives published generating set ratings for the Cummins GTA natural gas range, three phase, at 1800 RPM for 60 Hz and 1500 RPM for 50 Hz. All kVA figures are stated at 0.8 power factor.

Genset Model	Engine	60 Hz Standby kW / kVA	60 Hz Prime kW / kVA	50 Hz Standby kW / kVA	50 Hz Prime kW / kVA
CN-125	GTA8.3G1	125 / 156	115 / 144	105 / 131	95 / 119
CN-150	GTA8.3G2	150 / 188	135 / 169	120 / 150	105 / 131
CN-185	GTA855G1	185 / 231	165 / 206	150 / 188	135 / 169
CN-215	GTA855G2	215 / 269	190 / 238	175 / 219	160 / 200
CN-250	GTA855G3	250 / 313	225 / 281	215 / 269	195 / 244
CN-280	GTA855 (14 L)	280 / 350	235 / 294	240 / 300	210 / 263
CN-325	GTA19G1	325 / 406	285 / 356	275 / 344	250 / 313
CN-350	GTA19G2	350 / 438	315 / 394	295 / 369	275 / 344
CN-500	GTA28	500 / 625	450 / 563	430 / 538	390 / 488
CN-575	GTA38G1	575 / 719	520 / 650	480 / 600	430 / 538
CN-630	GTA38G2	625 / 781	570 / 713	525 / 656	475 / 594
CN-750	GTA50G1	750 / 938	675 / 844	620 / 775	560 / 700
CN-800	GTA50G2	800 / 1,000	725 / 906	670 / 838	605 / 756
CN-880	GTA50G3	880 / 1,100	800 / 1,000	760 / 950	665 / 831

Every kVA figure above resolves to its kW figure at 0.8 power factor. Ratings are reference ratings and are subject to derate for site altitude, ambient temperature, and fuel quality.

Three patterns are worth reading out of that table.

The 50 Hz penalty is real and it is large. Across the range, the 50 Hz standby rating sits roughly 12 to 16 percent below the 60 Hz standby rating for the same machine. The engine turns at 1500 RPM rather than 1800 RPM, so it completes fewer power strokes per minute. A project in a 50 Hz market that sizes from a 60 Hz datasheet will be short of capacity on day one.

Prime rating is roughly 8 to 12 percent below standby. That gap is not a safety margin you can spend. It reflects the durability derate required for unlimited hours of operation.

The whole GTA range sits between 95 kW and 880 kW. This is a distributed generation and standby band. It is not a utility scale band. Utility or heavy industrial duty is reached either by paralleling many sets, which brings synchronising and control complexity, or by moving to a larger medium speed platform.

Standby, Prime, and Continuous: Why the Rating Basis Changes the Number

The single most common specification error we see in the field is a set bought on its standby number and then run as base load. It fails early, and the warranty position is usually unrecoverable.

Rating Basis	Permitted Hours	Load Profile	Overload Capability	Correct Application
Standby	Duration of utility outage only	Varying	None	Emergency backup behind a reliable grid
Prime	Unlimited	Varying, with a load factor limit	Typically available for short periods	Off grid sites, unstable grids, peak shaving
Continuous	Unlimited	Constant, non varying	None	Base load and grid parallel operation

Cummins positions the GTA gas range primarily around standby duty, where the design priority is extreme transient capability through block load pickup and compliance with NFPA 110, and secondarily around prime power limited applications for locations with unstable grids. Continuous rated models are offered in two frequencies on the larger platforms.

The operational consequence is direct. If you are powering a remote mining camp, a processing plant, or an island microgrid, you are in prime or continuous territory, not standby territory, and you must size accordingly. Our operations and maintenance teams take over plants every year that were specified one rating class too optimistically.

Gas Quality, Altitude, and Temperature: The Derates Nobody Quotes

Published Cummins Natural Gas Generator Specifications are reference conditions. Four site variables move them.

Site Variable	Effect on Output	What to Supply the Engineer
Methane number	Low methane number forces ignition retard and derate to avoid detonation	Third party gas chromatograph analysis
Lower heating value	Fuel below the engine’s minimum BTU per cubic foot limits achievable load	Full gas composition report, not a single BTU figure
Altitude	Reduced air density reduces charge mass and output	Site elevation above sea level
Ambient temperature	Higher intake air temperature reduces output and raises detonation risk	Design maximum ambient, not annual average

Methane number is the gas equivalent of octane rating. It measures resistance to detonation. Pipeline natural gas typically sits high on the scale. Associated gas, wellhead gas, and gas with significant heavier hydrocarbons sit lower and force the engine controller to retard ignition timing, which costs output and efficiency. Because the GTA855 series is documented down to 33 methane number, it is one of the more tolerant options for non standard fuels, but tolerance is not indifference. The derate is still applied.

Never accept a single BTU number as a fuel specification. A full composition report is what an engineer can actually work with, because inert content, heavier hydrocarbon fractions, hydrogen sulphide, and moisture all drive separate decisions about gas treatment, materials, and oil change intervals.

Emissions rules are jurisdictional and they change. In the United States, stationary spark ignition engines are regulated under standards administered by the U.S. Environmental Protection Agency, with applicable limits depending on engine size, build date, and whether the unit is classified as emergency or non emergency. Lean burn combustion lowers oxides of nitrogen relative to rich burn, but it does not exempt a machine from permitting. Confirm the applicable rule with the regulator before capital is committed, not after.

On the comparison with diesel, the honest position is that it depends on what you are measuring. Natural gas combustion produces materially lower particulate matter and sulphur oxides than distillate diesel, and lower carbon dioxide per unit of energy, a difference visible in the fuel emission factors published by the U.S. Energy Information Administration. Against that, spark ignited gas engines generally show lower thermal efficiency than equivalent compression ignition diesels, and gas supply infrastructure is a fixed constraint that diesel does not have. The right answer is site specific, and both the fuel price differential and the delivered gas security have to be modelled before the environmental case decides anything. Broader fuel switching context is tracked by the International Energy Agency and, for renewables integration in hybrid power systems, by IRENA.

How to Specify and Verify Cummins Natural Gas Generator Specifications

These nine checks separate a set that will perform from one that will disappoint. They apply to new purchases, used purchases, and fleet takeovers alike.

Fix the rating basis in writing before you compare prices. Standby, prime, and continuous are three different products at three different prices. A quote that does not state the rating basis cannot be compared with one that does.
Commission a gas analysis before selecting the engine. Methane number, lower heating value, inert fraction, hydrogen sulphide, and moisture. A laboratory report, not a supplier assurance.
Calculate site rated output, not reference output. Apply the manufacturer derate curves for your actual elevation, design maximum ambient temperature, and power factor. Do this before the single line diagram is drawn.
Confirm frequency and voltage against the site, not the catalogue. 1800 RPM machines serve 60 Hz markets and 1500 RPM machines serve 50 Hz markets, at different published outputs.
Size for the block load, not just the running load. Motor starting and transient pickup often dictate a larger set than steady state demand suggests. This is where NFPA 110 compliance and block load capability become the binding specification.
Verify the engine designation against the nameplate. Displacement naming conventions differ between Cummins literature and packager literature. Confirm the physical stamped designation rather than a listing title.
Establish hours and overhaul history for any used unit. Original hours and hours since major overhaul are two separate numbers. Ask for both, plus oil analysis trend data.
Price the balance of plant, not just the genset. Gas train, pressure regulation, exhaust, radiators, switchgear, transformers, controls, and civil works routinely exceed the cost of the prime mover itself.
Confirm parts availability and lead time for the specific variant. The GTA platforms have a long production history and variant specific parts positions differ. A machine with a long parts queue is a machine that will sit idle.

USP&E performs this work as owners engineer and EPC contractor, meaning engineering, procurement and construction. Our power plant engineering team runs the derate modelling, gas train design, and interconnection studies, and our delivery record is set out in our project experience.

Frequently Asked Questions

What is the largest Cummins natural gas generator?

Within the GTA lean burn generating set range covered here, the largest published node is 880 kW standby at 60 Hz on the GTA50 platform, with Cummins describing the GTA50 as its largest engine platform at 50 litres displacement. Cummins genset configurations built on the GTA50 have been published up to 1,000 kW. Above that range, projects generally move to larger medium speed gas platforms or to gas turbines rather than staying on a single spark ignited set.

What is the difference between standby and prime rating on a Cummins gas generator?

Standby rating is the output available only for the duration of a utility outage, at varying load, with no overload capability. Prime rating is the output available for an unlimited number of hours at varying load, subject to a load factor limit. Across the GTA range the prime rating sits roughly 8 to 12 percent below the standby rating for the same machine. Any off grid or unstable grid application should be sized on prime or continuous, never on standby.

Why is the 50 Hz rating lower than the 60 Hz rating?

Because the engine turns more slowly. A four pole alternator produces 60 Hz at 1800 RPM and 50 Hz at 1500 RPM, so a 50 Hz set completes fewer power strokes per minute and delivers less power from the same displacement. Across the Cummins GTA range the 50 Hz standby rating runs roughly 12 to 16 percent below the 60 Hz standby rating. Sizing a 50 Hz project from a 60 Hz datasheet will leave the plant short of capacity.

Can Cummins GTA gas engines run on wellhead or non-pipeline gas?

Some can, within limits. The GTA855 Gas Series is documented as capable of operating down to 800 BTU per cubic foot and on fuels between 33 methane number and pipeline natural gas, including wellhead and non-standard fuels. The GTA38 Gas Series runs pipeline natural gas with a propane option. Tolerance is model specific and always comes with a derate, so a full gas composition analysis must precede engine selection rather than follow it.

What is methane number and why does it matter?

Methane number measures a gaseous fuel’s resistance to detonation, functioning much as octane rating does for petrol. Pure methane sits at the top of the scale, and heavier hydrocarbons drag it down. A low methane number forces the engine control system to retard ignition timing to avoid destructive knock, which reduces available output and thermal efficiency. It is one of the two or three numbers that determine whether a given engine is viable on a given gas supply.

Are natural gas generators cleaner than diesel generators?

On several measures yes, and on others the comparison is more complicated. Natural gas combustion produces substantially lower particulate matter and sulphur oxides than distillate diesel, and lower carbon dioxide per unit of energy delivered. Spark ignited gas engines typically show lower thermal efficiency than equivalent diesels, however, and gas supply infrastructure imposes a constraint that diesel does not. Emissions permitting also applies to gas engines regardless, so the environmental case should be modelled site by site rather than assumed.

Does USP&E supply and install Cummins natural gas generators?

USP&E Global is an EPC and O&M contractor with 350+ engineers and technicians, and Cummins is among the manufacturers represented in our inventory. Our current natural gas generator inventory is concentrated in the 1 MW and above band, so availability of specific GTA range units should be confirmed directly rather than assumed. We provide engineering, gas train and balance of plant design, installation, commissioning, and long term maintenance for gas generating plant across mining, industrial, and data centre applications.

Summary: Key Takeaways on Cummins Natural Gas Generator Specifications

The Cummins natural gas generator range is built on GTA series spark ignited lean burn engines from 8.3 litres to 50 litres displacement.
Published Cummins Natural Gas Generator Specifications span roughly 95 kW to 880 kW across standby and prime ratings at both frequencies.
The GTA855 and GTA14 designations refer to the same 14 litre engine under two naming conventions.
50 Hz ratings run roughly 12 to 16 percent below 60 Hz ratings for the same machine, because the engine turns at 1500 RPM rather than 1800 RPM.
Prime rating sits roughly 8 to 12 percent below standby rating, and off grid applications must be sized on prime or continuous.
Methane number, lower heating value, altitude, and design ambient temperature all derate published output, and all must be established before engine selection.
Balance of plant, gas train, and interconnection costs routinely exceed the cost of the generating set itself.

A rating table is where specification starts, not where it finishes. Every plant we deliver begins by establishing what the machine will actually produce on your gas, at your elevation, under your grid code, before a dollar of capital is committed.

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