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Design calculations

Design calculations establish minimum guidelines and requirements for generating electrical calculations on projects. Electrical calculations should be made for all projects that include electrical components and should be filed in the project notebook.

Electrical Design Calculations Needed For Projects
Electrical Design Calculations Needed For Projects

Design calculations may be made either manually or by computer programs. At a minimum, the following types of calculations should be made where applicable:

  1. Load calculations
  2. Conductor sizing
  3. Conduit sizing
  4. Motor branch circuit sizing
  5. Power factor improvement
  6. Transformer primary and secondary circuit sizing
  7. Voltage drop
  8. Motor starting voltage dip
  9. Short circuit analysis
  10. Lighting levels
  11. Grounding in substations (where step potentials are of concern)
  12. Harmonic distortion analysis
  13. Cable pulling calculations
  14. Generator capability/motor starting.


Load calculations should be made using applicable sections of NEC Articles 220, 430, and other sections of the NEC.

The following load calculations should generally be used for sizing:

  1. Feeder conductors and protective devices
  2. Transformers
  3. Panelboard and switchboard main busses
  4. Motor control center components
  5. Service entrance devices and conductors
Load calculations must include all loads. They should be made by summing all of the loads (using appropriate diversity factors allowed by NEC Article 220) that are connected to each panelboard, switchboard, and motor control center.

An allowance must be made for future load growth. The loads for each branch of the distribution system can then be summed back to the service entrance equipment.

Generator Sizing

The following information is intended to familiarize the design engineer with terms used by generator sizing software and underlying formulas.

Generator single or multi-sets must be sized to supply maximum starting (SkVA), stead-state running (RkVA) and non-linear (GkW) demands of connected and future electrical equipment.

Information critical to the sizing and selection of generator single or multi-sets include:

  1. Environmental conditions: elevation, temperature, indoor or outdoor
  2. Noise abatement requirements: mufflers, enclosure, silent models
  3. Fuel: diesel, gasoline, natural gas
  4. Fuel storage: skid mounted tank, day and remote tanks
  5. Cooling: liquid cooled radiator, forced air
  6. Voltage regulation: maximum allowable voltage dips
  7. Operation: prime, standby
  8. Voltage ratings: voltage, 3-phase, 1-phase, solid grounded, delta, wye
  9. Connected loads: Linear, non-linear, power factor
  10. Load operation: Motor starting methods, single step, single step with diversity, multiple steps of loading
  11. Future loads

Sizing Procedures for Generator Single or Multi-sets

The following is the sizing procedure for generator single or multi-sets:

  1. Prepare a load schedule.
  2. Enter individual load characteristics in software.
  3. Enter loads in step sequence in software.
  4. Have software calculate and select a generator set.

It is a good practice to request a verifying calculation from the preferred genset manufacturer.

Title:Electrical Design Calculations Needed For Projects – Seattle Public Utilities
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Electrical Design Calculations Needed For Projects

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    Mar 10, 2023

    Can you send me the Electrical Design Calculation pdf.

  2. warez ali
    Jan 27, 2023


  3. Shumbulo Shuta
    Jan 27, 2023

    thank you

  4. Chris Wu
    Jan 26, 2023

    Very nice article. Can you please send me the PDF of Electrical design calculation as well?
    Thank you very much.

  5. parisa rajaei
    Jan 26, 2023

    how I can get PDF?

  6. jameel
    Dec 11, 2022

    how can get the electrical calculations laws .. i’m electrical superviosor

  7. Okafor. Eleazer
    Dec 01, 2022

    This article is very informative.
    Please send me the PDF copy.
    Thank you.

  8. AKASH A S
    Oct 27, 2022

    please send the pdf of this topic: Electrical Design Calculations Needed For Projects

  9. Juan Carlos Guiardinu
    Oct 19, 2022

    Thank, Can you send me the PDF..

  10. William
    Sep 28, 2022

    I am beginner in Electrical engineering and would like to grow in terms of design. Please assist

  11. Arockia Das Middle Name Antony
    Sep 01, 2022

    Dear Sir,
    Thanks. This article is much useful.

      Sep 04, 2022

      Good Day dear Arokia
      Would you please to send me this file, I am interested to get this file
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    Sep 01, 2022

    Hello who may concern
    Would you please to send me this pdf (electrical design calculations needed for projects)
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    Jul 11, 2022

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      Mar 09, 2023

      I’m interested in electrical design calculation on AC, and DC, and Solar system

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