In walls or ceilings with a surface of concrete, tile, gypsum, plaster, or other noncombustible material, boxes employing a flush-type cover or faceplate shall be installed so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of. In walls or ceilings with a surface of concrete, tile, gypsum, plaster, or other noncombustible material, boxes employing a flush-type cover or faceplate shall be installed so that the front edge of the box, plaster ring, extension ring, or listed extender will not be set back of. The box on the right must be flush or have an extension ring installed to ensure that there are no exposed combustible materials behind the luminaire (not shown). Code Change Summary: Changes were made to provide clarity for flush-mounted box installations. The previous 2014 code language in. Done right, it ensures safety, compliance, and long-lasting performance. In this guide, we'll break down everything you need to know to install a distribution box correctly and confidently. Choose the right box based on environment (indoor/outdoor), load capacity, and durability. Electrical clearances are the minimum separation distances the National Electrical Code (NEC) requires between wiring, panels, overhead conductors. 370-20 In Wall or Ceiling Section 370-20 can easily be overlooked during the box installation procedure. At times, the box's front edge ends up more than 1/4 inch behind the finished surface. This is a violation, regardless of the type of material surrounding the box. The article includes table references that guide the electrician in the selection of the proper box size necessary to safely accommodate ele trical service requirements.