Guest Contributor: Jim Egan
This post reviews the securing and supporting requirements for MC Cable, NM Cable, PVC conduit, and EMT. A summary Table of the requirements is included.
The requirements for securing and supporting are similar in intent, but there are differences. Securing is done by installing a mechanical means of attachment to the cable and attaching it to a stationary structural member so the cable does not move. This will prevent the cable or conduit from becoming loose and pulling away from a box or fitting. Support requires a mechanical means to prevent the cable from sagging or drooping which may cause damage to the cable and create a hazard. A cable may remain loose while its support is maintained.
Throughout Chapter 3 in the NEC, Wiring Methods and Materials, the .30 section is reserved for securing and supporting. For example, 330.30, 334.30, 352.30, 358.30 are all sections about securing and supporting.
330.30 Securing and Supporting Metal-Clad Cable: Type MC
Securing MC Cable.
MC cable must be secured at intervals not exceeding 6 ft. Cables that contain conductors no larger than No. 10 AWG must be secured within 12 inches of every box, cabinet, fitting, or other cable termination.
MC cable must be supported every 6 ft. MC cable is considered supported when run through metal or wooden framing members.
MC cables may be unsupported when (1) fished through concealed spaces in finished buildings, (2) when not more than 6 ft. in length from the last point of cable support to the point of connection to a luminaire or other equipment, and (3) when not more than 3 ft. from the last point of support where flexibility is necessary for equipment that requires movement after installation.
334.30 Securing and Supporting Non Metallic Sheathed Cable: Type NM, NMC, and NMS
NM cable must be supported and secured every 4 ½ ft. and within 12 inches of every outlet box, junction box, cabinet, or fitting.
Secure and support.
Where nonmetallic cable is installed horizontally through holes or notches in framing members, it is considered supported and not required to be secured. Also, where nonmetallic cable is fished within walls, it does not require support or fastening. In dwelling units within an accessible ceiling, support for the cable can be 4 ½ ft. from the last point of support to a connection to a luminaire or other piece of electrical equipment.
352.30 Securing and Supporting Rigid Polyvinyl Chloride Conduit: Type PVC
Expansion fittings for PVC conduit.
PVC conduit must be secured within 3 ft. of each outlet box, junction box, device box, conduit body, or other conduit termination. PVC must be supported every 3 ft. for trade sizes ½ inch through 1 inch. The support distance increases as the conduit size gets larger. For example, 2 inch PVC conduit can be supported every 5 ft; 6 inch PVC conduit can be supported every 8 ft. See Table 352.30.
PVC conduit is subject to thermal expansion or contraction. When the outside temperature gets hotter, PVC conduit will expand, pushing on fittings, boxes, and straps. When the temperature cools down, PVC conduit will contract, causing the conduit to separate or pull apart from fittings, boxes, and straps. For example, if a 100 ft. run of PVC was installed when it was 30°F outside, and in the summer the outside temperature reached 90°F, that is a 60°F change, and the conduit would expand 2.43 inches. See Table 352.44.
Expansion fittings are required for PVC conduit to allow the conduit to move back and forth inside the expansion fitting. Without the expansion fittings installed, the conductors inside the conduit can be damaged. Since a temperature change of only 20°F can cause the conduit to expand or contract more than 1 inch, expansion fittings are necessary to protect the conductors inside the conduit.
358.30 Securing and Supporting Electrical Metallic Tubing: Type EMT
EMT must be secured at least every 10 ft. and within 3 ft. of every outlet box, junction box, device box, cabinet, conduit body, or other termination.
An exception allows EMT to be fastened up to 5 ft. apart if structural members like joists or rafters do not permit fastening every 3 ft. A second exception allows EMT to be fished in concealed spaces without support in unbroken lengths without a coupling.
Horizontal runs of EMT are considered supported if installed through openings in framing members that are spaced not more than 10 ft. apart.
|Wiring Material||Secure and Support|
|330.30 Metal-Clad Cable: Type MC||Shall be secured within 12 inches of a box, cabinet, or fitting and supported every 6 feet thereafter.|
|334.30 Nonmetallic-Sheathed Cable: Types NM, NMC, and NMS||Shall be secured and supported within 12 inches of a box and every 4 ½ feet thereafter. The cable is considered to be supported in horizontal installations through framing members.|
|352.30 Rigid Polyvinyl Chloride Conduit: Type PVC||Shall be securely fastened within 3 feet of a box, conduit body, or other point of termination. Support shall comply with Table 352.30, based on the size of the conduit.|
|358.30 Electrical Metallic Tubing: Type EMT||Shall be secured and fastened within 3 feet of a box, conduit body, or other point of termination and within every 10 feet thereafter.|
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