Critical Facilities & Data Center Design Consideration: Generator Systems Design (3)

Further to the the topics of Paralleled Generator System and Generator Ratings, generator sizing is important for your critical facilities. Design engineers should learn how generator-sizing calculations are performed, whether by hand or using software.

Most generator manufacturers provide generator-sizing software to assist design engineers and generator system specifiers. Although the generator-sizing software is a very handy tool, the design engineer must evaluate the load and performance characteristics before selecting one generator set over another.

It should be noted that generator rating results are manufacturer-specific and may need to be derated for ambient temperature, altitude, and harmonics. Voltage dip and frequency response will vary between generators from different manufacturers.


To perform manual generator-sizing calculations, the following information is required for each load:-

Load starting information:
starting kilowatts (SkW), starting kilovolt-amperes (SkVA), and starting power factor (PF)

Load running information:
running kilowatts (RkW), running kilovolt-amperes (RkVA), and running power factor (PF)


For motor loads, this information can be derived from nameplate data:
horsepower, efficiency, locked-rotor kVA/horsepower, motor-starting PF, and running PF

In addition, nonlinear load (miscellaneous load) characteristics would be required to appropriately size the generator alternator and select the optimum exciter type. The generator loading sequence will determine how the SkW, SkVA, RkW, and RkVA are summed to find the generator's total SkW, SkVA, RkW, and RkVA. The generator is subsequently selected to meet the minimum RkW, RkVA, SkW, and SkVA required from the manufacturer's generator specification sheets.


Here is the sizing calculation method (roughly):-


MOTOR LOAD: Equations to calculate SkVA, SkW, RkVA, and RkW:

SkVA = motor hp x locked-rotor kVA/hp (Motor Code, refer to the table of  below)
SkW = SkVA x starting motor PF (Power Factor)
RkW = motor hp x 0.746 kW/hp/efficiency (1 Electrical Horsepower (hp) = 0.746 KW)
RkVA = RkW / running motor PF

MISCELLANEOUS LOAD: Equations to calculate SkW and RkW:

SkW = SkVA x starting PF
RkW = RkVA x starting PF (Load kVA remains constant: kVA = SkVA = RkVA)

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SMA connects IT, Facilities and Design. For the other design considerations, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection,
(8) Raised Floor,
(9) Code & Standards,
(10) Transformers and Harmonic Distortion,
(11) Multi-mode UPS Systems,

(12) Electrical Rooms,

(13) Generator Systems, etc.

All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.