National Electrical Installation Standards

Standards as High as Your Own

Tuesday, June 11, 2019


Raceway Fill and Adjustment Factor Part 1.

I need to know how many #10 awg thhn wires and #18 awg dimming wires derated I can fit in a 1" conduit? How would you derate and split up the different wires to know how many we can have in each pipe.

Michael Henson


Hey Michael thanks for your question(s). Depending on the type of conduit you are using the .22 section of the applicable Article references Chapter 9, Table 1 for the percentage of interior area of the conduit that can be used to determine the number of conductors allowed. For over 2 conductors it is limited to 40% as shown in Table 1. Note 5 to that table refers to Table 5 for conductor dimensions and Table 4 for the conduit dimensions. Using Rigid Metal Conduit (RMC Article 344) as an example find that table within Table 4 (the 9th one). The second column has the conduit trade sizes and the fourth column has the cross sectional area in square inches that can be used based on 40%. The fourth line is for 1 inch RMC and in the fourth column 0.355 square inches can be used to determine the number of conductors allowed. In Table 5, locate THHN in the first column then use the row with 10AWG size then go to the center column for the area in square inches for a single conductor which is 0.0211. You don't mention the insulation type for the 18 AWG conductors but for example if they are TFN, four lines up is their area which is 0.0055.

If we install 9 THHN conductors that are 10AWG they have an area 0.1899 square inches (9 X 0.0211). Using the 40% area of 0.355 minus the 0.1899 leaves us 0.1651 of area for the 18 AWG conductors. Dividing 0.165 by 0.0055 allows 30 of the 18 AWG TFN conductors to also be installed. Checking our math 9 X 0.0211 = 0.1899 + 30 X 0.0055 = 0.165, 0.1899 + 0.165 = 0.3549 which is just less than the allowable area of 0.355. Tomorrow we will address your derating question.

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