Accurate Generator Set Sizing

by Visitor bakersj ‎11-30-2009 10:52 AM - edited ‎03-12-2010 08:54 AM

Steve Baker of Caterpillar

Proper sizing is important. A genset under-sized for the application can fail to start or run the site's electrical loads. Adding additional storage room for larger or additional gensets, where none was initially planned or none can be added, can be costly. Also, over-sizing your genset can lead to lightly loaded engine conditions, which can result in wet stacking.

How can you improve accuracy when sizing? Here are some things to think about:

1. Have you considered the differences when sizing motor loads versus static loads? Motor loads have higher starting requirements (inrush); if inrush is not accounted for properly, a genset can fail to start.

2. Has the sizing calculation provided a full accounting for transient response? For example does it calculate genset:
     · frequency dip
     · voltage dip
     · recovery time

It's important that the sizing accounts for more than just the alternator-caused voltage dip, but for the engine-related, or frequency-induced, voltage dip as well. Otherwise, predicted voltage dips can be half as much as actual.

3. Can the sizing calculation account for the lower genset frequency dip that results from an increase to the genset's adjustable voltage regulator setting?

It's important to know if and when this volts/hertz optimization can help size a smaller genset.

What are your experiences with sizing... and are there any tools or tips that you would like to share? Please post below.

by New member renosteinke
on ‎12-08-2009 08:45 AM

I think there is an NEC problem with proper sizing.


Most generastors I have installed have been purely optional generators. When I've installed generators under Articles 700 and 701, sizing has nit been an issue; only with the hew Article 702 requirements does the debate start.


702.5(B)(2)  states that if you have an automatic transfer switch, the generator 'shall be capable of supplying the full load.'


Here is where the debate starts:

It has been my practice to measure the actual load, as best as I can, and to size the generator to be slightly larger. Add an automatic transfer switch and an excerciser, and you generator will be properly maintained.


Some have argued, however, that the 702 language means that the generator must be capable of supplying either the calculated load of the dwelling, or the maximum service supply.


Here's where the trouble starts. Many homes have 200-amp services, yet the highest load I've ever actually measured -in the Nevada summer, with the air conditioning running- has been 70-amps. Furnishing a 48KW generator to power a17KW 'worst case' load is contrary to every instruction I have received from an generator manufacturer.


What says Cat?



by Visitor mrmediumvoltage
on ‎12-08-2009 09:41 AM

I deal mostly with larger medium voltage control products - are there any specific guidelines for Genset sizing for loads that are started with soft starting technologies, i.e. ramp voltage - using a thyristor based convertor or with  Variable frrequency drives. There seems to be conflicting sizing information. Some say a 4:1 genset output capacity to motor load ratio for ramped voltage systems (soft starters) and 2:1 for VFDs.


Does Cat have specific details about spec'ing gensets for these types of LV AND MV applications?


Also, I have heard of issues on the regulation systems when soft starters are applied to the genset due to the harmonic contents being generated as the voltage to the load is increased. Apparently this has to do with analog versus digital regulators on the gensets and that analog is preferred for these types of applications. Can you confirm whether or not we need to specificy analogue regulators on the genset for these types of application?



by New member K9CK
on ‎12-08-2009 11:13 AM
What additional considerations must be taken into account when sizing in the hot dry climate of the middle east?
by Contributor wlj1943
on ‎12-08-2009 01:31 PM

I think Steve Baker has it right, with a slight quibble. When you are dealing with induction motors in theory and acutual practice, there are two inrush values that need to be considered. It is my experiece you need to look at both with motors around 50 HP and larger, maybe much lower depending on generator size and motor load percentage.


The first is the magnetizing inrush, which lasts for about the first 3-4 cycles ( about 0.05 seconds on a 60Hz system). This is the worse case number for most nema design b motors, and can 8 to 16 per unit. Once the the motor starts to rotate and the transformer action is complete, the current value reduces to a lower value while the load accelerates. That current is often calculated based on the nema locked rotor nameplaqte data. The load characteristics for the application should also be considered, for instance high inertia loads like big fans and some older refrigeration compressors my accelerate very slowly before the motor reaches rated speed; this means more time at high current. ISO standard motors are in my opinion much more difficult than NEMA motors due to lack of standard data.


The bottom line on all this is many commercial and probably all industrial applications for power generation need in my opinion the services of an experieced Application Engineer to select and size the equipment. 


Re K9CK's issues,

Temperature,both high and low;  low,and in coastal areas high humidity, sand and dust contamination, lack of qualified maintainers, even high altitude are all issues that occur in the middle east. All the environmental conditions must be properly specified and equipment designed and sized accordingly. Again, this is a  application engineering required for these jobs in my opinion. The CAT application manual referenced elsewhere if a good starting resource for specifiers.




on ‎12-08-2009 09:23 PM

I work with Cat dealer.

Cat do improve the "sizing pcapability" with  software tool to select the genset and a erries application instruction.

I have experienced some sizing process. The major problem is to know the "electrical loads", to know the details about SKVA,SPF,SKW,harmonic load,  the apllication mode, then to find the fitable gensets. Sometimes, the generator manufacturer knows more about the affect of electrical load to generator  from the academic degree, such as X"d. But they do not know the affect of load to engine while the electrical load thansfer to mechanica torque addting to flywheel. So different gensets manufacturer provide different tool for sizing. Sometimes the manufacturer of electrical

appliance could give good suggestion or exact data in sizing, such as UPS, VFD. All help much in calculating load.

Additional, the morden engine is designed for high efficiency, small dimensition, high power density, light weight and so on. The capability of  accepting "Block Load" is worse. The actual site expeerience is important to accumulate the sizing experience.


by New member sunil7090
on ‎12-09-2009 02:31 AM
A new brown field project has connected load of 22807 KVA. Total 13 generators each of 2000KVA (11KV) are proposed.Main loads are chillers (9nos--900KW AC capacity each).Most other loads (like lift,fan,escalators ) are VFD driven. Even chillers have softstarters to limit starting current to 2 times full load current.Let me know whether proposed size of generators is OK or if not, what is the correct size
by Visitor bakersj
on ‎12-09-2009 03:56 PM

sunil7090: Without complete data on the project, I’m not able to estimate size. I recommend that you contact your power systems dealer and/or manufacturer for an estimate. For reference, this is a link to Caterpillar’s online dealer locator:

on ‎12-10-2009 02:32 AM


According to my experience, firstly the Runing KVA of total gensets is enough for the load requirements.The second step is to caclulate if the Paralleling system can meet Starting requirements, so you have to give  the permitted Volt dip and Frequency dip. The third step is to consider the applcation mode, such as intermitted, prime or continuous.

by Visitor sd
on ‎12-22-2009 07:51 AM

Hi David!


Good to hear from you on Genset sizing. Very recently we were evaluating some offers of Gensets for a power plant project. We came across CAT's offer also.


We faced the similar problem of determining accurate size of the generator. We have ETAP in-house. But it does not support DG simulation in an isolated condition. Moreover, both load data from the purchaser and DG parameters from the bidders to simulate accurate conditions were hardly available.

by Visitor sd
on ‎12-22-2009 08:00 AM
Sorry Steve! Typed the name wrong in earlier post.
by Contributor see-deif
on ‎01-19-2010 05:25 PM

Add to WLJ’s  comment about motor starting: another factor is the motor load characteristics. That is why the inrush “rule of thumb” varies from 5x to 12x. High inertia loads, like some fans, some pumps, unloaded compressors, will be much higher.  


You can also use the voltage ramp up feature in the AVR utilizing a technique called Close Before Excitation or Dead Bus Paralleling.

Here is the sequence: Close Before Excitation (CBE) or Delayed Excitation Mode. This is an old mode of starting a generator that has found new life with the need to meet a requirement of NFPA 110, often called the 10 second rule. Life Safety devices must have power restored in 10 seconds. When the total load in this category exceeds the capacity of one generator, CBE can be used to start multiple generators in less than 10 seconds. This is the sequence of CBE:

a.       A start command is sent to all generators at the same time.

b.       A short time after the gensets’ RPM exceeds the crank RPM, the output breaker is closed. Note: the excitation is not allowed to automatically start at this time like on standard applications.

c.       Once the first genset speed reaches about 70-85% of rated 

                                                  i.      any engines still on crank are locked out

                                                 ii.      the excitation is started on all running generators 

                                                iii.      excitation is gradually ramped up, typically 1-2 seconds from zero to rated volts.

d.       Gensets that did not start within this time may be brought on-line via standard synchronization practices later.

e.       As the excitation increases, the generators will be pulled into synchronization. Tests have shown that the current typically stays below 20% of rated, where a standard synchronization of even 15 degrees out can lead to 80% or more of rated current flow. This also reduces the mechanical stress on the generator windings, the coupling, and the engine.

f.         Once voltage and frequency reach 95% of rated values, the load breaker is closed and the system is on line.

g.       Gensets that did not start in time – step c – may be added to the power system with the standard synchronization process.  


This is written from the fast start perspective, but the same basic steps apply for starting motor loads or transformers with high inrush except in step c wait until closer to 85-90% speed to start the excitation. This effectively makes the generator(s) the soft starter.  


Good luck!



by Visitor craiovan
on ‎05-21-2010 04:21 AM

Hi Steve,we have a sitiuation here:

  -3 CAT gen. sets,( 1825kva,1460kw,95.9 amps)each, synchronised .

  -1 electric motor,600kw, 131amps running current..

   Questions;- can these gen sets start this motor.

                       - if not, can we install a soft start , and what size.

     What do you recommend.


    best regards,


by Visitor bakersj
on ‎05-21-2010 09:13 AM

Craiovan, my recommnedation is that you size your loads and site parameters using a genset sizing tool.  Here is a link to order a sizing tool I recommend:


Once you've sized your application, feel free to continue dialogue with respect to the results.


I'd like you to take full advantage of all the engineering that's gone into a genset sizing tool.  Since you have (3) Cat gensets, the SpecSizer software can provide all the answers you need.  The tool has a paralleling feature on the Generator Set Selection tab that will allow you to parallel (3) gensets for your input site conditions and motors / loads.  You'll be able to see the smallest genset kw/kva of (3) paralleled gensets that is required.

by Visitor craiovan
on ‎05-21-2010 03:29 PM

Thanks, Steve, but time is realy a problem, the job is in Africa, and to order the tool , before it gets here, sometime, with your experience what can we do , these gennys have been installed , pannel connected,...etc.., but i doubt they can start the motor, should we go for a soft start?




   best regards,


by Visitor bakersj
on ‎05-24-2010 09:05 AM



If you request the software today, I can authorize the download link be sent to you today.  Once you have the software installed, post here if you still need my assistance with the sizing. When you order the software at 

indicate that it is being requested by Craiovan.


I am contacting our Help Desk now to ask them to watch for your (Craiovan) software request, and to email you the download URL today.

by Contributor Donsumudu
on ‎05-28-2010 04:05 PM

Inorder to size a generator properly for a particular application you need to have;


1. Better understanding of the application process ie say type of loads such as standing loads, block loads,  DOL, SS, VSD, UPS, transformers, and loading sequence or pattern, load shedding capabilities.

2.Data sheets of the motors and starting methods. Say Soft Starting 6 pulse or 12 , you need to understand that motor starting with load or without load, transformer inrush.

3.Future load growth; Ex: 10% or 20 % reserve capacity for load growth.

4.Better understanding of application type ie Prime, continuous, standby.

5.Site conditions ie Max ambient temp, altitude etc.

6.Type of fuel ( for this discussion I assume Diesel)

7.Voltage, frequency. then G1, G2, G3, G4 class refer to ISO.

8.Load response for frequency variations. (frequency limited)


10,Load factor

11.Multiple unit applications possibility (separate circuit or parallel operation)

12.Type of cooling of the generator you proposed (package mounted radiator or remote radiator)


Most of the time we prefer to start high starting current motors (DOL) at first step , then VSDs, and SS, and at last UPS loads. But in practice this sequence may not be possible as some loads are continuous and some loads are intermittently ex: air conditioning plants are intermittent, ventilation blower fans may be continuous. Water pump station pumps could be intermittent or conituous.


Now a days we have sizing software that help us to quickly size the generator that we required. But it is very important that you should use these tools as only a guide to understand the kVA range of  the generator required and should consider the other factors such as operating procedures at the site. Also, It should be noted that sizing software have certain limitations that they have limited data base which is pre entered and if that is not connected to on - line updates it can indicate over size generators for the application. For example if you have minium capacity of 200kVA unit in your data base then it will give you 2 x 200kVA units for multiple applications and would be an oversize for the application.


I prefer to gather the information as much as possible from the site and use the software only as a guiding tool , and always discuss with the end user to understand their operation at the site. 

If it is a existing site you can ask for a load profile which will be able to understand the load peaks and maximum demand, operating pf  at the site.

If it is a new site you can estimate the maximum demand and running kVA using calculation methods indicated in standards such as IEE, AS.

Then you can enter the data in to the software and check various options that you could apply .Better to send a copy of sizing file to site engineer for his feedback before decide, and discuss with him.


For high starting and block load applications better to have permanaent magnet excitation systems which gives 300% short cct for 10sec, and it is better than AREP excitation system as AREP system take slightly longer time to buid up the voltage of the control winding. Remote radiator option of the generator may give better block loading capability , than package mounted radiator (engine fan driven) option?


Self excited system is cost effective solution for failry flat load pattern (general office, without lifts and escalators).

For  block load applications, if you donot have sufficient data available it is better if you can carryout site testing using a temporary generator (hired) to understand the behaviour of the load.


Also, you need to consider the regenerative loads when you are sizing generators for the applications such as cranes, lifts, escalator loads,and suggests load banks where it is necessary.



Thanks & Regards












by New member jimmy-chance
on ‎10-31-2010 05:52 PM

Does anybody have advice on reasonable light load limits.  I am looking at C4.4 99 kW units or C6.6 units of 150 or 170 kW. I have always used 30% as my light load limit but I have heard 2nd hand that now says 70%, which does not seem right.

by Trusted Contributor
on ‎02-03-2011 06:24 AM
this comment was moved to a separate thread in the discussion board Power Generation: Site Design, Installation, and Connection

by New member JK1956
on ‎02-13-2011 01:48 PM

How accurate is SpecSizer - can the load acceptance figures, recovery times, volt and frequency dips be taken as completely verifiable and reliable - is there any margin for conservatism built in?

by Visitor bakersj
on ‎03-01-2011 02:44 PM

Dear JK1956,

I apologize for my delay in response.  

Significant efforts are made to provide actual and accurate gen-set transient performance data for gen-set models in SpecSizer; our goal is to document the gen-set transient performance within a 5% variation, which is a typical gen-set model-to-model performance.  In managing the SpecSizer tool, we evaluate the primary, secondary and tertiary effects on gen-set transient response, and factory gen-set test data is calculated for the SpecSizer tool to update any gen-set model's transient performance, for new product introduction, and for changes to the gen-set iron that would cause a notable change to the influencers on transient performance.

As with any tool, we recommend that a licensed contractor/engineer is directly involved with sizing determinations.  Please see our SpecSizer software disclaimer below:
"Caterpillar makes no express warranties and disclaims all implied warranties including merchantability and fitness for a particular purpose regarding program.  Caterpillar shall have no liability in law or equity for damages consequential or otherwise arising from use of program and related material or any part thereof.  The analysis provided from SpecSizer is only for the expected results at the generator terminals.  Analysis of transient conditions of any device downstream is the responsibility of the system designer."


by New member randyleesteel
on ‎05-12-2011 02:11 PM

Steve, any chance that Specsizer can be updated to provide sizing for Natural Gas Generators?  We specify 80% of our Generators for Natural Gas, about 20% for Diesel Gensets.   thanks.

by Visitor bakersj
on ‎05-13-2011 10:10 AM

Randy, our Cat dealers can size the gas generator sets for you. The Cat dealer can also work with our Electric Power A&I group if they need sizing assistance. We intermittently look at adding the small gas gen-set sizing back into the SpecSizer tool and I will again review the cost and supplier data concerns. I am working with our new EP gas group to develop a foundation for either adding the large Cat-brand gas gen-sets to SpecSizer, or to develop a standalone tool.  For now, you can provide your sizing information for either small or large gas gen-sets to your Cat dealer and I am certain that they can size your gen-set with you. Please let me know if you would want me to identify your Cat dealer via the "Find Cat Dealer" link on You actually can use SpecSizer to calculate all the sizing parameters for your site conditions and load scenario, but you would need our dealer or the Cat A&I group to marry those sized parameters up with the gen-set list (NG gen-set would derate differently vs. diesel, and transient performance is specific to the gen-set iron. Thanks for your inquiry, 

by Visitor edspl
‎05-26-2011 01:57 PM - edited ‎05-26-2011 02:03 PM

Hi Steven Baker,


I am working on "Light Up Bamyan" project of the Agha Khan Foundation. The project is for providing evening lighting to a few hundred rural village houses and small shops in the Bazar area in the provincial capital of Bamyan in Afghanistan. The expected load per connection is 200watts for residential houses and 300watts per small shop in bazar.


The Project intends to utilize existing Cat diesel gen set (Model 650F, serial number CAT00000L4BZ04884, yr of mfr 2003, rated prime power 600KVA, max altitude 152.4m, 40 deg celcius). It is to run in parallel with another existing diesel gen set (MARAPCO, P165E1, Serial number FGWPEPO5JCOA07871, yr of mfr 2007, rated standby power 165KVA, max altitude 152.4m, 27 deg celcius).


The end uses will comprise lights (mostly tube lamp, CFL lamps and may  be a few LED street lamps), household TVs and small refrigerators in some of the small shops. The generators are to provide basic evening lighting until a larger supply source becomes available. A 1 MW minihydro is being planned for development soon, and the national grid might be arriving in the next 5 to 10 years. Once the hydro supply becomes available, these generators might be used as add-in during peak load hours.


I am working from home base (Nepal) on the design of MV distribution, LV lines and consumer service connections. I may be travelling to the project site later to support field works. Right now, on the generation side, except for the DG itself and the name plate details on it, i have no documented information. I need to know the following.

(a) Maximum KVA load I can subject the CAT generator to at the site altitude of 2500m and 25 deg celcius max ambient summer temp.

(b) Parallening prospects and problems. With the other MARAPCO gen set, and with the 1 MW hydro unit in future.

(c) Based on the serial number and the yr of mfr, can i get the technical documnents and instruction manuals pertinent to the CAT set? This will enable me to understand the sets and the generator control & instrumentation panel better for proper installation and putting into service.

(d) Any other information that might be useful for me to accomplish the task of using the CAT gen set to electrify the remote villages in Afghanistan.


I look forward to hear from you. And please let me know if you need any further information, which I can relay to my colleague at site and try to get. Thank you.


Shankar Vaidya


PS Kindly send an email to with any attachment/documents relating to the CAT genset.


Existing Cat gen name plate

by Garotas
on ‎10-10-2016 08:52 AM

Very article !

by charliewatts
on ‎01-23-2017 03:48 AM

What additional percentage should I add to the genset rating when the harmonic contribution is 10-15% THDi?


How much does that range of THDi de-rate the genset?




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