Your questions answered: Generator Set Sizing and Specification

The April 12, 2018 “Generator Set Sizing and Specification” webcast presenter addressed questions not covered during the live event.

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This presentation is designed to educate consulting specifying engineers regarding the various factors that influence the sizing and selection of a generator set, and how familiarity with these factors can contribute to more economical and reliable facility designs. The presenter discusses NEMA MG 1-2016: Motors and Generators standards for generators used in standby, prime, and continuous power applications. The speaker considers assumptions that sizing programs from the manufacturer use default values—such as starting and running power factor, motor starter types, motor code, etc.—and their implications in terms of generator sizing.

During this webcast, the presenter compares various starting methods including across-the-line starting and soft-starting solutions. Load sequencing, also known as load steps, are investigated and discussed. Intermittent loads, such as air conditioners and compressors, are reviewed and considered, including the implications when specifying generator sets.

Presenter Nicholas Paolo, business development manager, MTU Onsite Energy, responded to questions not answered during the live Generator Set Sizing and Specification webcast on April 12, 2018.

Question: If I have only 30% load on a generator, what could be done to allow the genset to run for a period of time?

Nicholas Paolo: A load bank could be added to the installation to add additional load to a diesel generator set so the engine reaches the recommended operating temperature or minimum load recommended by the manufacturer.

Q: I need to size a generator for a small free-standing emergency department building (11,000 sq ft). The diesel generator needs to serve the CT scan machine, the MRI scan machine, and the general X-ray machine. Does the software permit this modeling?

Paolo: These loads could be entered as MISC loads but the starting/running kVA/kW, voltage dip requirements, and harmonic details would need to be known for accuracy.

Q: Is there a standard range for voltage dip?

Paolo: Yes, 15% to 30% is a common voltage dip requirement, which will be defined as the requirement of the loads but the requirement can be higher than 30% or lower than 15%.

Q: Per which NEMA code did you quote regarding 80°C?

Paolo: NEMA MG-1.

Q: Where can I find descriptions on the generator end numbers shown in PSSPEC?

Paolo: By contacting your local MTU Onsite Energy dealer or by referencing Marathon Electric’s website for the examples from this presentation.

Q: Please explain why the fire pump running (facility on fire?), yet other loads, such as air conditioning units are allowed to start?

Paolo: This ultimately comes down to how the system is designed and if there are controls in place to shed the air conditioner if the fire pump is running or called to start.

Q: How do you specify generators across different manufacturers (equivalents)? Often, kW from manufacturer A isn’t the same size of manufacturer B.

Paolo: If the kW from different manufacturers isn’t the same, a sizing exercise from each manufacturer can be performed to determine the size of genset selected or a spec can be written with the loads defined and the party bidding the job is required to provide a sizing report. For example, some manufacturers provide 2,800 kW generator sets as opposed to 2,750 kW generator sets so both options could be listed in the spec or the bidder is required to confirm compliance via a sizing report.

Q: Is the genset sizing tool free?

Paolo: Yes.

Q: I understand that this presentation refers to MTU generators, but is the software applicable/usable to other brands?

Paolo: Yes, but it is recommended to produce a sizing report for the project loads to confirm compliance of other brands.

Q: What is the difference between an “emergency” generator and a “standby” generator?

Paolo: International Organization for Standardization (ISO) 8528-1-2005: Reciprocating internal combustion engine driven alternating current generating sets—Part 1: Application, ratings and performance defines emergency standby-, prime-, and continuous-rated generator sets. NFPA 70-2017: National Electrical Code (NEC) references emergency and standby systems.

Q: Does the temperature rise rating of the alternator affect the generator’s ability to handle harmonics?

Paolo: It can. For example, a 176°F (80°C) alternator will have more copper and iron in it because it is “oversized” when compared to a 266°F (130°C) alternator and the 176°F (80°C) alternator can handle harmonics better. Ultimately, a sizing exercise will need to be performed with the loads to determine the size/temperature rise of the alternator required.

Q: What resource do you recommend to become more informed on the technical aspects of generator set temperature rise?

Paolo: NEMA MG-1 and also reach out to your local MTU Onsite Energy dealer.

Q: Does the MTU sizing software display harmonic distortion values, and does this vary the recommended sizes?

Paolo: The sizing software can account for total harmonic current distortion of an entered load and size the generator set accordingly.

Q: Does a UPS-type load require a greater generator capacity than a similar motor load with the same kVA requirement? In in the past, the rule of thumb was 1.5 times the UPS load to assign a generator capacity.

Paolo: Due to differences in UPS technology, we do not recommend using the 1.5 times the UPS load rule of thumb to assign a generator capacity. We recommend to enter the actual UPS load with rectification type, recharge rate, etc., and complete a sizing report.

Q: I usually put HVAC units in step 2, because most have an automatic time delay, if running. Is this OK?

Paolo: If the time delay will allow for the generator set to recover to stable voltage/frequency and the HVAC unit will start in step 2 not 3, 4, 5, etc., then yes.

Q: Is it possible to select genset size as per total running kW, max starting kW, and cumulative step load rather than going with size being suggested by generator sizing tool?

Paolo: We recommend using a generator set sizing tool to address power factor/kVA, nonlinear loads, and harmonics in addition to your values listed.

Q: Is it cost effective to add variable frequency drives (VFDs) to reduce generator sizes?

Paolo: This would be an engineering economics exercise where the cost of the VFD must be compared to the savings in generator set size.


Nicholas Paolo, PE, is the Business Development Manager for MTU Onsite Energy.

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