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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.
Q7. How is the conductor ampacity for Type MC cable determined according to the NEC?
A7. Conductor ampacity is calculated on the 90°C insulation rating of the conductors; however, the conductors must be sized to the termination temperature rating in accordance with 110.14(C)(1) [330.80].
Q8. What are the Code requirements for Type MC cable to serve as an equipment grounding conductor?
A8. If Type MC cable is to serve as an equipment grounding conductor, it must comply with 250.118(10) and 250.122 [330.108].
Author’s Comment:
The outer sheath of:
• Traditional interlocked Type MC cable isn’t permitted to serve as an equipment grounding conductor, therefore this cable must contain an equipment grounding conductor in accordance with 250.118(1) [250.118(10)a].
• Interlocked Type MCAP cable containing an aluminum grounding/bonding conductor running just below the metal armor is listed to serve as an equipment grounding conductor [250.118(10)(b)].
• Smooth or corrugated-tube Type MC cable is listed to serve as an equipment grounding conductor [250.118(10)(c)].
Q9. What are the uses not permitted by the NEC for Type NM cable?
A9. Types NM and NMC cables aren’t permitted [334.12(A)]:
(1) In any dwelling or structure not specifically permitted in 334.10(1), (2), (3), and (5).
(2) Exposed within a dropped or suspended ceiling cavity in other than one and two family, and multifamily dwellings.Figure 01
(3) As service entrance cable.
(4) In commercial garages having hazardous locations, as defined
in 511.3.
(5) In theaters and similar locations, except where permitted in 518.4(B).
(6) In motion picture studios.
(7) In storage battery rooms.
(8) In hoistways, or on elevators or escalators.
(9) Embedded in poured cement, concrete, or aggregate.
(10) In any hazardous location, except where permitted by other sections in the Code.
(B) Type NM cables aren’t permitted to be used under the following conditions, or in the following locations:
(1) If exposed to corrosive fumes or vapors.
(2) If embedded in masonry, concrete, adobe, fill, or plaster.
(3) In a shallow chase in masonry, concrete, or adobe and covered with plaster, adobe, or similar finish.
(4) In wet or damp locations.
Author’s Comment:
• A raceway in a ground floor slab is considered a wet location.
Q10. What does the Code require when installing Type NM cable for exposed work?
A10. (A) Surface of the Building. Exposed Type NM cable must closely follow the surface of the building [334.15(A)].
(B) Protected from Physical Damage. Nonmetallic-sheathed cable must be protected from physical damage by rigid metal conduit, intermediate metal conduit, Schedule 80 PVC conduit, Type RTRC-XW conduit, EMT, guard strips, or other means approved by the authority having jurisdiction.
Author’s Comment:
• When installed within a raceway, the cable must be protected from abrasion by a fitting installed on the end of the raceway [300.15(C)].
Type NMC cable installed in shallow chases in masonry, concrete, or adobe, must be protected against nails or screws by a steel plate not less than 1⁄16 in. thick [300.4(F)] and covered with plaster, adobe, or similar finish.
Author’s Comment:
• If Type NM cable is installed in a metal raceway, the raceway isn’t required to be connected to an equipment grounding conductor [250.86 Ex 2 and 300.12 Ex].
(C) In Unfinished Basements and Crawl Spaces. If Type NM cable is installed at angles with joists in unfinished basements and crawl spaces, it’s permissible to secure cables containing conductors not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables must be installed through bored holes in joists or on running boards.
Type NM cable installed on a wall of an unfinished basement or crawl space subject to physical damage must be protected in accordance with 300.4, or be installed within a raceway with a nonmetallic bushing or adapter at the point where the cable enters the raceway, and the NM cable must be secured within 12 in. of the point where the cable enters the raceway.
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