ECN Forum Forums General Topics of the Trade General Discussion Area Masonite boxes

See: http://tnbelectricalworld.tnb.com/contractor/docs/union.pdf

They are "Union-brand reinforced-phenolic wall boxes"
as opposed to thermoplastic boxes.

Originally, the manufacturer called themselves Union Insulating Co.

I agree that “masonite” is a composite wood-fiber material, and likely flammable as most other lumber. It’s probably not intended for electrical enclosures.