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Wiring a series of HI HATS, do I need dedicated green wire?

Wiring a series of HI HATS, do I need dedicated green wire?

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  #1  
Old 08-13-11, 07:46 AM
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Wiring a series of HI HATS, do I need dedicated green wire?

OK I have wired two sets of hi hat lights and I have been doing it one way, but now I think the way I am doing it is redundant.

My house is wired with all EMT conduits, originally was all rigid conduits and when I remodeled I used EMT where I modified. Typically a junction box in the attic, then I use MC cables to go to the individual devices like lights, fans, etc...

The way I used the MC cable, from the attic junction box if I have three fixtures, then I connect three 12-3 MC cables to it. I tie the three green wires together and connect to the metal box. The rest of the house is all conduits so it's grounded properly.

The individual 12-3 cables then go to the fixtures, and the green wire ties to the green wire from the fixture. That's how I did it and it's good.

However, when I did the hi hat lights, all the hi hat lights I have are metal cans, and they have this metal box with it and there is a green wire in it that's screwed to that metal box. That metal box is connected to the metal MC cable, that cable goes to the junction box. So do I really need a 12-3 when I say wire six or eight hi hats in a series? Can I use 12-2 and simply ground the light's own green wire to it's own metal box, and rely on the MC cable and the rest of the system to do the grounding?

I can see if Romex is used, then the green wire needs to be connected through a dedicated conductor to where it is grounded, but in my case, is a 12-3 really necessary?

I have three more sets to wire and there is a significant cost difference between 12-2 and 12-3. Thanks,
 
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  #2  
Old 08-13-11, 07:56 AM
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You will need to connect all the green conductors from the MC to the grounding lead provided in the recessed junction box. The sheath of the MC will not count as the grounding means.
 
  #3  
Old 08-13-11, 09:05 PM
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OK thanks. So I was doing it the correct way.

Just out of curiosity why is the metal on the MC cable not valid to be used as grounding but the EMT pipe is?
 
  #4  
Old 08-13-11, 09:44 PM
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Originally Posted by MiamiCuse View Post
OK thanks. So I was doing it the correct way.

Just out of curiosity why is the metal on the MC cable not valid to be used as grounding but the EMT pipe is?
Short et sweet answer is the MC cable metal band itself is not listed for grounding purpose at all due one is there is no interal bonding at all. you may noted why it is lighter than BX cable which it will have steel cable shroud with interal grounding strip and MC typically come in alum shroud and it do not have grounding band and it will included green THHN/THWN grounding conductor in there.


Merci.
Marc
 
  #5  
Old 08-14-11, 05:48 PM
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At the local Home Depot they sell MC cables in 12/2, 12/3 and 12/4. Under what situation will 12/2 be used?
 
  #6  
Old 08-14-11, 06:12 PM
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MC and Romex is commonly called xx/2, xx/3, etc. with the ground being assumed. Technically it’s 12/2 with ground, 12/3 with ground, etc., but most everyone calls it 12/2, 12/3, etc.

12/2 MC actually has 3 conductors in it…black, white, green. 12/3 MC has black, red, white and green. 12/2 NM (Romex) cable has black, white, bare. Both the green and the bare are the ground wires. When I do commercial work in my area, 12/2 MC is actually used for most of the general wiring. We’ll use 12/3 or 12/4 at times, if it’s a 3-way switch or some special situation.
 
  #7  
Old 08-15-11, 05:32 PM
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12-3 and 12-4 MC cables are also frequently used for multi wire branch circuits using common neutrals.
 
  #8  
Old 08-15-11, 06:03 PM
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12-3 and 12-4 MC cables are also frequently used for multi wire branch circuits using common neutrals.
Let's not forget switch loops.
 
  #9  
Old 08-16-11, 05:32 PM
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While it is not sold in the big box stores there is a new comer in the MC world called MCAP (Metal Clad All Purpose) This has 2 insulated copper conductors and an aluminum #10 bonding strip. This bonding strip is in direct contact with the outer sheath and is UL listed for the outer jacket to be used for grounding because of this strip. It is the same cost as MC with an insulated ground but we save on time and box fill. There is also a version of MCAP for health care facilities called MCAP/HCF which has both an insulated ground and the aluminum bonding strip.
 
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