This rule has been revised to be more technically accurate.

300.4 Protection Against Physical Damage. Conductors, raceways, and cables must be protected against physical damage [110.27(B)].

Analysis: Previous editions of the NEC have required protection of conductors where subject to physical damage. While most Code users understand this rule is intended to apply to all conductors in all wiring methods, it didn’t clearly state that. This revision makes it clear that all conductors in all wiring methods must be protected from physical damage.

The rule on protecting raceways under metal-corrugated sheet roof decking has been expanded.

300.4(E) Wiring Under Roof Decking. Cables, raceways, and enclosures under metal-corrugated sheet roof decking must not be located within 1½ in. of the roof decking, measured from the lowest surface of the roof decking to the top of the cable, raceway, or box. In addition, cables, raceways, and enclosures aren’t permitted in concealed locations of metal-corrugated sheet decking type roofing.

Ex: Spacing from roof decking doesn’t apply to rigid metal conduit and intermediate metal conduit.

Analysis: New to the 2008 NEC was a requirement for the protection of most raceways when installed within 1½ in. of the roof deck. Although this 2008 rule change went a long way toward protecting wiring systems from damaging roofing screws that can penetrate the raceways, it left out one critical part of the installation — boxes. With this change, it’s clear that the Code is concerned not only with protecting the raceways, but also the boxes.

This rule was also changed to prohibit wiring methods from being installed in concealed locations above the roof decking. In some instances, installers place raceways above the roof deck prior to the insulation being installed, which results in the same potential for damage from roofing screws.

The requirement for protection of conductors 4 AWG and larger has been changed to add clarity.

300.4(G) Insulating Fittings. If raceways contain insulated circuit conductors 4 AWG and larger that enter an enclosure, the conductors must be protected from abrasion during and after installation by a fitting identified to provide a smooth, rounded insulating surface, such as an insulating bushing. (click here to see Fig. 9)

Ex.: Insulating bushings aren’t required if a raceway terminates in a threaded raceway entry that provides a smooth, rounded, or flared surface for the conductors. An example would be a meter hub fitting or a Meyer’s hub-type fitting.

Analysis: The term “substantial fitting” has been replaced with the term “identified” so that inspectors won’t have to interpret the Code unnecessarily. The term “identified” is clearly defined in Art. 100, is used throughout the NEC, and takes interpretation out of the requirement. Although fittings that are designed to provide this protection are typically used to achieve compliance with this requirement, it could have been argued that fittings designed for another application could satisfy this rule. This change removes that argument by making the NEC a more prescriptive document.

A new protection requirement for structural (expansion) joints was added.

300.4(H) Structural Joints. A listed expansion/deflection fitting or other approved means must be used where a raceway crosses a structural joint intended for expansion, contraction or deflection.

Analysis: In larger commercial/industrial buildings, it isn’t uncommon to see an expansion joint inside of the building. When these are encountered, the Code has never offered any guidance to the installer as it pertains to wiring methods. With this change, it becomes clear that a fitting or other approved means must be used to allow for expansion or deflection of the wiring method.