Q. When installing cable- or raceway-type wiring methods, installed in exposed or concealed locations under metal-corrugated sheet roof decking, the cable- or raceway-type wiring method shall be installed and supported so that the nearest outside surface of the cable or raceway is not less than 1½ inches from the nearest surface of the roof decking. This rule applies to all cable- or raceway-type wiring methods except which one of the following?
- No exceptions. It applies to all cable- or raceway-type wiring methods.
- MI cable. It does not apply to MI cable with a 2-hour fire rating.
- Type TC – ER cable as noted in 336.10(7).
- Rigid metal conduit (RMC) and intermediate metal conduit (IMC).
This is a new requirement in the 2008 NEC [300.4(E)]. As noted in the FPN, due to some methods of securing roofing materials to roof decking, long screws have been used without consideration of what lies beneath the roof decking (which often times may be cables, conduits, boxes and other wiring methods). Some of these long screws have penetrated electrical conduits and boxes, thus creating electrical problems and fire hazards. This new requirement is for the protection of these wiring methods.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at firstname.lastname@example.org.