Metal and Nonmetallic Wireways — Articles 376 & 378

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Unlike conduit and tubing, sizing wireways and calculating conductor fill must follow a different set of requirements according to the 2023 National Electrical Code. Wireways can serve both as raceways and at times J-boxes and pull boxes. Wireways are typically made from sheet metal but can also be made with nonmetallic materials such as PVC or even fiberglass. These are also known in the field as troughs or gutters. Auxiliary Gutters are covered in Article 366 and will not be included in this column.

Section ***.22 Number of Conductors, Wireways, both metallic and nonmetallic, are required to be filled at not more than 20 percent of the interior cross-sectional area in inches squared. For example, a 6” X 6” wireway, at 20 percent equals 7.2 square inches. Not more than 7.2 in² at any cross section of this size wireway shall be filled with conductors. Splices and taps are permitted up to 75 percent fill. Fill calculations for conductors in the wireway shall be in accordance with Chapter 9, Table 5 for conductor area.

Adjustment factors of section 310.15(C)(1) only apply if the number of current carrying conductors exceeds 30. However, this rule does not apply to Nonmetallic Wireways. Section 376.22(B) exposes another issue. While no adjustments or derating is applicable up to and including 30, once another conductor is installed, then all the conductors must be adjusted as per Table 310.15(C)(1) at 40 percent!  This could result in a 12 AWG, THHN copper wire being compliant or not. There are no gradual adjustments. All or nothing! If a wireway were to be installed in an article 520 location such as a stage to a theater or a performing arts center, then no adjustments need to be made regardless of how many current carrying conductors contained within the 20 percent fill. See 520.6

What about sizing a wireway? Not including the length, there are three dimensions that must be considered when calculating the physical size of any wireway. The top, the bottom and the back or the wall opposite to a removable cover. If the conductors are 4 AWG and larger, then the requirements of 314.28 for straight pulls, angle-pulls and U-pulls shall apply. The back wall or the wall opposite a removable cover, shall be calculated in accordance with Table 312.6(A) using the “One Wire per Terminal” column of the Table.

 ***.23(A) Deflected Insulated Conductors – This section directs us to use Table 312.6(A) under the column “One Wire per Terminal” based on the size of the largest conductor in the set. If you installed a set of 350 Kcmil compact aluminum conductors, the minimum distance in the wireway should be 5 inches to the opposite wall. No need to increase the calculations if paralleled circuits are run. Paralleled circuits must be grouped in sets when installed in any wireway, cable tray or gutter. See ***.20

Be aware that the wireway is not permitted to extend into the required working space of a 110.26 dimension more than 6 inches. Planning and proper layout is key.

***.56 Splices, Taps, and Distribution Blocks – These sections contain rules for splices and taps. We are permitted to fill the wireway up to 75 percent but only at the splices and taps and they must be left accessible. This stands to reason if one needs to gain access to the splice, tap or listed distribution block, it could pose a hazard if other conductors were an obstacle and prohibited the access to such splices or taps for future inspection or repairs. 378.56 does not have rules for distributions blocks in nonmetallic wireways.

If distribution blocks are to be installed in metal wireways, 376.56(B) directs us to follow the manufactures instructions for the minimum size of the wireway that is required according to manufactures specifications and the distribution block used. They must be listed and if used on the line side of service equipment then marked “suitable for use on the line side of service equipment”.

All associated fittings shall be listed for all wireways. This will ensure that grounding and bonding requirements will be maintained and compliance with section 376.60 is in accordance with Article 250 and section 250.118(A)(13). Nonmetallic wireways must have equipment grounding conductors installed unless used for service entrance and the grounded conductor is used to ground equipment in accordance with 250.142

Fittings would include 90-degree angles left, right and inside or outside. Flanges, T’s crosses, 45-degree bends, and couplings that will join sections together.

***.30 Securing and Supporting – Horizontal supports shall not exceed 5 feet for metal and not more than 3 feet for nonmetallic. In no case shall the distance between supports exceed 10 feet for both.

Vertical supports shall not exceed 15 feet but 4 feet for nonmetallic wireways.

Wireways can be a useful wiring method that can help transition from a conduit to an enclosure. They can contain buses or conductors and splices or taps. Wireways can also help save space in electrical rooms when real estate is a premium. Uses permitted is only limited to locations where subject to severe physical damage or severe corrosive environments.  If nonmetallic wireways are used in lengths were expansion and contraction is expected to be greater than ¼ inch, listed expansion fittings must be installed.