Section 300.4 in the National Electrical Code (NEC) provides requirements for protecting raceways, conductors and cables from physical damage with comprehensive details on various installations where damage could occur without the proper required protection techniques. Cables and raceways installed through bored holes or notches in joists, rafters, and wood or metal framing members (studs) are required to be installed and supported not less than 1 inches from the nearest edge of the framing member. Standard steel protection plates that are 1/16-inch minimum thickness or listed and marked plates less than 1/16-inch thick can be installed to provide extra protection against nail or screw penetration.
Where raceways cannot be installed at least 1 inches from the edge of the framing member, rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit or electrical metallic tubing can be installed as that extra protection against damage to the conductors. Cables, raceways and boxes cannot be installed in metal roofing, based on 300.4(E), or if they are installed under the metal-corrugated sheet decking-type roofing, they must be installed or supported no closer than 1 inches from the lowest surface of the metal roofing to the top of the cable, raceway or box. This required installation depth is meant to provide protection for the cable, raceway or box against penetration from large hold-down screws designed to fasten the roofing material to the building structure.
While 300.4 provides requirements for protection of physical damage for raceways, conductors and cables, 300.7 provides requirements for raceways to be able to compensate for thermal expansion and contraction. Since there was a necessity within the NEC to also deal with deflection of raceways in all directions, a new subsection 300.4(H) was added in the 2011 NEC. This new text deals specifically with structural joints where there are probabilities of lateral or angular deflection. Section 300.4(H) states that “a listed expansion/deflection fitting or other approved means shall be used where a raceway crosses a structural joint intended for expansion, contraction, or deflection, used in buildings, bridges, parking garages, or similar structures.”
The proposal that was submitted for this new requirement provided the following substantiation for the change: “Raceways can be damaged when improperly installed in structural construction joints leaving conductors or cables exposed. Structural construction joints may experience shear and lateral loads due to gravity, expansion and contraction and movement of the structure.”
The data provided in the proposal contained a drawing detailing a deflection of 30 degrees from the center point of the expansion joint, but each different deflection joint could be designed for a different deflection as well as expansion. To use this requirement for any building, bridge or structure where deflection may occur, a structural engineer or architect would provide the details of all construction joints with the maximum deflection in the construction plans and documentation. Once the vertical deflection and the horizontal expansion is known, the installer/electrician should check the expansion and deflection fitting manufacturer’s installation instruction sheet for the maximum movement that a particular deflection fitting can provide. Careful installation of the fitting should ensure both expansion and deflection can occur without damaging the raceway, but determining the proper adjustment for the fitting during installation also requires understanding the expansion and contraction of the raceway material.
The informational note in 300.7(B) provides details on the expansion and contraction rates in Table 352.44 for polyvinyl chloride (PVC) and in Table 355.44 for reinforced thermosetting resin conduit (RTRC). A nominal number for the expansion and contraction for steel conduit can be determined by multiplying the expansion length in Table 352.44 by 0.20. The coefficient of expansion for steel electrical metallic tubing, intermediate metal conduit and rigid conduit is 0.650 10–5 per inch or 0.0000065 per inch of conduit for each degree Fahrenheit in change of temperature. A nominal number for aluminum conduit and aluminum electrical metallic tubing can be determined by multiplying the expansion length in Table 352.44 by 0.40. The coefficient of expansion for aluminum electrical metallic tubing and aluminum rigid metal conduit is 1.30 10–5 per inch or (0.000013 per inch) of conduit for each degree Fahrenheit in temperature change.
The expansion and deflection fitting must also have connection hubs that will not permit water or other liquids from entering into the fitting for all different expansion and contraction—as well as deflection—positions. The fitting must also have an internal flexible-bonding jumper installed at the factory that will maintain the grounding integrity of the raceway to the maximum degree of deflection designed into the deflection fitting. Determining the expansion and deflection of a raceway as discussed above will ensure safety of the installation under all considerations.
ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and firstname.lastname@example.org.