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#12876 - 08/20/02 09:42 AM Romex vs. BX or MC cable
SvenNYC Offline
Member

Registered: 08/19/02
Posts: 1685
Loc: New York City
OK. First off, I should say that I prefer MC (or BX) cable. Also, I live in New York City, where BX and MC are apparently required by code (from all the new construction I've seen going on that sports it).

Are there any advantages (besides price and ease of working) of Romex over metal clad cables?

I wonder why NYC is insistent on metal clad.

Probably because we have a large rodent population and there's always the possibility of these critters chewing through Romex insulation.

At least that's my argument for preferring MC -- rats aren't as likely to chew through what is, in essence, a metal pipe. Not to mention that it's probably less prone to damage as you're fishing it behind a lath and plaster wall and god only knows what else hides back there...

A friend of mine in Jersey City, NJ bought a 1950s house and rewired some 15-amp circuits with AWG 12-gauge Romex (AWG 14 being the norm for 15 amp, I understand). The existing wiring was all BX.

He liked the Romex because it was easier to fish through walls (and cheaper, of course)...although I do imagine that Jersey City has the same problems with rodents that NYC has.

P.S.: Is it good practice to "over engineer" a 15 amp circuit by wiring it with 12 gauge instead of 14 gauge?

I should state that I am not a licenced professional electrician, but am interested in how wiring gets done (and like to stick by code).

Thanks

Sven

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#12877 - 08/20/02 04:06 PM Re: Romex vs. BX or MC cable
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6804
Loc: Brick, NJ USA
Sve:
The place to get a lot of your answers is the current edition of the NEC (2002) that has been adopted by NY.
A key issue to RX (NMC) is the three story (or 4 story) limit, also, "subject to physical damage", also fire ratings of the construction.
HotLine1
John
_________________________
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#12878 - 08/20/02 04:51 PM Re: Romex vs. BX or MC cable
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
MC or BX (properly installed)have the advantage of better RF shielding, which can help reduce radio interference, and some of the "unexplained flakiness" of stuff like X-10 systems, powerline networking, etc.

NM cables can also be dangerous when hit by an errant nail by a careless builder or homeowner. I got a call one time from a person being shocked by a metal picture frame. A check showed that the nail that hung the picture (on a metal wire) was hot! It was driven partway into a piece of NM cable, managing to hit only the hot conductor. Had ben that way for quite a while, from the rusted appearance of the nail.

At least with the metal-clad cables, such a nail would have to pierce the grounded armor first, resulting in a ground fault that either trips the breaker and/or makes itself known with a flash/bang. No easy way for a hot nail/staple to "lie in wait" for a victim.

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#12879 - 08/20/02 05:26 PM Re: Romex vs. BX or MC cable
CanadianSparky Offline
Member

Registered: 03/05/01
Posts: 86
Loc: Alberta Canada
Sorry to go off on kind of a tangent here, but is the armour of bx bonded? I've seen many instances where people tape the bushing in therefore the armour isn't touching anything else that is bonded.

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#12880 - 08/20/02 07:01 PM Re: Romex vs. BX or MC cable
NJwirenut Offline
Member

Registered: 09/15/01
Posts: 816
Loc: Bergen County, NJ
BX (properly called AC) cable contains a separate bare metal strip between the metal armor and the craft paper wrapping over the conductors. Usual practice is to fold this strip back over the end (holding the anti-short bushing in place), and wrap it around the armor several times before trimming the end off. The bonding strip is then clamped to the armor (and to the box) by the connector or box clamp.

The armor (and bonding strip) is the EGC, and as such MUST be bonded with a proper connector at all terminations.

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#12881 - 08/20/02 07:25 PM Re: Romex vs. BX or MC cable
CTwireman Offline
Member

Registered: 02/07/02
Posts: 839
Loc: Connecticut, USA
I don't believe it is necessary to backwrap the bond strip over the sheath, other than to hold in the anti-short. I know this is commonly done but I just cut off the bond strip.

It seems that MC cable is being used far more frequently than AC now.
_________________________
Peter

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#12882 - 08/21/02 06:17 AM Re: Romex vs. BX or MC cable
JBD Offline
Member

Registered: 07/12/01
Posts: 599
Loc: WI, USA
According to my "AC & MC Cable Pocket Guide", from AFC Cable Systems (http://www.afcweb.com):

To terminate an AC Cable, insert an anti-short bushing and bend back the exposed length of bonding strip. The bonding strip can be bent back before or after the bushing is inserted.

Several techniques for this prcedure are:
Method 1 - Bend back over anti-short
Method 2 - Bend back under anti-short
Method 3 - Back-wrap under anti-short
Method 4 - Back-wrap over anti-short

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#12883 - 08/21/02 08:18 PM Re: Romex vs. BX or MC cable
CanadianSparky Offline
Member

Registered: 03/05/01
Posts: 86
Loc: Alberta Canada
Wow, sorry guys. I've never seen this metal strip on our BX(we call it AC90 too). Must be a south of the border thing(PaulUK might help me out here.)

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#12884 - 08/26/02 06:12 PM Re: Romex vs. BX or MC cable
jes Offline
Member

Registered: 07/12/02
Posts: 103
Loc: CT
The bond strip in AC cable is NOT an equipment grounding conductor! It is simpally an add-on to reduce the impedance of the armor sheath when the armor functions as an EGC. It does this by running along the underside of the armor and has casual contact with it. Old AC cable (pre '60's) did not typically have it and the impedance was very unpredictable, particularly as the cable aged and corrosion occured. Even with the bond strip, the impedance of the armor can grow to an unacceptable level on long runs and those with numerous terminations. Hence the benefit of MC and AC with a separate EGC. I know of not requirement to terminate the bond in any particular manner although I was always taught to back-wrap it under the clamp. That becomessomewhat of a moot point with some of the newer push-on connectors.

Both metallic and nonmetallic wiring methods have pros and cons. AC and MC are not immune to punch thru and I have seen more than one sheet rock screw perfectly run-thru aluminum armor. I would consider corrosion and poorly made terminations the biggest problems for armor because it attacks the EGC, a problem that can be avoided with MC or AC with a seperate EGC. Even locations that are originally 'dry' can become wet enough over the life of the installation to cause deteriation of the armor.

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#12885 - 08/27/02 06:53 PM Re: Romex vs. BX or MC cable
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
when you mention BX , the old knarly stuff comes to mind.


someone did something really bad for the trade to have ended up with BX......

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