Wiring With MC – WHAT YOU CAN AND CAN’T DO WITH MC CABLE
In this episode we talk in depth about how to wire with MC cable, what the uses permitted and uses NOT permitted are, as well as securing and strapping and a whole host of other tips for use.
MC vs AC
First off I suppose it’s good to point out the difference between AC and MC. A lot of people think that it’s the jacketing that separates the two, but actually both come in steel and aluminum jacketing. Really it’s what’s inside that sets the two apart.
MC cable typically has either an insulated or bare FULL SIZED equipment grounding conductor inside of it – or sometimes both depending on the purpose of the MC. AC has a small #16AWG equipment grounding conductor in it. The codes in the 2017 NEC have variations in them between MC and AC on things such as bushings as well as the bonding conductor requirements, that is all to say, they are not the same thing. Think Metallica and Megadeth or Audioslave and Soundgarden. Essentially the same…but different.
USES PERMITTED AND NOT PERMITTED
There are a lot more uses both permitted and not permitted in Article 330 covering MC cable than what I mention in the above video. But here are they are in totality (2017 NEC)
330.10 Uses Permitted.
330.10(A) General Uses.
Type MC cable shall be permitted as follows:
330.10(A)(1) For services, feeders, and branch circuits.
330.10(A)(2) For power, lighting, control, and signal circuits.
330.10(A)(3) Indoors or outdoors.
330.10(A)(4) Exposed or concealed.
330.10(A)(5) To be direct buried where identified for such use.
330.10(A)(6) In cable tray where identified for such use.
330.10(A)(7) In any raceway.
330.10(A)(8) As aerial cable on a messenger.
330.10(A)(9) In hazardous (classified) locations where specifically permitted by other articles in this Code.
330.10(A)(10) In dry locations and embedded in plaster finish on brick or other masonry except in damp or wet locations.
330.10(A)(11) In wet locations where a corrosion-resistant jacket is provided over the metallic covering and any of the following conditions are met:
(a) The metallic covering is impervious to moisture.
(b) A jacket resistant to moisture is provided under the metal covering.
(c) The insulated conductors under the metallic covering are listed for use in wet locations.
Where single-conductor cables are used, all phase conductors and, where used, the grounded conductor shall be grouped together to minimize induced voltage on the sheath.
330.10(B) Specific Uses.
Informational Note: The “Uses Permitted” is not an all-inclusive list.
330.10(B)(1) Cable Tray.
330.10(B)(2) Direct Buried.
330.10(B)(3) Installed as Service-Entrance Cable.
Type MC cable installed as service-entrance cable shall be permitted in accordance with 230.43.
330.10(B)(4) Installed Outside of Buildings or Structures or as Aerial Cable.
330.12 Uses Not Permitted
Type MC cable shall not be used under either of the following conditions:
330.12(1) Where subject to physical damage
330.12(2) Where exposed to any of the destructive corrosive conditions in (a) or (b), unless the metallic sheath or armor is resistant to the conditions or is protected by material resistant to the conditions:
(a) Direct buried in the earth or embedded in concrete unless identified for direct burial
(b) Exposed to cinder fills, strong chlorides, caustic alkalis, or vapors of chlorine or of hydrochloric acids
SECURING AND SUPPORTING
330.30 Securing and Supporting
Type MC cable shall be supported and secured by staples; cable ties listed and identified for securement and support; straps, hangers, or similar fittings; or other approved means designed and installed so as not to damage the cable.
Unless otherwise provided, cables shall be secured at intervals not exceeding 1.8 m (6 ft). Cables containing four or fewer conductors sized no larger than 10 AWG shall be secured within 300 mm (12 in.) of every box, cabinet, fitting, or other cable termination. In vertical installations, listed cables with ungrounded conductors 250 kcmil and larger shall be permitted to be secured at intervals not exceeding 3 m (10 ft).
Unless otherwise provided, cables shall be supported at intervals not exceeding 1.8 m (6 ft).
Horizontal runs of Type MC cable installed in wooden or metal framing members or similar supporting means shall be considered supported and secured where such support does not exceed 1.8-m (6-ft) intervals.
330.30(D) Unsupported Cables.
Type MC cable shall be permitted to be unsupported and unsecured where the cable complies with any of the following:
330.30(D)(1) Is fished between access points through concealed spaces in finished buildings or structures and supporting is impractical.
330.30(D)(2) Is not more than 1.8 m (6 ft) in length from the last point of cable support to the point of connection to luminaires or other electrical equipment and the cable and point of connection are within an accessible ceiling.
330.30(D)(3) Is Type MC of the interlocked armor type in lengths not exceeding 900 mm (3 ft) from the last point where it is securely fastened and is used to connect equipment where flexibility is necessary to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation.
For the purpose of this section, Type MC cable fittings shall be permitted as a means of cable support.
WET OR DAMP ENVIRONMENTS
330 of the NEC mentions that MC cable can be used indoor and outdoor but does not make mention of it being allowed or disallowed for use in damp environments. It does state that when used in wet locations there must be a corrosion-resistant jacket to protect the conductors. However if you look into the UL listing for MC cable it is designed to be a dry-environment cable assembly. Use your discretion on this, but it’s always better to err on the side of caution when interpreting code. For wet and dry environments there is an MC cable assembly that is listed and approved for both.
PVC-jacketed MC has a coating over the length of it that makes the inside of the conductor impervious to moisture. It also allows for the conductor to be in direct contact with the earth for use in direct-burial applications.
ANTI-SHORT BUSHINGS (READ-HEADS)
As of 2017 there is no requirement in the National Electric Code for MC to use anti-short bushings in MC cable installations. There is, however, a code which calls for them with the use of AC cable (320.40). Most wire-manufacturers include bundles of these bushing with every roll of MC cable they sell, as an courtesy in the event you choose to use them. You do not need to with MC. I find this a little bizarre, since AC and MC are fundamentally the same thing. The same problems exist in both instances, so I still prefer to use them. The national electric code is a minimum standard, which means you can go above and beyond them…you just can’t fall short.
There exists a problem with the methods employed in the field to strip the jacketing from MC and AC cable. The roto-splitter seen below is a tool designed to be used for this purpose. It leaves a very clean cut, however the pointed edge that results is extremely sharp and can pierce the insulation of a conductor if folded a certain way. For this reason NEMA recommends you cut and square the edge of the jacket after using a roto-splitter. Many people choose not to use this tool for this reason, and instead use either dikes (diagonal cutting pliers) or channel-locks to bend and snip the jacketing. This too gnarles the end of the jacketing where the conductors emerge, so either way you go you need to clean up the jacketing and protect the wire.
330.24 Bending Radius.
Bends in Type MC cable shall be so made that the cable will not be damaged. The radius of the curve of the inner edge of any bend shall not be less than required in 330.24(A) through (C).
330.24(A) Smooth Sheath.
330.24(A)(1) Ten times the external diameter of the metallic sheath for cable not more than 19 mm (3? 4 in.) in external diameter
330.24(A)(2) Twelve times the external diameter of the metallic sheath for cable more than 19 mm (3? 4 in.) but not more than 38 mm (11? 2 in.) in external diameter
330.24(A)(3) Fifteen times the external diameter of the metallic sheath for cable more than 38 mm (11? 2 in.) in external diameter
330.24(B) Interlocked-Type Armor or Corrugated Sheath.
Seven times the external diameter of the metallic sheath.
330.24(C) Shielded Conductors.
Twelve times the overall diameter of one of the individual conductors or seven times the overall diameter of the multiconductor cable, whichever is greater.
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**Disclaimer – These videos are for training purposes alone, all work done on electrical systems should be done by a licensed and insured electrical contractor. If you are not an electrician, do not attempt any of the work you are seeing in these videos.**