D.I.Y. question

[Hawkmoon]

The house I live in is 68 years old, and has real plaster walls (not sheetrock). I recently had to run a new electric line to supply a new outlet I installed. The house was originally wired with BX (steel armored) cable, not Romex. Any time I do electrical repairs or expansion, I use MC cable to remain consistent with the original installation.

For this latest project, I had to break open a small "porthole" in the plaster to snake the MC cable past a 2x4 shoe. Now it's time to repair the plaster and get everything put back to normal. MC cable uses aluminum for the outer covering. I know you can't embed aluminum in Portland cement-based concrete. Does anyone know if patching plaster is equally corrosive to aluminum? If so, I'll have to find some way to patch the plaster without allowing it to contact the MC cable.

[Perd Hapley]

Can you wrap some plastic around the cable?

[Kingcreek]

You don't have to patch plaster with plaster.
I would use ready patch or a similar canister patch, not a dry mix which is more likely to have cement component.

[zxcvbob]

No, it is not as corrosive.  How big a hole are you patching?  (I assume it's pretty small; a few inches)  I'd put a plastic backer behind the hole and fill it with a Plaster of Paris type joint compound.  (the powdered kind you mix yourself.)  I think the brand I've used it called Easy Sand 45.  The number is how many minutes before it sets.  I usually use cardboard for the backer, but I don't have armored cable back there.

If it's a larger hole, maybe patch it with a too-thin piece of sheetrock and then level it with the joint compound to make it sort of match the plaster.  It doesn't need to match perfectly, the patch will disappear when it's painted or wallpapered as long as you get it smooth and level.

[K Frame]

Get one of these. Available just about anywhere.

I used on in my kitchen. Did a fantastic patch.

https://www.truevalue.com/drywall-patch-kit-8-x-8-in?ctplacement=157081-44622122499&cid=gooshop

Sent from my SM-G960U using Tapatalk

[Hawkmoon]

Hole is approximately 4" wide and 3-1/2" high. I'm using pre-mixed patching plaster.

Mike, the surface-mount patch screen won't work. They do raise the surface, and you then have to feather the joint compound out for a modest to considerable distance to make it look right. The hole is behind the wood baseboard, so if I do anything that will set out from the plane of the original plaster, the baseboard won't flush up when I reinstall it. The baseboard will hide the hole, but that doesn't help with firestopping. I've investigated a couple of structure fires and I've seen how even a small gap becomes a de facto blowtorch inside the wall, so I want to patch the hole before I reinstall the baseboard.

I think I'll just wrap the MC cable with a strip of duct tape before I start shoveling plaster into the hole.

[zxcvbob]

Hole is approximately 4" wide and 3-1/2" high. I'm using pre-mixed patching plaster.

Mike, the surface-mount patch screen won't work. They do raise the surface, and you then have to feather the joint compound out for a modest to considerable distance to make it look right. The hole is behind the wood baseboard, so if I do anything that will set out from the plane of the original plaster, the baseboard won't flush up when I reinstall it. The baseboard will hide the hole, but that doesn't help with firestopping. I've investigated a couple of structure fires and I've seen how even a small gap becomes a de facto blowtorch inside the wall, so I want to patch the hole before I reinstall the baseboard.

I think I'll just wrap the MC cable with a strip of duct tape before I start shoveling plaster into the hole.

Put some kind of backer to hold the plaster.  Like I said earlier, I usually use cardboard.  Put in enough plaster to almost fill the hole but not quite, and leave it rough.  You can make it nice and smooth with the second coat, then sand it flush and smooth.  I've done this several times with holes almost as large as yours.  (Phrasing!)

[K Frame]

"so if I do anything that will set out from the plane of the original plaster, the baseboard won't flush up when I reinstall it."

Been there, done that, sanded the back of the baseboard so that it sat nice and level over the patch.

[charby]

Piece of lathe and plaster over it?

[Kingcreek]

You can usually anchor a fill piece of drywall in a bigger hole by working a piece of plywood backer into the hole that is big enough to put a couple screws into it on each side of the hole through the old plaster or wallboard. Run the screws a little deeper than flush. Then place your fill piece and screw to the backer plywood and then patch.
I did this with a recent remodel where I had big round holes from ceiling can lights and a dryer vent hole.

[Brad Johnson]

Would some self-vulcanizing silicone tape be an option? I would think a couple of wraps where the sheathing comes into contact with the plaster should provide plenty of separation in a static mounting scenario.

Brad

[Nick1911]

What do your local codes have to say about it?

[zxcvbob]

It does seem like you're looking for a complex solution to a simple problem.  

Fill the hole with a piece of too-thin drywall (probably 5/16 or 3/8) anchored in place with wooden tabs on the back.  Plaster over it with a setting joint compound to fill the seams and bring the surface flush.  Sand it and paint it, and it will match the original plaster even if you weren't covering it with a baseboard.

I have patched 2 inch holes without using a piece of drywall; just setting joint compound.

[K Frame]

Flood the wall with paxilon hydrochlorate.

The aluminum and the plaster will be too chill to interact with each other.

Or they'll develop a violent rage that will result in your home being destroyed...

[Hawkmoon]

What do your local codes have to say about it?

No local code. The state uses the International Residential Code and the International Existing Building Code. That code says repairs can be made using like materials.

[RocketMan]

Flood the wall with paxilon hydrochlorate.

The aluminum and the plaster will be too chill to interact with each other.

Or they'll develop a violent rage that will result in your home being destroyed...

This is the best solution offered so far.  I'd go with this one.

[brimic]

Q?s
I have BX in my house (or maybe its MC? where it terminates in boxes it has a thin ribbon wire that I'm assuming is for grounding- at least on the boxes I've opened.). I've tested every recepticle   in my house with a multimeter, and they all are grounded.
Can I branch off circuits from these junction boxes using NM, grounding to the junction box?

[Hawkmoon]

Q?s
I have BX in my house (or maybe its MC? where it terminates in boxes it has a thin ribbon wire that I'm assuming is for grounding- at least on the boxes I've opened.). I've tested every recepticle   in my house with a multimeter, and they all are grounded.
Can I branch off circuits from these junction boxes using NM, grounding to the junction box?

The thin ribbon is for bonding, not grounding. Do your cables have two conductors (black and white), or three (black, white, and green)? The old BX (my house was built in 1950) didn't have any grounding, and the receptacles ("plugs" or "outlets") didn't have a third hole for the grounding conductor. If your wall receptacles have a grounding conductor, the cable should have three conductors in addition to the bare bonding jumper.

No, you cannot ground Romex to a metal junction box.

[Hawkmoon]

Here's a quick cheat sheet on the difference betwen BX cable and MC cable:

http://www.ncwhomeinspections.com/BX+Cables+on+exterior+installations

For electrical questions, I like to refer to Mike Holt's forum:

http://forums.mikeholt.com/showthread.php?t=123801


More:

https://www.electriciantalk.com/f2/mc-bx-41342/

The first link suugests that BX with the bonding strip can use the armored outer shell for grounding. The latter two suggest that it cannot. I've always understood that the outer shell is not to be used for the ground.


[Edit to add]So you made me open the NEC (2014 National Electric Code)"

250.118 Types of Equipment Grounding Conductors.
The equipment grounding conductor run with or enclosing
the circuit conductors shall be one or more or a combination
of the following:

(8) Armor of Type AC cable as provided in 320.108. [AC is BX. MC, which is what you can buy a Lowe's and Home Depot, is not BX]

(10) Type MC cable that provides an effective ground-fault
current path in accordance with one or more of the
following:
a. It contains an insulated or uninsulated equipment
grounding conductor in compliance with 250.118(1)
b. The combined metallic sheath and uninsulated equipment
grounding/bonding conductor of interlocked
metal tape–type MC cable that is listed and identified
as an equipment grounding conductor
c. The metallic sheath or the combined metallic sheath
and equipment grounding conductors of the smooth
or corrugated tube-type MC cable that is listed and
identified as an equipment grounding conductor.

You haveBX, so then we need to look at what 320.108 says:

320.108 Equipment Grounding Conductor. Type AC
cable shall provide an adequate path for fault current as
required by 250.4(A)(5) or (B)(4) to act as an equipment
grounding conductor.

And, of course, it tells us nothing. It sends us back to Chapter 2, section 250 -- which is where we started.

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what
grounding and bonding of electrical systems are required to
accomplish. The prescriptive methods contained in Article
250 shall be followed to comply with the performance requirements
of this section.

(A) Grounded Systems.

(5) Effective Ground-Fault Current Path. Electrical
equipment and wiring and other electrically conductive material
likely to become energized shall be installed in a
manner that creates a low-impedance circuit facilitating the
operation of the overcurrent device or ground detector for
high-impedance grounded systems. It shall be capable of
safely carrying the maximum ground-fault current likely to
be imposed on it from any point on the wiring system
where a ground fault may occur to the electrical supply
source. The earth shall not be considered as an effective
ground-fault current path.

(B) Ungrounded Systems.

(4) Path for Fault Current. Electrical equipment, wiring,
and other electrically conductive material likely to become
energized shall be installed in a manner that creates a low-
impedance circuit from any point on the wiring system to
the electrical supply source to facilitate the operation of
overcurrent devices should a second ground fault from a
different phase occur on the wiring system. The earth shall
not be considered as an effective fault-current path.

Got it?

[Nick1911]

I recommend that you use like materials. 

[Hawkmoon]

I recommend that you use like materials.  

I agree. My house is 68 years old and was built using the old BX cable, with no grounding for the outlets and no bonding strip in the BX. There's just no way I'm going to try using Romex to extend or add circuits to that system. I recently added a couple of circuits, and I used MC, which has a green ground conductor in the bundle. I used all metal boxes, so all the new work is compatible with the original installation.

Reading what the code says, I may have been incorrect and it may be code-compliant to connect Romex to a BX system. But the code doesn't say you can or can't -- it says the armored cable (BX)

"shall be installed in a
manner that creates a low-impedance circuit facilitating the
operation of the overcurrent device or ground detector for
high-impedance grounded systems. It shall be capable of
safely carrying the maximum ground-fault current likely to
be imposed on it from any point on the wiring system
where a ground fault may occur to the electrical supply
source."

How to verify what constitutes "low-impedence" or "safe" when you use Romex to extend an old BX system is above my pay grade.

[Nick1911]

/shrug/ you may need to have a license PE evaluate and sign off on it then, if the procedure you're undertaking isn't listed as a qualified procedure allowed under code.

In any case, if you can't find a reference to it under applicable code, why should you trust us to?  You either need a code inspector to evaluate your work and cite it as a code violation, or have a licensed engineer to provide you with an approved remediation for your siutation.

[Hawkmoon]

/shrug/ you may need to have a license PE evaluate and sign off on it then, if the procedure you're undertaking isn't listed as a qualified procedure allowed under code.

In any case, if you can't find a reference to it under applicable code, why should you trust us to?  You either need a code inspector to evaluate your work and cite it as a code violation, or have a licensed engineer to provide you with an approved remediation for your siutation.

   Is this addressed to me, or to brimic? The work I did is to code. As to brimic's question, I AM a licensed building inspector. Even licensed professionals don't always agree on what the NEC says (or means). Spend a little time on Mike Holt's forum and your head will be spinning. For brimic's question, I found the reference. It says what it says. You can use BX cable IF the metal sheath provides a low-impedence path for ground current back to the load panel. But it doesn't define what "low-impedence" is. Even now that I know that Romex can be added onto a BX installation, if it were my house I would use MC cable. It doesn't cost that much more, and it's not as difficult to work with as some people make it out to be. But you have to use metal boxes, not plastic.