8 Factors you Should Consider When Sizing Conductors

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I found that cable sizing is a very important skill that every electrical engineer has to be aware of. So today I’m sharing what I learned about conductor sizing from The electrical systems design specification handbook. In this article, I’m going to present a summary of some main points on cable sizing.

Factors in Sizing Conductors

There are many different factors we should take into account when sizing conductors including:

  • The amount of current drawn by the load we want to serve. The cable must be sized based on ampacity (current-carrying capacity). Cable ampacity is the amount of current that the conductor can carry under condition of use.
  • Physical distance between the source and the load.
  • The ambient conditions in the area where the conductors will be installed.
  • The type of method used to mechanically protect the conductors. Popular wiring methods include conduit, cable tray, encasing in concrete and direct burial.
wiring method: Cable tray which is mechanical support used in manage & protects cables
  • The resistance of the cable. Cable resistance results in electrical losses (P = I^2*R = heat), which results in a voltage drop.
  • Ease of installation. Heavier conductors for large power distribution systems are harder to install or remove.
  • Access for maintenance and inspection. It should be noted that stranded conductors are easier to terminate and bond.
  • Future growth. It is common practice to add 25% to the installed current-carrying capacity to main feeder circuits when performing a design.

Formula for Sizing Conductors

The selection of the conductor size is based on the above considerations including, cables ampacity under actual site installation condition, voltage drop, etc. . For example, size of underground cables is different in case the cable was in air.

Multiply by root three in case 3 Phase
Current Rating

Formula For Voltage Drop:

where VD = voltage drop, L = length, I = FLA, R = AC Resistance, cos(theta) = power factor, X =Reactance.
  • 14.53/400*100=3.6325% which is lower than the maximum permissible voltage drop of 5%.

Processes of Sizing Conductors

1- Calculate the VA to identify the load (V x A x 1.73 for three-phase loads or V x A for single-phase loads).


1- Efficient Electrical Systems Design Handbook by Albert Thumann.



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Electrical Engineer KAU 2019 👩🏻‍🎓✨