Nonmetallic Wireways

Nonmetallic wireways serve the same purpose as metal ones — an accessible, coverable trough for grouping conductors — but they're molded from flame-retardant plastic so they survive corrosive and wet atmospheres that would eat steel. They're common in water-treatment plants, chemical processing and food facilities.

The fill and splice rules mirror the metal article, but because the material is nonmetallic it provides no grounding path, and it expands with temperature so expansion fittings are needed on long runs. Support spacing is closer than for metal wireway because plastic sags more.

When it applies

Nonmetallic wireways are the plastic version of the trough, used where a steel wireway would corrode, wet locations, wash-down areas, and chemically aggressive rooms. They give you the same open, hinged-cover access for splicing and adding conductors, without the rust.

Common mistakes

• Ignoring expansion, plastic moves a lot with temperature, so long runs need expansion fittings the metal version would not.
• Overfilling, the wireway fill and the derating for many current-carrying conductors still apply just like a metal trough.
• Forgetting that nonmetallic wireway carries no ground, every circuit needs its own equipment grounding conductor pulled inside.

In the field

In a wash-down food plant I reach for nonmetallic wireway and then leave room for expansion fittings on the long straight runs. Skip those and a hot summer will bow the trough and pop the cover joints right open.