How to Size Conduit for Cable

Written by alex smith
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How to Size Conduit for Cable
Use conduit to protect wiring from the weather and hostile environments. (electrical spools image by jimcox40 from

Conduit is used in construction to provide a safe path for wiring to pass through. When selecting the size of conduit, plan for the number of wires that will pass through it. Due to safety laws, a conduit must contain a certain portion of empty space. With a bit of math, quickly determine the exact size the conduit needs to be in order to properly support electrical requirements.

Skill level:
Moderately Easy

Things you need

  • Calculator
  • Conduit size chart

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  1. 1

    Determine the diameters of each cable that must pass through the conduit. This information is available on the wire's spool.

  2. 2

    Square each diameter (multiply it times itself) and add up all of the squared numbers into a grand total.

  3. 3

    Find the area of the cable in square inches by multiplying the total by: .5927 for (one cable, by 1.0134 for two cables and by .7854 for three or more cables.

  4. 4

    Check the calculated area of the cable against a conduit size chart to determine the size conduit that you must use.

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