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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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