I am confused by parts of the code. As I understand it properly installed MC can be used as the EGC. This would then be acceptable to ground an outlet to a properly installed metal box if the outlet has no washer behind the screw. If so why does MC typically come with a ground wire? Or am I reading the code wrong? Could I pigtail from the Ground screw of an outlet to the box with only 2 wire system in the box?(if the box and MC/EMT is properly installed)
MC as EGC
- Thread starter stardust
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xformer
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Since you asked 3 questions.... .
1. MC is now manufactured with a separate insulated grounding conductor. My guess is to keep fault current off of the outer metal sheath.
2. It does not appear that you are reading the code erroneously.
3. One could do that only if the metal box is properly bonded to the EGC system. Running a pig tail from the grounding terminal of a receptacle to a box does not a Ground make.
Hope this helps
winnie
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http://forums.mikeholt.com/showthread.php?t=162208
I think that you might be confusing MC with AC.
In MC cable, the sheath is not rated to be used as the EGC. MC cable always has a separate wire EGC.
In AC cable the sheath is rated to be used as the EGC. AC cable will have a little 'bonding strip' in direct contact with the sheath.
AC cable for healthcare use has both the sheath and an insulated wire EGC, to provide redundant grounding.
A proprietary type of MC cable, called MC-AP has a full size wire EGC, but it is made of aluminium and not insulated, and is in contact with the sheath.
-Jon
I think that you might be confusing MC with AC.
In MC cable, the sheath is not rated to be used as the EGC. MC cable always has a separate wire EGC.
In AC cable the sheath is rated to be used as the EGC. AC cable will have a little 'bonding strip' in direct contact with the sheath.
AC cable for healthcare use has both the sheath and an insulated wire EGC, to provide redundant grounding.
A proprietary type of MC cable, called MC-AP has a full size wire EGC, but it is made of aluminium and not insulated, and is in contact with the sheath.
-Jon
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If the cable type is permitted to ground the box then you would need a self-grounding receptacle or a bonding jumper to the box for the device.
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- Bremerton, Washington
MC-AP has an aluminum bonding strip under the cable armor, buts its not an EGC, the cable armor is and the neutral is labeled as such. With this type of MC there is no green EGC, and the installation is 30% faster as the green EGC does not have to be bonded at each box. it is available with an insulated EGC and then can be used in healthcare applications
winnie
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I absolutely agree that the cable armor is what you connect to for the ground in MC-AP, and you don't have to bond a wire EGC at each box.
But I believe that the thing which makes this _MC_ rather than _AC_ cable is that the 'bonding strip' is in fact a _full size_ wire. It is an uninsulated aluminum wire, rather than an insulated copper wire, and it is 2 awg larger than the corresponding copper EGC would be. It is much larger than the bonding strip found in AC cable. I don't believe there is a prohibition against using this wire as an EGC, but doing so would pretty much defeat the reason for using MC-AC in the first place.
-Jon
But I believe that the thing which makes this _MC_ rather than _AC_ cable is that the 'bonding strip' is in fact a _full size_ wire. It is an uninsulated aluminum wire, rather than an insulated copper wire, and it is 2 awg larger than the corresponding copper EGC would be. It is much larger than the bonding strip found in AC cable. I don't believe there is a prohibition against using this wire as an EGC, but doing so would pretty much defeat the reason for using MC-AC in the first place.
-Jon
DrSparks
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Mc types
Mc types
There are many different manufacturers and types of MC. It is always best to use ann insulated copper EGC sized according to the rating of the overcurrent protection device ahead of the circuit. There are types of MC, however, that are listed for using the outer jacket as part of the EGC. Some manufacturers color code the jacket green. One key difference is that the helical corrugations are interlocked so ensure ground continuity.i might add that this type of MC would be ill-advised for use with sensitive electronic equipment such as IT equipment. There are MC types available with an addition IG wire though.
Mc types
There are many different manufacturers and types of MC. It is always best to use ann insulated copper EGC sized according to the rating of the overcurrent protection device ahead of the circuit. There are types of MC, however, that are listed for using the outer jacket as part of the EGC. Some manufacturers color code the jacket green. One key difference is that the helical corrugations are interlocked so ensure ground continuity.i might add that this type of MC would be ill-advised for use with sensitive electronic equipment such as IT equipment. There are MC types available with an addition IG wire though.
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mjf
Senior Member
You must be certain that the old 2 wire cable at least has a bonding strip in it, otherwise bonding from the device ground screw to the steel box is prohibited.
MasterTheNEC
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Added note...not all manufacturers call it MC-AP. For example, Encore Wire calls it MC-SG but is the same type of product. The SG stands for Smart Ground in case anyone was wondering.
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- Bremerton, Washington
Southwire invented the MC-AP, with an added EGC it can be used in heath care apps. Traditionally heath care was a
AC cable market, made by AFC. This MC-AP gives southwire a bit more market share
AC cable market, made by AFC. This MC-AP gives southwire a bit more market share
As already noted the sheath of mc cable isn't listed for grounding. Ac cable on the other hand is.
If the recep you're wiring is in a surface mounted box using mc cable, as long as the box is grounded using the EGC and you remove at least one of the insulating washers from the recep mounting screws then you wouldn't need a pigtail. No pigtail needed if you install to a plaster ring using a self grounding recep either. 250.146 A & B
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