Q. What are the NEC requirements for sizing raceways?
Find the Answer
A. Raceways must be large enough to permit the installation and removal of conductors without damaging the conductor’s insulation [300.17]. When all conductors in a raceway are the same size and of the same insulation type, the number of conductors permitted can be determined by Annex C. When different size conductors are installed in a raceway, conductor fill is limited to the percentages in Table 1 of Chapter 9.
| Table 1, Chapter 9 | |
| Number | Percent Fill |
| 1 Conductor | 53% |
| 2 Conductors | 31% |
| 3 or more | 40% |
The above percentages are based on conditions where the length of the conductor and number of raceway bends are within reasonable limits [Chapter 9, Table 1, Note 1].
When sizing a raceway, first determine the total area of conductors (Chapter 9, Table 5 for insulated conductors and Chapter 9, Table 8 for bare conductors). Then, select the raceway from Chapter 9, Table 4, in accordance with the percent fill listed in Chapter 9, Table 1.
Let’s run through an example problem to make sure you understand the concepts noted above.
Question: What trade size Schedule 40 PVC conduit is required for the following conductors?
- Three 500 THHN
- One 250 THHN
- One 3 THHN
Step 1: Determine the total area of conductors [Chapter 9, Table 5]:
500 THHN — 0.7073 x 3 = 2.1219 in.2
250 THHN — 0.3970 x 1 = 0.3970 in.2
3 THHN — 0.0973 x 1 = 0.0973 in.2
Total Area = 2.6162 in.2
Step 2: Select the raceway at 40% fill [Chapter 9, Table 4]:
Trade size 3 Schedule 40 PVC = 2.907 sq in. of conductor fill at 40%.
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