Nonmetallic Electrical Enclosures

I’ve installed (and replaced) a lot of enclosures over the last 30 
years. And I’ve probably used just about every type of enclosure, from 
small metal EMT boxes with knockouts to stainless steel behemoths that 
needed drilled or punched. Allied Moulded Products, Inc.

Steve Maurer, IME

I’ve installed (and replaced) a lot of enclosures over the last 30 years. And I’ve probably used just about every type of enclosure, from small metal EMT boxes with knockouts to stainless steel behemoths that needed drilled or punched.

The biggest pain for me is when I need to replace an enclosure that’s been corroded. Harsh chemicals and damp environments are NOT your friend. Add to that the almost inevitable scenario that it’s never the box at the end of a run and replacing an enclosure can be a nightmare.

All the conduits and conductors must be carefully labeled and set aside to make sure everything gets put back in the right place, right?

I label everything. But I’ve taken to snapping a photo with my smartphone… just in case.

However, nonmetallic enclosures help combat the evils of chemical degradation. Two of the basic types are polycarbonate and fiber reinforce polyester (FRP).

FRP boxes are thicker than poly boxes and have some advantages. For example, if they’re constructed properly, they are suitable for both indoor and outdoor use in corrosive environments. Resistance to UV degradation is possible if they adhere to UL 50, UL 50E, and UL 508A standards.

Many FRP enclosures are constructed by using a hot compression-molded process. However, larger enclosures may need additional impact tolerance. After all… they’re usually in the way of something.

As a result, they can be built with a hand lay-up gel coat fiberglass process. Another option is construction using a resin transfer mold process. Either gives them high impact tolerance and an overall excellence in strength.

FRP enclosures hold their shape well, too. Very important if you need to open them. You want all the screws and latched to line up later, right?

FRP enclosures are hard to beat when physical strength and corrosion resistance are needed.

Polycarbonate boxes (often called plastic boxes) are a little lighter. They are often constructed using injection molding, using a polycarbonate thermoplastic material.

If constructed to the same standard mentioned above, they will also have acceptable UV resistance and stress ratings.

Because they’re lighter than their FRP cousins, they can be warped if your conduit shifts when you open the box. Shouldn’t be too much of a problem if you supported your conduit runs properly.

Both types are easy to work with. They’re much easier to configure for conduit holes and button or device knockouts than their metallic counterparts.


Photo courtesy of Allied Moulded Products, Inc.

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