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Wednesday, June 21, 2017
Question:
Hi, on the 2011 NEC Chapter 9, Table 1 Percent of Cross Section of Conduit and Tubing for Conductors, it gives 53 % for 1 conductor, 31% for 2 conductors, and 40% for over 2 conductors. It means that the percentage is increased when there are more than 2 conductors.
Please, can you explain the reasoning behind those values?
Thank you for your everyday help and for spreading your knowledge.
Marco Perez
A
Answer:
Hey Marco thanks for your question and the kind words. Raceways must be sized to allow conductors to be installed and withdrawn without damaging their insulation as stated in 300.17. As you said a single round conductor in a round raceway can take up 53 % of the space as stated in Chapter 9 Table 1 because it is a fairly efficient use of space. Conductors are typically round and many raceways are round so it is about the relationship between diameter and area.
To help illustrate draw a circle with a 2 inch diameter, in the circle draw two 1 inch diameter circles. Notice the "wasted space" next to the 1 inch circles that is not large enough for a third 1 inch circle. So you get to use more of the space (40%) for more than two conductors.