Generator Ratings

This question-and-answer article outlines how to use manufacturer ratings for generators to choose the proper equipment for the job. Also covered: How to consider conditions that could "de-rate" the generator.

By Amara Rozgus, Editor in Chief September 13, 2010

Our Roundtable Participants (from left)

  • Brian Artzer, Vice President, Henderson Engineers Inc., Lenexa, Kansas
  • David J. Courtemanche, PE, Principal, X-nth, Boston
  • Rosa Lazebnik, PE, Electrical Technology Manager, Primera, Chicago
  • Mak Wagner, PE, Associate Chief Electrical Engineer, Stanley Consultants, Muscatine, Iowa

CSE: What are the critical parameters for generator ratings?

David Courtemanche: The critical parameters for sizing a generator all start with the load to be served. These parameters include: the size of the load in kW and kVA; the maximum allowable voltage and frequency dips that the load can withstand; the type of load, such as motors, lighting, UPS, VFD, etc.; the character of the service expected of the generator, meaning is this a standby power application or is the generator to be the only power supply for this load; any requirement for spare capacity for the future loads; and, of course, voltage, phase, and frequency requirements. All of these parameters can affect the generator sizing.

Rosa Lazebnik: Genset parameters are usually published by manufacturers, following EGSA (Electrical Generating Systems Assn.) Performance Standards, Pub. 101P-1995, and Guideline Specifications, Pub. 101S-2005a. The specifications include rated capacity in kW at power factor equal to 0.8, voltage system configuration, and frequency.

Additional parameters include mode of operation, duration of running time, and overload capability. For these parameters the following ratings are identified: emergency standby, limited running time, prime power, and industrial.

For correct selection of operating mode, it is critical to determine the specific types of load to be served by the generator, peak demand, required hours of operation per year, and required limitations for starting and load acceptance conditions. For example, unit starting time and load acceptance for generators designed for emergency operation are defined by NFPA 101 and National Electrical Code and shall not exceed 10 sec. Runtime and overload requirements are different for generators operating in emergency standby or prime power mode. It is important for correct selection of the generator to determine correctly how many hours per year the generator will operate: will it be constantly connected to utility (cogeneration), supplying power to the site with no utility, or working only a limited number of hours during a power outage?

Brian Artzer: The major genset manufacturers generally provide applicable standby and prime ratings for most gensets that would apply to these applications. These ratings are established by international standards that all manufacturers have to comply with so they can be used for comparison. The manufacturer’s data sheets include ratings, prototype testing data, and other performance criteria. The data sheets are typically available online or otherwise readily available from the manufacturer for most applications.

Information regarding limited run time and base load ratings typically is not as readily available. Applications that really lead to these ratings will require a more direct contact and coordination with the generator manufacturer. Limited running time ratings typically include a load management application such as peak shaving, load curtailment, and cogeneration. Besides utility applications, base load ratings may also apply to cogeneration or main source of power applications.

Mak Wagner: There are multiple generator rating parameters (power output, voltage, frequency, and duty ratings) that are critical to the generator application projects. Generators are designed to provide a given output at rated fundamental frequencies of 50 or 60 Hz. The generator power output and duty ratings must be properly selected to meet the demand of the connected loads. The relative size of the connected load compared to the generator capacity will impact the generator performance. Too much load on the generator may cause the generator to not start when needed. Too little load will result in poor operating performance and high maintenance requirements. The generator’s voltage and frequency output must also operate within tolerable limits of the connected system.

Keep in mind that the generator ratings are based on a certain set of standard ambient conditions, such as a maximum ambient temperature of 40 C and altitude less than 3300 ft (1000 meters) above sea level (per NEMA MG1). Ambient temperatures in excess of 40 C and altitudes above 3,300 ft will affect generator performance and may require the generator to be de-rated. Refer to the generator manufacturer performance data for appropriate de-rating factors. As you can see, any one of these generator rating parameters can be critical to the generator project.

Artzer: The most common ratings for a generator are standby and prime ratings. Standby rating is intended for supplying emergency power for the duration of a normal utility power source failure. There is essentially no overload capability for a standby rated generator. Prime rating is the maximum power available at a variable load for an unlimited number of hours. In addition, a 10% overload capability is also available for a limited time. Limited run time and base load (continuous) are other examples of generator ratings but are less commonly specified. These ratings are defined by international standards that all manufacturers are required to follow in regards to published performance data for their equipment.

Generators’ ratings are based on their capability of delivering a specified amount of power for the duration or number of hours per year anticipated. In general, a generator can deliver more power for a limited amount of hours per year, or less power continuously. Duration of anticipated generator use throughout the year is a key factor in choosing the appropriate rating. Determining whether a normal utility source (or other reliable power source) is available or if the generator is the only source of power for the site is another key factor. Many generators have both standby and prime ratings for the same set, so the portion of continuous and variable loads also needs to be considered and evaluated to determine which size would be applicable for each rating.

Applicable codes are another factor in determining generator size ratings. Emergency systems (NEC 700) are required to be sized for connected load, unless the application is an NEC 517 healthcare application; in that case, the load is sized based on anticipated load levels and prudent design. If the application is NEC 701 legally required systems, the generator is sized for loads that are expected to operate at one time. For optional standby sizing, the code requires compliance with NEC 220 calculations or another authority having jurisdiction-accepted load calculation/sizing process.

Finally, spare or future capacity must also be considered when making initial genset selections or recommendations while balancing the owner’s first cost and long-term needs.

CSE: In your experience, do manufacturers’ generator ratings provide enough detail for you to make informed decisions? Why or why not?

Wagner: Sometimes yes and sometimes no, depending on project application requirements. For a simple project application, the nameplate ratings information and published manufacturer generator data are sufficient. For a more complex project (project involving larger generators and/or utility grid interconnection), generator performance data, electrical characteristics, capability curves, excitation and governor system, system load characteristics, and utility system grid data may also be required, even before bidding. This data is sometimes harder to get, but the manufacturer generally will provide this data if you require it as part of your project submittal. For smaller size generators (standard designed motor), the “typical data” needs to be more readily available. Understanding the generator rating standard (such as NEMA MG1), project application requirements, and type of motor data needed, and working with the generator manufacturer representative will help to ensure the success of the project.

Courtemanche: From strictly a generator standpoint, I have found that manufacturing cut sheet information is generally limited to kW, kVA, voltage, phase, and frequency of the gensets that they offer. Although this is required information for any project, this information alone is not sufficient to determine that a generator is correctly sized for a particular application. Luckily, the major genset manufacturers provide sizing software that allows the engineer to model all of the load parameters and select the optimum genset for the project.

From the standpoint of other information needed to engineer and design a generator installation, at least the major manufactures provide fairly comprehensive information including physical size and weights, air requirements, noise information, emissions information, fuel consumption, and connection requirements.

Lazebnik: Manufacturer data sheets provide basic information about ratings, dimensions, emissions, data, etc, which is sufficient for small generator sizes with simple loads and preliminary planning. However, for most projects the design process typically requires us to size the engine-genset using manufacturer provided software or manufacturer assistance based on the load profile and characteristics. After the engine-genset is selected, the data sheets are used to determine fuel, ventilation/exhaust, and space requirements.

CSE: When considering a generator, you have to look at the type of particular rating you’re interested in (standby, prime, etc.) that manufacturers provide. It’s often difficult to compare them. How do you determine which rating you actually use when sizing a generator?

Artzer: Before making any rating selection, you must first determine how the generator is intended to be used. For a few hours at a time? Continuously for a week? As the primary source of power for the facility? A standby rated generator can be used continuously for several days to get through an emergency outage, but the environmental requirements must be adhered to. Also, after a long run, maintenance should be performed. If outages lasting multiple days are frequent or if outages are for hours at a time on a more or less daily basis, a generator size based on its prime rating should be used.

The fuel source and fuel storage size are important in the selection of rating type (i.e., what is the availability of re-fueling under emergency conditions), and the rating type is important in the selection of the generator rating. If the plan is for long runs on a regular basis, then natural gas may be the choice (if used for emergency loads, the authority having jurisdiction, or AHJ, will need to approve its use as an emergency power source to comply with NEC 700.12(B)(3)) if re-fueling is a problem or large storage tanks are undesirable, or if emission regulations are an issue.

The published technical data should be compared to aid in the decision making process. Fuel consumption at the anticipated load can affect ongoing operating costs. Physical size differences can impact overall space requirements, especially if installed indoors. Sound or exhaust data may be an important factor to consider for your site. Other technical differences may have various impacts for your application. Room design is critical to make sure the air requirements for the units are provided.

After comparing published technical data, there are other factors to consider when determining which rating to specify. Initial cost and future capacity are probably the two biggest factors to consider. Too much focus on either of these will likely lead to nearly opposite results. But finding the correct balance between first cost and realistic future needs will lead you to the best recommendation.

Lazebnik: I have found some manufacturer technical information that defines the genset ratings guidelines and load factors for standby, prime, and continuous ratings for their equipment; however, as you’ve noted, it’s not always readily apparent. The best approach is to define the intended or expected loads and hours of operation per year and consult with the manufacturer’s representative for proper selection/sizing of the generator, as well as include these requirements in the specifications for the generator. Typically standby generator ratings are suitable for operation of 400 h per year or less, while prime ratings are used for more than 400 h of operation under varying load. Continuous ratings are used for non-varying base load operation.   

Courtemanche: The generator ratings that I’m familiar with are standby, prime, and continuous. For the most part, the major generator manufacturers define these as:

  • Standby: with varying load, operates for the duration of the normal source outage, with an average output of 70% of the standby rating, for up to 500 h per year. This unit has no overload rating.
  • Prime: with varying load operates for an unlimited time, with an average output of 70% of the prime rating, peak demand of 100% of the prime rating with 10% overload capability for 1 h in 12, not to exceed 25 h per year.
  • Continuous: with unvarying load, operates for an unlimited time, with an average of 70% to 100% of continuous rating, peak demand of 100% of the continuous rating for all operating hours. No overload rating.

The selection of which rating to use is a function of how the unit is intended to operate. For example, for a commercial facility relying on the generator to provide life safety power in the event that the utility fails, a standby rated unit is in order. For a data center, or perhaps a facility with a utility agreement to run generators at peak load times, a prime generator rating should be considered because usage may exceed 500 h per year. For that pumping station in the boonies, a continuous power rated unit may be required because it is the only power source.

Generally, for the same gensets, a standby unit will have the highest kW rating; prime will be rated about 10% less than standby, and continuous will rated around 10% less than prime.

Wagner: A generator’s continuous, prime, or standby ratings pertain to the definition of duty for which the generator will operate. Operating above these rating definitions will result in shorter life and higher generating costs per year. Below are general descriptions of duty ratings as described by a generator manufacturer (Caterpillar). Depending on the manufacturer, the actual definition may vary from the general definitions presented below.

  • Standby Rating: Output available with varying load for the duration of the interruption of the normal power source. The generator with the Standby Rating is typically used for the building standby power services. (Typical Load Factor = 60% or less; Typical Hours per Year = 500 h; Typical Peak Demand = 80% of standby rated ekW with 100% of rating available for the duration of an emergency outage. 1)
  • Prime Rating: Output available with varying load for an unlimited time. The generator with the Prime Rating is typically used for heavy industrial loads, pumping station applications, peak shaving applications or cogeneration plant applications. (Typical Load Factor = 60% to 70%; Typical Hours per Year = no limit; Typical Peak Demand = 100% of prime rating used occasionally. 1)
  • Continuous Rating: Output available without varying load for an unlimited time. The generator with the Continuous Rating is typically used for utility parallel operation, on-site generation or base load cogeneration plant applications. (Typical Load Factor = 70% to 100%; Typical Hours per Year = no limit; Typical Peak Demand = 100% of continuous rating used 100% of the time. 1)

Operating a generator beyond the constraints for which it was designed will result in a shorter life and increased operating and maintenance costs.

CSE: How much do you need to “de-rate” a generator when you’re powering harmonically rich loads, such as uninterruptible power supplies (UPS) or variable frequency drives (VFD)?

Artzer: Not as much as in the past. Factors of 1.8 were often used when UPS units were the bulk of the load. Now, the factor is more like 1.2. An often forgotten item is the impact of battery recharging load. UPS systems and telecommunications dc plant rectifiers often go into charging mode if any battery time was used, even only a few seconds. The charging current load is based on the full load rating times 1.1 to 1.25, not based on the actual running load. As an example, six 400 A rectifiers carrying a normal load of 200 A each (1200 A total) suddenly becomes a load of 2640 A at 110% load. Use typical generator calculations and manufacturer sizing tools. Then consult the manufacturer for your specific application.

Wagner: There is no hard-and-fast rule on how much the generator needs to be de-rated when serving nonlinear, harmonic generating loads. Nonlinear loads may cause harmful harmonic currents. The harmonics can cause internal heating of the generator, limiting its capability and shortening its life. For electrical systems where the harmonic content seen by the generator is high, significant de-rating of the generator may be required to prevent overheating or premature generator failure. However, harmonic or other power quality issues should first be mitigated to limit the de-rating or oversizing of the generator. For example, provide filtering or other harmonic attenuating options such as isolation transformers at the source of distortion. Where loads are connected line-to-line to the generator bus, without a neutral connection or supplied through delta-wye transformer, any triplen harmonics, caused by the load, are not seen by the generator. It is always better to mitigate the harmonic through proper design, rather than to significantly oversize the generator to compensate for harmonic affects. Significantly oversizing the generator above the load requirements will typically reduce performance efficiency, and increase capital investment, operation, and maintenance costs.

Courtemanche: The best way to determine how much a unit needs to be de-rated in order to cope with harmonic loads is to use the genset manufacturer’s sizing software. The software allows you to input the exact character of UPS and VFD harmonic signature (6 pulse, 12 pulse, 18, pulse, filters) and provides a selection that is compatible. That being said, and in the absence of any better method, I’ve had good luck in performing a preliminary estimate of generator rating using the following:

  • 160 % of the nonlinear load (UPS, VFD, electronic ballast lighting, office equipment, etc.), plus 100% of the linear load (motors, electric heat, etc.) OR 115% of the total load, whichever is greater.

This usually gets you to within one generator size, plus or minus, of the final selection when it is calculated using the sizing software.

Lazebnik: If the generator is not sized and selected properly, both the generator and loads (especially such “harmonics-rich” loads as VFDs or UPS) can experience problems while operating from generator power. The generator shall provide stable power with minor voltage and frequency variations to assure stable operation of VFDs or UPS.

Consideration shall be given to the unit size and nonlinear loads connected relative to the generator size. When a few large nonlinear loads are connected, the generator experiences more disturbances than when only a few small VFDs or UPS are present in the system.

In general, the generator shall be sized so that nonlinear loads will not exceed 40% of its capacity. The percentage depends also on the type and design of the generator and controls, specifically voltage regulation. For example, an isochronous speed governor can help to synchronize the generator speed, thus restricting voltage variations to limits acceptable for normal operation of UPS or VFD. Manufacturers usually specify a generator with 105 F temperature rise, Class H insulation, and permanent magnet generator (PMG) excitation. Certain options (for example, filters) can be specified as part of VFD or UPS to minimize their effect on the generator. Our experience is that the best results can be achieved when manufacturers of UPS, VFD, and generator are involved in the early stages of the design. They can provide recommendations specific to the project to achieve the best possible results.

CSE: Taking into account the motor starting capability of a particular generator, how do you make sure you’re able to start all the motors connected to the generator? If, for example, you have a 65 kW generator and a 50 hp motor, the motor draws 65 kW, so in theory, the generator should start and run the motor.

Courtemanche: (Note: A typical 50 hp motor draws about 40 kW, not 65 kW. I have assumed 40 kW in my response.) Again, but I don’t want to harp on it, the best way to ensure that your selected generator will start and run your motor loads is to model the starting and operation of the loads using the aforementioned generator sizing software.

The software allows you to specify the motor type, starting method, and allowable dips in voltage and frequency that can be tolerated by the load. Also, the ability of a generator to start a motor, or other load with an inrush characteristic, is dependent on the unit’s SKVA (starting kVA). The SKVA rating of a generator is somewhat dependent on the type of excitation system that you specify. A unit with PMG excitation will offer more SKVA than the same unit with static excitation. As you can see, there are a number of variables that must be considered to determine that a genset will start and run its motor load. And it is best to utilize software that will take it all into account.

Using “rules of thumb” to estimate if a 65 kW genset can start and run a 50 hp motor, I would say it depends on the starting method. For an across-the-line start, my rule of thumb is the generator kVA rating should be at least 3 times the largest starting motor, so a 120 kW/150 kVA generator would be required. For a VFD (without bypass) start, my rule of thumb is the generator kVA rating should be at least 1.6 times the largest motor starting, so a 65 kW/81 kVA generator should be adequate.

Just to check myself, I ran these two scenarios using sizing software. The units selected were a 90 kW genset with an oversized generator rated 145 kVA for the across the line start, and a 50 kW genset with an oversized generator rated 88 kVA for the VFD start.

As you can see, rules of thumb will get you in the ballpark, but the sizing software will help you hit the home run.

Wagner: Perform a motor starting study with generator unit(s) as the power supply source using a motoring starting analysis program. The purpose of performing a motor starting study is twofold: to investigate whether a motor can be successfully started under the appropriate operating conditions, and to see if starting the motor will seriously impede the normal operation of other loads on the system. During the motor starting period, the starting motor appears to the system as an impedance connected to a bus that changes through time to reflect the starting of the motor-load combination. It draws a large current from the system, typically about six times the motor rated current, which therefore results in voltage drops in the system and imposes disturbances to the normal operation of other system loads. Since the motor acceleration torque is dependent on motor terminal voltage, in some cases the starting motor may not be able to reach its rated speed due to extremely low terminal voltage. A general rule when starting unloaded induction motors from generators is to keep the voltage dip to less than 0.8 per unit rated voltage. However, if the motors are loaded before start up, especially with other motors operating, a smaller allowance may be required. If operating in island mode, isolated from the utility grid, the generator frequency will also dip. If the engine generator frequency drops too much (typically around 25% below the rated frequency), the engine generator may not be able to recover and will drop offline. This makes it necessary to perform a motor starting analysis.

By design, most modern engine-driven generators can provide 300% of rated KVA for 10 seconds. For most induction motor applications (unloaded starting application), this is sufficient time to get the motor to its rated speed. Thus, as a rule of thumb, a generator sized 2.5 to 3 times the KVA rating of the motor should be able to supply the required starting KVA to successfully start the motor.  

Artzer: When specifying a generator, both the engine and alternator need to be sized for the intended load or motor in this case. The starting “inrush” current for large motor loads has a major impact on genset sizing. The starting power may drive the appropriate size of the alternator beyond the standard alternator size offered for a particular genset by the manufacturer. The manufacturer will often be able to make adjustments to the alternator size as long as it fits on the standard frame size for the particular genset model. Custom arrangements may not be as economical as standard gensets offered with larger matched engine and alternator. 

Across-the-line starters will require high starting power for the genset to overcome. Reducing the starting current may help in reducing the genset size. For example, a solid state starter will reduce starting current but will create voltage distortion due to the SCRs, which is typically compensated with a larger genset. VFDs are current limiting and reduce starting power, but the reduced current is nonlinear, which induces harmonics onto the system. Since VFDs are nonlinear they may also require additional generator capacity to keep voltage distortion to a reasonable level. Use typical generator calculations and manufacturer sizing tools. Then consult the manufacturer for your specific application.

CSE: How would you sequence the loads onto a generator to allow it to operate more emergency load?

Artzer: In some cases, load sequencing may help in reducing the size of the generator. In order to do so, it is necessary to have load steps to minimize the size of the generator. Load steps can be accomplished with multiple transfer switches or with other strategies such as timers on motor starters or BAS integration. The load with the largest starting “inrush” current would need to be sequenced to start first, then adding remaining load steps using the same largest motor first concept. The load steps in the sequence should be long enough for the alternator to recover before adding the next step. This is typically a couple of seconds for larger equipment loads.

This concept works when the large motor loads are started and then run continuously. But when on/off cycling large motor loads exist, the system still needs to be sized as if it were the last load to start with the other loads already energized. However, with cycling motor loads, reducing the starting current (and starting kW) by use of a solid-state starter with bypass contact can also help reduce the overall generator size.

Also, the load priority needs to be considered when sequencing loads. For example, in a hospital, the emergency system loads consisting of the life safety and critical branch loads are typically the highest priority because they must be online within the required 10 sec. Code requirements for load priority can also have an effect on generator system sizing. Depending on the size of system, the largest load first method may still be applied after code required loads are energized to help in overall system sizing.

When large continuous motor loads are present, proper load sequencing and managing the starting KW of the generator can assist in reducing the size of the generator.

Wagner: The control of load pickup and removal becomes an important factor in maintaining emergency power supply system stability and power quality. Starting of certain load feeders may be inhibited when the on-line generators have insufficient capability. When the on-line generators have sufficient capability, load feeders may be automatically re-closed in a controlled manner after load shedding. Avoid the addition and removal of large blocks of emergency loads. If response time and load priority permit, the largest emergency load should be added first, followed by smaller emergency load blocks. Allow sufficient time delay in the sequencing of emergency load pickup and load shedding. Load priority is a decision that the facility operator must make based on impact to the emergency load supporting functions, with consideration given to load characteristics and emergency power system limitations.

Courtemanche: If the application allows loads to be sequenced on, you can usually achieve the smallest generator size for a given total load by sequencing the largest motor on first and then the remaining load in multiple steps. The theory is that the large motor will assist the generator in starting other loads with inrush characteristics in later load steps. The caution here is that the largest motor needs to be a constant, not a cyclic, load. In other words, once started, it stays running. Otherwise, cyclic motor load should be sequenced on last to ensure that the generator is sized correctly to start the cyclic load at any time.

CSE: What do you do to make certain that your generator has adequate load during an exercise during wet stacking or reduction in operating capacity?

Lazebnik: Properly sized and selected generators should have an available load of at least 70% to 80% of the rated output of the generator. So what happens when you are exercising your generator or it is running during off-hours with very little load on the system? Diesel generators that run for extended periods of time with less than 40% load tend to have problems with wet stacking.

Wet stacking is caused when diesel generators are run under light loads and the engine is not able to get up to operating temperature. If the engine does not get up to the proper operating temperature, unburned fuel gets exhausted and accumulates in the exhaust system. Frequent wet stacking can shorten the life of a generator, increase maintenance costs, and increase the level of emissions produced. Wet stacking can be alleviated by running the generator under a load for several hours.

Having an adequate building load available for the generator is the best way to ensure that your generator will operate properly, but that is not always easy to do. For hospitals, data centers, and other facilities with critical loads, it is not always practical to use building load for generator exercising. Load banks can provide the necessary load for generator tests, but are an additional investment if they are permanent and can be expensive to rent and connect for frequent tests. Supplemental load can be provided by connecting noncritical loads to the generator through manual transfer switches that can be switched onto the generator for exercising or testing.

Artzer: Wet stacking is typically associated with diesel generators. Diesel generators need to run at least 30% of rated load to avoid wet stacking effects. The wet stacking effects are due to the engine not running at a high enough temperature to burn all of the fuel.

In order to avoid potential wet stacking conditions, the designer can implement possible remedies. A permanent load bank can be incorporated into the design to add necessary load to the system during routine testing. Provisions for a portable load bank to be easily connected during routine maintenance may also be provided.

Auxiliary loads, either automatic or manual, can be very beneficial to the facility. This would typically consist of a large mechanical load served by a separate ATS that could assume the load necessary to avoid wet stacking. On a standby generator this may be a non-essential load that could be added to the system when capacity is available, or be load shed when reaching the limits of the rated genset. For example, a chiller and associated components may be an available but non-required emergency load for hospital or other applications. However, regardless of any designed opportunities to eliminate wet-stacking effects, the maintenance and testing personnel need to be aware of and take advantage of these provisions. Adequate education and communication with the maintenance staff is necessary to ensure that the intended methods to avoid wet-stacking are implemented.  

Courtemanche: In order to maintain the reliability and longevity of a genset, it is my opinion that you should maintain a minimum of 30% rated load during any operation, be it exercise or supplying load. The least expensive way to achieve this is judicious sizing of the genset so that minimum load on the generator is at least 30%, allowing the use of the system load during exercise or actual operation. But then, that tends to be in the perfect world.

In those cases where the 30% minimum load cannot be maintained, such as a purposely oversized genset for future load or an application where the load is all or nothing, the use of a supplemental load bank should be considered. My preference is to size the load bank for full generator rating with the capability of dialing it down. This allows for both maintaining a minimum 30% load and full load testing of the generator. At the very least, provisions for adding a load bank should be included in the design.

Wagner: Wet-stacking is a common problem with diesel engines, which are operated for extended periods with little or no loads applied. When a diesel engine operates without sufficient load, it will not operate at a high enough engine temperature to completely burn all of the fuel. The unburned fuel tends to accumulate in the exhaust system, which can foul the fuel injectors, engine valves, and exhaust system, including turbochargers, and reduce the operating performance. Generally, the major manufacturers suggest a minimum load of 30% of nameplate to prevent wet stacking.

How to address this problem? Relying on the building load for generator exercise and load testing are not practical for facilities with critical computer, life safety, or communication equipment. Any interruption of power to these loads may cause a loss of data or operations, or jeopardize personal safety. Instead, it may be appropriate to design provisions to connect portable or rental load banks to the generator bus or to provide permanently installed load banks so that the building operators can perform regular, periodic generator testing under load. At a minimum, a load bank with a capacity of approximately 30% to 50% of the largest generator unit nameplate capacity should be provided. The more desirable option is to design the generator switchgear and control system to also allow parallel operation with the connected utility grid. This will allow load testing of any combination of generator units at full load without the need for load banks. The electrical system configuration should be designed to allow testing of single or multiple generator units and be designed such that the critical facility loads are not impacted during on-line generator testing.

CSE: In your opinion, what impact does the use of various types of fuel have on the performance rating of the generator?


Natural gas engines

Advantages:  They perform better for base loading applications, they are cleaner burning, there is an unlimited fuel source during power outages, no onsite fuel storage is required, they are more emissions compliant, and they start well in cold temperatures.

Disadvantages: They do not respond well to transients, have lower power output than gasoline or diesel, have higher first costs, have shorter life expectancy than diesel generators, and natural gas may be unavailable during natural disasters.

Diesel engines

Advantages: Diesel engines perform better in situations with large load fluctuations, diesel is the least flammable fuel source, diesel engines are less expensive to operate than natural gas engines, and they are the lowest first cost generators.

Disadvantages: Lightly loaded engines are subject to “wet stacking,” fuel may not be available during power outages, the shelf life of diesel is 18 to 24 months without additives, they are more difficult to start in extreme cold weather, emission laws may limit run time of diesel generators, storage tanks might require monitoring or double-walled construction, and diesel spills require environmental cleanup.

Wagner: Generator engines are typically designed to burn specific type fuels: No. 2 diesel fuel, heavy fuel oil, natural gas, or JP-8 jet fuel. Some manufacturers design engines to burn multiple fuels. Each fuel type has advantages and disadvantages, including starting speed, emission performance, and economic performance. For example, natural gas engine generators, as compared to diesel generators, will have better emission performance and lower operation and maintenance costs. Another advantage of using natural gas engine generators, when a natural gas supply is available, is that the cost of fuel storage tank construction and refueling operation can be avoided. However, natural gas engine generators are more expensive and do not respond well to fluctuating loads. Engine generators burning No. 2 oil or No. 2D diesel fuel are well suited for applications with large load fluctuations. Diesel engine generators are an excellent choice for base loading and standby applications. Engine generators burning heavy fuel, although they may have lower operating fuel costs, will have poor emission performance. Heavy fuel engine generators have slower response and thus are not well suited for standby application. When selecting an engine generator and the fuel type that it will burn, consideration should be given to the capital investment cost, fuel costs, operation and maintenance costs, emission performance requirements, and availability of fuel for a particular project location.

Courtemanche: It is my opinion that the use of other than straight diesel fuel in a diesel engine will have an effect on the engine performance, usually reducing the engine output and response. That is not to say that you should not consider more environmentally friendly biofuels or blends. You simply need to consult with the manufacturers on how the use of these fuels will affect the engine performance and, therefore, sizing of the unit for the load.

Historically, the use of natural gas fired engines requires a larger kW rated engine than a diesel engine to achieve the required response from the generator to properly serve the load.

1Caterpillar Publication: LEBX0026 (05-00)—Electric Power Application and Installation Guide—Engine and Generator Sizing.