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Figure 01
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For EC&M Magazine
By Mike Holt, NEC® Consultant
Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.
Note: The answers to these questions are based on the 2017 NEC.
Underlined text indicates a change in the rule for the 2017 NEC.
Q1. What are the uses permitted by the NEC for Metal-Clad cable (Type MC)?
A1. General Uses [330.10 (A)].
(1) In branch circuits, feeders, and services
(2) In power, lighting, control, and signal circuits
(3) Indoors or outdoors
(4) Exposed or concealed
(5) Directly buried (if identified for the purpose)
(6) In a cable tray
(7) In a raceway
(8) As aerial cable on a messenger
(9) In hazardous locations as permitted in 501.10(B), 502.10(B), and 503.10
(10) Embedded in plaster or brick in dry locations
(11) In wet locations, where a corrosion-resistant jacket is provided over the metal sheath and any of the following are met:
a. The metallic covering is impervious to moisture.
b. A jacket is provided under the metal covering that’s moisture resistant. Figure 01
c. The insulated conductors under the metallic covering are listed for use in wet locations.
(12) If single-conductor cables are used, all circuit conductors must be grouped together to minimize induced voltage on the sheath [300.3(B)].
Author’s Comment: Installing the conductors of a circuit close together will minimize the circuit impedance.
(B) Specific Uses.
Informational Note: The “Uses Permitted” isn’t an all-inclusive list, which indicates that other suitable uses are permitted if approved by the authority having jurisdiction.
(1) Cable Tray. Type MC cable installed in a cable tray in accordance with Article 392.
(2) Direct Buried. Direct-buried cables must be protected in accordance with 300.5.
(3) Installed as Service-Entrance Cable. Type MC cable is permitted for service entrances when installed in accordance with 230.43.
(4) Installed Outside of Buildings. Type MC cable installed outside of buildings must comply with 225.10, 396.10, and 396.12.
Q2. What are the uses not permitted by the NEC for Metal-Clad cable (Type MC)?
A2. Type MC cable isn’t permitted to be used where: [330.12]
(1) Subject to physical damage.
(2) Exposed to the destructive corrosive conditions in (a) or (b), unless the metallic sheath or armor is resistant to the conditions, or protected by material resistant to the conditions:
a. Direct burial in the earth or embedded in concrete unless identified for the application.
b. Exposed to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine or of hydrochloric acids.
Q3. What does the Code require when installing Metal-Clad cable (Type MC) in exposed work?
A3. Exposed runs of Type MC cable must closely follow the surface of the building finish or running boards. Type MC cable installed on the bottom of floor or ceiling joists must be secured at every joist and not be subject to physical damage [330.15]
Q4. What does the NEC require for the protection of Type MC cable installed through or parallel to framing members?
A4. Type MC cable installed through or parallel to framing members or furring strips must be protected against physical damage from penetration of screws or nails by maintaining a 1¼ in. separation, or by installing a suitable metal plate in accordance with 300.4(A), (C), and (D) [330.17].
Q5. What are the Code requirements when installing Type MC cable in accessible attics or roof spaces?
A5. Type MC cable installed in accessible attics or roof spaces must comply with 320.23 [330.23].
Author’s Comment:
• On the Surface of Floor Joists, Rafters, or Studs. In attics and roof spaces that are accessible, substantial guards must protect cables installed across the top of floor joists, or across the face of rafters or studding within 7 ft of the floor or floor joists. If this space isn’t accessible by permanent stairs or ladders, protection is required only within 6 ft of the nearest edge of the scuttle hole or attic entrance [320.23(A)].
• Along the Side of Framing Members [320.23(B)]. When Type MC cable is installed on the side of rafters, studs, or floor joists, no protection is required if the cable is installed and supported so the nearest outside surface of the cable or raceway is at least 1¼ in. from the nearest edge of the framing member where nails or screws are likely to penetrate [300.4(D)].
Q6. What are the NEC requirements for bends made in Type MC cable?
A6. Bends must be made so the cable won’t be damaged, and the radius of the curve of any bend at the inner edge of the cable isn’t permitted to be less than what’s dictated in each of the following instances [330.24]:
(A) Smooth-Sheath Cables.
(1) Smooth-sheath Type MC cables aren’t permitted to be bent so the bending radius of the inner edge of the cable is less than 10 times the external diameter of the metallic sheath for cable up to ¾ in. in external diameter.
(B) Interlocked or Corrugated Sheath. Interlocked- or corrugated-sheath Type MC cable isn’t permitted to be bent so the bending radius of the inner edge of the cable is less than seven times the external diameter of the cable.
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