How standby generators work and how to dimension them?
How standby generators work and how to dimension them? (photo credit:

Standby generators

Standby generators (Figure 1 below) are used in order to provide consumers in the system entitled to standby service in the event of breakdown or shutdown of the normal power supply. They consist of the generator, the flywheel and the motor.

For the dimensioning of the generator power to supply a large number of consumers, the connected load with the coincidence factor must be taken as the basis.

This connected load should be about 60 % of the rated load for the standby generator. The prime mover must be designed for the effective power and the generator for the apparent power. The feed-in of the network to the automatic standby power control takes place through the secondary distribution, to which other consumers are also connected.

The functional and operational reliability of standby generators is ensured only when the planning takes account of all requirements for channeling air intake and exhaust air, room ventilation, sound attenuation and fuel supply!

Schematic of circuit for standby generators
Figure 1 – Schematic of circuit for standby generators (click to expand schemes)

The main components are:

  1. Generator
  2. Battery system
  3. Emergency standby control
  4. Fuel system
  5. Exhaust system with sound attenuation
  6. Fans for room ventilation
  7. Air intake and exhaust ducts

The starting power (Figure 2) and the correction factor (Figure 3) for consumers with a high starting power, taking into account the still permissible voltage dip, are calculated as follows and an additional 20 % reserve is planned to cover future energy requirements.

Starting power / rated standby generator power
Figure 2 – Starting power / rated standby generator power

Correction factor
Figure 3 – Correction factor

The starting power:

Generator starting power

Generator power:

Generator power

Motor drive power:

Generator motor drive power

The meanings of the symbols in above formulas are:

  • IA – Starting current in A
  • Sst – Consumer starting power in kVA
  • In – Rated current in A
  • k – Correction factor for operating conditions
  • P – Effective power in kW
  • PrM – Motor drive power in kW
  • SrG – Generator power in kVA
  • cosφ – P Power factor

Generator specific limiting operational values

Application // Communal facilities

After a switching time of maximum 15 seconds, 100% of the consumer power for the required safety equipment must be supplied (Table 1).

Table 1 – Electrical installations in hospitals and communal facilities

Characteristic parameterCommunal facilitiesIn hospitals
Static frequency deviation5%4%
Dynamic frequency deviation± 10%± 10%
Frequency adjustment time5 s5 s
Static voltage deviation± 2.5%± 1%
Dynamic voltage deviation± 20%± 10%
Voltage adjustment time4 s4 s
Steady state short circuit current3×In (3 s)3×In (3 s)
Total harmonic distortion of voltage3 ∼ < 5%3 ∼ and 1 ∼ < 5%

Application // Electrical installations in hospitals

After a switching time of maximum 15 seconds, 80 % of the consumer power (consumers of the required safety equipment and operationally important consumers) in a maximum of two steps and after a further 5 seconds 100 % of the total consumer power must be supplied (Table 1 above). For inductive consumers the large starting current must also be considered (Table 2 below).

Table 2 – Standard values for starting currents

Startup procedureIA/IrM
Direct startup4 … 6
Squirrel cage rotor via star delta2 … 3
Squirrel cage rotor via frequency converter1.5 … 3
Squirrel cage rotor via phase control2 … 4

Type classes 2 and 3 – Connecting and disconnecting loads takes place in accordance with the regulations

Table 3 – Type class 2 and 3

Characteristic parameterClass 2Class 3
Static frequency deviation5%3%
Dynamic frequency deviation± 10%± 7%
Frequency adjustment time5 s3 s
Static voltage deviation± 1%1%
Dynamic voltage deviation+ 22%; – 18%+ 20%; – 15%
Voltage adjustment time6 s4 s
Steady state short circuit current3×In (3 s)3×In (3 s)
Total harmonic distortion of voltage3 ∼ < 5%3 ∼ < 5%

Planning a standby generator

The following features must be considered during the planning of standby generators:

  1. Characteristic parameters of the network, such as type and number of active conductors of the feed-in, type of ground connections (mostly TN-S systems)
  2. Determination of the power requirements with the coincidence factor
  3. Expected three-pole and single-pole short circuit currents at the feed-in point
  4. Distribution of phase loads for the operational equipment of the electrical system
  5. Switching between network and standby power with mutual mechanical or electrical locking
  6. Connection of the standby generator to the existing network takes place up to 125 A through a 5-pin CEE plug-and-socket device with HO7RN-F line.
  7. System ground can be in the form of a concrete footing electrode, ring conductor or buried grounding electrode.
Example of a priority services switchboard supplied (in an emergency) from a standby generator set
Example of a priority services switchboard supplied (in an emergency) from a standby generator set (schematic credit:

Example – Calculation of standby generator power

The following data are known for a three-phase motor:

  • Speed = 1480 rpm
  • Rated power = 22 kW
  • Rated current = 42 A
  • Power factor cosφ = 0.88
  • Starting current direct: factor 5.5
  • Starting current star-delta: factor 2.3
  • Voltage dip 15 % in accordance with communal facility regulations

For the correction factor k, from Figure 3 we find a value of 0.97. Starting power with direct startup:

Standby generator starting power formula

Starting power with star delta connection:

Starting power with star delta connection

For 15 % voltage dip, from Figure 2 we find a factor of 1.25. The required standby power for the delta connection is:

Required standby power for the delta connection

and for the Y/Δ connection:

Required standby power for Y/Δ connection

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Reference // Analysis and design of low voltage power systems by Ismail Kasikci (Get hardcopy from Amazon)

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

One Comment

  1. KOUDJIL Mohamed
    Aug 01, 2016

    how could you find the value k=0.97 of the correction factor ?

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