Furnace grounding question

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  #1  
Old 11-18-14, 10:44 AM
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Furnace grounding question

Theres a small box inside my furnace that connects the house wiring (from the switch on the side of the unit) to the furnace wiring. Inside the box there is a green ground wire from the furnace that is not connected to anything.

I have continuity between the furnace and my grounded metal conduit, continuity between the duct work and the conduit, continuity between the gas piping etc. There is also continuity between that green wire that's not connected and the metal of conduit, furnace etc.

However, with that ground wire not connected, does that mean the electronics are not grounded?

Should that wire be connected to a ground wire and jumpered to the switch box on the side of the unit? Do I need to go to the trouble of having a separate ground wire installed?
 
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  #2  
Old 11-18-14, 11:14 AM
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All of the ground wires should be connected together with a wirenut and there should be a ground wire pigtail from that bundle that connects to the furnace chassis. Sometimes there will be a screw terminal or ground wires with ring terminals around a screw on the chassis, which is OK too.
 
  #3  
Old 11-18-14, 11:28 AM
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Maybe I didn't clarify. The EGC is provided by EMT.

Is that ground wire provided with the furnace in case you have a separate EGC run with cable or does it have to be jumpered to the box?

Again, there is continuity between that wire that is not connected and my grounded electrical system.
 
  #4  
Old 11-18-14, 11:53 AM
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It should be jumpered to the box. Otherwise you're only getting incidental ground because the j-box is screwed to the furnace case (through painted surface). Fastening the ground pigtail ensures good low resistance continuity.
 
  #5  
Old 11-18-14, 02:06 PM
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Without that ground wire connected the furnace is still bonded thru the flexible conduit, the gas line and the duct work attached. Is that additional ground for the electronics? And without it connected, does that mean the control board etc are not grounded?

Also, what does the code say in regards to my inquiry? Should this be jumpered to the box according the NEC?
 
  #6  
Old 11-19-14, 05:10 AM
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According to the code all ground wires (as equipment grounding conductors) coming into the box are tied ... (see #2 above)

So in your case with just one ground wire (the green wire from the furnace) entering the metal box with other grounding to the panel etc. provided by metal conduit, you attach that wire to the box via a screw that does not serve any other purpose such as hold on the box lid. That's it.

A flexible metal conduit such as BX does not count as an EGC unless it has a bare wire or metal strip running the full length inside but outside any paper lining inside. This wire or strip does not have to come into the box but if it does far enough then it is connected to other ground wires as if it were the EGC. Do not pull that strip to provide some length into the box because then it will no longer go all the way to the other end of the flex conduit. The green wire coming from the furnace through the flex conduit counts as the EGC so the flex conduit itself doe not need to have a metal strip inside or act as EGC.
 

Last edited by AllanJ; 11-19-14 at 06:13 AM.
  #7  
Old 11-19-14, 06:27 AM
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The bond wire can be cut off flush with the sheath or back wrapped. It does not need to connect to any egc's in the box.
 
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Old 11-19-14, 11:10 AM
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So PC says it doesn't need to be connected, AllanJ differs.

I do know that some furnaces depend on a solid ground for some of their electronics to work. So you missed the point of my question.

IBPOOKS advice about a having a flakey bond thru paint and sheet metal screws makes the most sense.... so what I did was connect a #14 ground to the back of the disconnect box and ran it into the furnace and connected with the dangling ground from the furnace.

AllanJ mentioned flexible metal conduit is not suitable for a EGC but I was under the assumption it was and did not need an extra grounding conductor if under 6' in length. One of us are wrong.

So from an NEC standpoint I think I am compliant???
 
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Old 11-19-14, 11:44 AM
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Flexible metal conduit is different than AC cable. Which are we talking about. AC cable is prefilled with conductors.
 
  #10  
Old 11-19-14, 12:03 PM
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Flex, conductors pulled thru....................
 
  #11  
Old 11-19-14, 01:28 PM
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I don't remember a change but flex metal conduit used to be good as a grounding means up to 6'. If a grounding conductor is pulled it needs to connect to any other grounds.
 
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