Type MC cable

[electrofelon]

Also, MC "smart cable"? What's the difference between that and type AC??

Yeah Im wondering that too. I dont understand why it needs such a big "bonding strip" and AC doesnt. Why not just use AC if you dont want to deal with the EGC?

For me, the advantage of MC is the integral EGC that provided a reliable ground, unlike AC that relied on the integrity of the connectors. So now, in the name of cutting labor costs they are back pushing what is actually AC cable and again relying on the connectors? I ain't buying it!! If they want to reinvent AC fine, but don't call it something it isn't and say how great it is just to sell product.

Personally I am all for it. I dont buy all the over-hype on "good grounds" being of paramount importance. I think the extra conductor is awaste of time and money that doesnt add any value. I dont even know where all this came from. I am not aware of any history of problems with raceway EGC's etc.

 

[mbrooke]

Still waiting for that article. You can link to wherever you got that pdf. That magazine archive only goes back to 2014.

Link no longer works, the PDF is from my hard drive. This forum won't let me upload PDFs PM me and I'll Email to you. In the mean time can mods here pull a few strings? Please

Also, MC "smart cable"? What's the difference between that and type AC??

Good Question. To my knowledge the bonding strip is only 18 or 16 AWG aluminum.

https://www.mysouthwire.com/medias/Armorlite-Type-AC-Cable.pdf?context=bWFzdGVyfHByb2R1Y3Qtc3BlY2lmaWNhdGlvbnN8NDY5MjA5fGFwcGxpY2F0aW9uL3BkZnxwcm9kdWN0LXNwZWNpZmljYXRpb25zL2hkNS9oZDkvODg1NDA4OTAwNzEzNC5wZGZ8NzMzNWE4MzM0NzBiM2UyMDk0ZDRmZTQ5MWVkNzA2ZmFkNjAzMjRkYzg1NjMxZTU4N2VhMWZhZGNmMjgxZWM3Nw

Smart cable has a full size bonding strip.

For me, the advantage of MC is the integral EGC that provided a reliable ground, unlike AC that relied on the integrity of the connectors. So now, in the name of cutting labor costs they are back pushing what is actually AC cable and again relying on the connectors? I ain't buying it!! If they want to reinvent AC fine, but don't call it something it isn't and say how great it is just to sell product.

The insulated EGC is a reliable ground for wiring devices and that which plugs into them. But what protects the cable armor itself from a short circuit? Thats where the bonding strip comes in.

I think the benefit of a separate EGC conductor far outweighs the very remote possibility that the EGC could be severed without damaging the current carrying conductors thereby tripping the breaker.

-Hal

Why not have both? Is an aluminum bonding strip really that expensive?

 

[mbrooke]

In that case, you are a perfect example of last sentence of Section 90.1(A) and have been for a long time.

90.1 Purpose.

(A) Practical Safeguarding. The purpose of this Code is the
practical safeguarding of persons and property from hazards
arising from the use of electricity. This Code is not intended as a
design specification or an instruction manual for untrained
persons.

Practical Safe Guarding involves opening a breaker for a fault- and doing so in a manner that doesn't result in fire or shock- being what we are trying to mitigate in the first place by opening that breaker.

 

[mbrooke]

Yeah Im wondering that too. I dont understand why it needs such a big "bonding strip" and AC doesnt. Why not just use AC if you dont want to deal with the EGC?

Now I want to ask. Why do people even bother with MC when AC is wire?

I am not aware of any history of problems with raceway EGC's etc.

Think BX...

 

[electrofelon]

Now I want to ask. Why do people even bother with MC when AC is wire?





Think BX...

Not sure when AC fell out of favor in many parts of the country. Many places you never see it and most people don't even know it exists.

Yeah bx had issues, I was referring to approved raceway and armors without a wire EGC

 

[mbrooke]

Not sure when AC fell out of favor in many parts of the country. Many places you never see it and most people don't even know it exists.

Yeah bx had issues, I was referring to approved raceway and armors without a wire EGC

Article 250 has always had issues... and still does...

FWIW BX and AC is very common in NYC.

 

[electrofelon]

Article 250 has always had issues... and still does...

FWIW BX and AC is very common in NYC.

Yeah some places it is. I did a job for a friend of a friend down in maryland years ago, and Big orange had AC on the shelf.

 

[mbrooke]

Yeah some places it is. I did a job for a friend of a friend down in maryland years ago, and Big orange had AC on the shelf.

Best part is when the bring the bond strip into the box and splice it lol.

 

[hbiss]

Not sure when AC fell out of favor in many parts of the country. Many places you never see it and most people don't even know it exists.

It fell out of favor when quality construction took a back seat to labor costs and less skilled "rope pullers".

My house was built in 1955 with AC and to date all work I did was done with AC. Ok. there might be some MC circuits in there too. A lot of the old AC was even replaced with new when I renovated rooms (probably 2/3 of the house). There was a mix of AC with the bonding strip and without.

The only NM there is was installed by the HVAC guys who used UF to connect the indoor and outdoor split system units.

-Hal

 

[texie]

I agree with mbrooke that logically, it is kinda odd. Generally something needs to be an approved egc per 259.118, but not for itself ?

I have to admit that I have always wondered the same thing that the OP mentions and find it disingenuous.

 

[hbiss]

The insulated EGC is a reliable ground for wiring devices and that which plugs into them. But what protects the cable armor itself from a short circuit? Thats where the bonding strip comes in.

Why not have both? Is an aluminum bonding strip really that expensive?

The whole point of MC is that it was developed to eliminate all the problems caused by the cable armor carrying fault current by providing a EGC conductor with the current carrying conductors. The armor of MC WAS NEVER INTENDED TO CARRY FAULT CURRENT OR BE USED AS AN EGC. That's why there is no bonding strip!!

So now somebody wonders what would happen if a run of MC gets hit by a meteor and the EGC gets severed.
Ya think maybe it should have a bonding strip just in case...

-Hal

 

[electrofelon]

. The armor of MC WAS NEVER INTENDED TO CARRY FAULT CURRENT OR BE USED AS AN EGC. That's why there is no bonding strip!!

So now somebody wonders what would happen if a run of MC gets hit by a meteor and the EGC gets severed.
Ya think maybe it should have a bonding strip just in case...

-Hal

But like it or not it very well may carry fault current if there is a fault in the middle somewhere. I don't consider that much more unlikely than a fault anywhere else. It's like emt where people dont trust it as an egc, but a fault in the middle somewhere from, whatever, a chaff on a coupling, knock in insulation, separated pipe, MUST use the pipe as the fault path. Why is mc any different?

 

[mbrooke]

The whole point of MC is that it was developed to eliminate all the problems caused by the cable armor carrying fault current by providing a EGC conductor with the current carrying conductors. The armor of MC WAS NEVER INTENDED TO CARRY FAULT CURRENT OR BE USED AS AN EGC. That's why there is no bonding strip!!

So now somebody wonders what would happen if a run of MC gets hit by a meteor and the EGC gets severed.
Ya think maybe it should have a bonding strip just in case...

-Hal

What problem was there with armor carrying fault current? Surely the bonding strip in AC did its job?

 

[hbiss]

What problem was there with armor carrying fault current? Surely the bonding strip in AC did its job?

The problem was with AC before the bonding strip was required. Old AC, particularly after years and oxidation or corrosion of the armor that ruined the wrap to wrap continuity, would heat up under fault conditions. There have been instances of where it actually got cherry red.

But like it or not it very well may carry fault current if there is a fault in the middle somewhere. I don't consider that much more unlikely than a fault anywhere else. It's like emt where people dont trust it as an egc, but a fault in the middle somewhere from, whatever, a chaff on a coupling, knock in insulation, separated pipe, MUST use the pipe as the fault path. Why is mc any different?

I don't consider EMT the same, as it's a field assembly with several ways and places where the conductors can be damaged as you say. On the other hand MC, because it's a factory assembly of wrapped conductors, it's very unlikely that anything is going to cause a hot to armor fault, short of something like a well placed screw or nail. And even if that were to happen, the fault current through the armor would divide two ways towards both of the boxes it runs between owing to the EGC. I just don't see it as enough of an issue to warrant a bonding strip.

The fault current that caused the problems with the old AC was at the load, wiring device or box and involved the entire length of the run.

-Hal

 

[hornetd]

I'll point out that the original AC was labeled BX because it was a patented product of the General Electric Company (GE). Like all GE products it was labeled with a code for the GE factory were it was made before shipping. That earliest AC was made in the GE Bronx, NY factory and had a BX tag wired to every roll or reel. So when you sent an apprentice out to a supply stockpile to get a roll of AC cable you made it light on yourself and them by telling them to get you a roll of the BX cable. The UL listing mark was in small print on a paper label wrapped around the cable and was therefore no were near as easy to see. By the time GE's patent expired everyone in the craft had become used to the nickname BX. It was always type AC as far as UL was concerned.

 

[hornetd]

What problem was there with armor carrying fault current? Surely the bonding strip in AC did its job?

The original AC cable; as first made in the General Electric factory in Bonx, NY (BX) did not have a bonding strip. When tested after manufacture it had a perfectly adequate ground current path. The problem with that came from the layer of initially invisible corrosion which formed between the turns of the cable armor thus converting it into an impedance coil. The problems that caused took several years to show up and come to the attention of the industry. During that interval AC acquired a bad reputation among electrical inspectors, whom the larger cities would call in from the permitting office, when a fire was thought to be electrical. Finding partially melted AC and origin points that were obviously the overheating of the cable the electrical inspection community became hostile to that cable type and local prohibitions began to appear some of which are still enforced in a few localities today. That may have been the impetus for the addition of the bonding strip and for UL including corrosion testing of all future electrical products containing non-noble metals which functioned as a current carrying pathway. I'm guessing that may have been the reason steel electrical boxes with cylindrical sides disappeared. The contact between lock nuts and the curved wall of the box had always been only 2 or 4 locknut teeth.

 

[hornetd]

Why doesn't type MC cable have a bonding strip? What if the middle of the run was damaged or drilled into?

Have you considered that the portion of the cables circumference were the bonding strip would be encountered, in a manor centered enough to break it, by the foreign object is only 20 degrees of 360 degrees of arc or less. Additionally a single break in the bonding strip does not increase the impedance of the cable armor enough to be problematic. The role the strip plays in improving the armor to a low impedance pathway for fault current is shorting out the turns of the armor tape so that the armor itself then has a low enough impedance to carry the fault current. A single open turn would not increase the impedance enough to cause dangerous heating of the armor. The combination of circumstances you have raised this concern about would be very rare indeed.

 

[mbrooke]

The original AC cable; as first made in the General Electric factory in Bonx, NY (BX) did not have a bonding strip. When tested after manufacture it had a perfectly adequate ground current path. The problem with that came from the layer of initially invisible corrosion which formed between the turns of the cable armor thus converting it into an impedance coil. The problems that caused took several years to show up and come to the attention of the industry. During that interval AC acquired a bad reputation among electrical inspectors, whom the larger cities would call in from the permitting office, when a fire was thought to be electrical. Finding partially melted AC and origin points that were obviously the overheating of the cable the electrical inspection community became hostile to that cable type and local prohibitions began to appear some of which are still enforced in a few localities today. That may have been the impetus for the addition of the bonding strip and for UL including corrosion testing of all future electrical products containing non-noble metals which functioned as a current carrying pathway. I'm guessing that may have been the reason steel electrical boxes with cylindrical sides disappeared. The contact between lock nuts and the curved wall of the box had always been only 2 or 4 locknut teeth.

Sadly, the NEC does not address this issue remaining mute.

 

[mbrooke]

Have you considered that the portion of the cables circumference were the bonding strip would be encountered, in a manor centered enough to break it, by the foreign object is only 20 degrees of 360 degrees of arc or less. Additionally a single break in the bonding strip does not increase the impedance of the cable armor enough to be problematic. The role the strip plays in improving the armor to a low impedance pathway for fault current is shorting out the turns of the armor tape so that the armor itself then has a low enough impedance to carry the fault current. A single open turn would not increase the impedance enough to cause dangerous heating of the armor. The combination of circumstances you have raised this concern about would be very rare indeed.

Right. But what is the per foot R+jX impedance of the armor itself?

 

[hornetd]

Right. But what is the per foot R+jX impedance of the armor itself?

Well if I were not retired I would take a 100 foot roll and measure it but that is not practical for me to do. I still own a ground loop impedance tester so it wouldn't be hard to do. What I don't have is a roll of the original BX AC cable to compare it to. One of the supply houses I used to do business with might be willing to let me use a roll of cable long enough to test it but I would have to risk exposing myself to Corona 19. Since that is a clotting disease and I, like all persons of Irish decent, have an elevated risk of clotting I think I'll wait until I've been vaccinated. My Brother in law spent a couple of months on a ventilator with that disease. He is now severely disabled and is trying to learn to walk all over again. I think I'll skip that if I can!

--
Tom Horne