Grounding Of Hot Air Ductwork ?

Hello:
Checked the hot air ductwork from a forced hot air furnace, and find that it is not grounded.
Probably because there is a non-metallic "flex-joint" between the furnace and the main outgoing duct.
What does the Code say regarding this ?
Should I just run a ground wire between the furnace shell (which is grounded, probably via the the AC power ground) and the Duct ?
This should work well, I guess, as most furnaces that don't have a non-metallic joint are probably being grounded, in effect, in this manner.
What about the electrical Code, though ?
B.
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I just took a quick look at the 2002 NEC section 250. and the phrase that I found over and over was "may become energized"
Practically I have not seen duct work grounded except for a project that I can not talk about. That not residential and it was more than 10 years ago. There was not a piece of metal in the whole building that was not grounded more than once. Door frames were grounded once on the inside and once on the outside. We installed 250 MCM bare copper on the duct connections near the air handlers. I protested that the connection would eventually break the ductwork do to the ductwork moving and the copper would not. I was over ruled by the EE. He just graduated so consider the source.
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I also worked in some places I can't talk about and I agree with you about the EEs spec on the wire. When we were doing RF shielding we used very fine stranded cables, not garden variety THHN. "Skin effect" makes a finely stranded conductor far better than coursely stranded cable. As a secondary benefit they are a lot more flexible. Just be sure your terminations are listed for this kind of cable. Regular lugs are not.
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Our code, states that you dont use ductwork as a ground, ever. And really, since you would have to bond each joint to be effective, its not worth the time to.

No. Why do you want to ground the ductwork?

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If you just need something to fill your time, I bet some local charity would love to have you volunteer. I know of no reason for you to ground your ducts.
Stretch
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The code says that you must not use ductwork as a _grounding conductor_.
That's different than _grounding_ ductwork.
I suppose it's up to debate.
Both US and Canadian codes require "grounding of major metal components of structures".
Both US and Canadian codes require that you ground plumbing and steel building frames. [But in _neither_ case can you use plumbing or steel structure as a grounding conductor for other things.]
Is not ductwork just as major a component of a structure as, say, plumbing is? In fact, there's a lot more of it, and much bigger contact area.
[And the code especially notes duct bonding in some circumstances.]
Isn't the consequences of a hot-to-ductwork fault just as potentially lethal as, say, a hot-to-plumbing fault?
I'm fully aware, as most other people would be, that duct-to-duct electrical conductivity wouldn't be perfect. But, at least having the majority of it having a least nominal connectivity is better'n nothing.
I suspect that officially, it _should_ be grounded, but given that most ducting is more or less effectively grounded anyway, the incidence of problems _at present_ is sufficiently low that inspectors and code isn't getting anal about bonding between components.
In other words, it's certainly legal to do, may not be _necessary_ in practise, but it won't hurt, and whether the OP wants to do it is up to their own risk avoidance preferences.
--
Chris Lewis, Una confibula non set est
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On Mon, 15 Aug 2005 19:51:58 -0000, snipped-for-privacy@nortelnetworks.com (Chris Lewis) wrote:

The code says metal likely to be energized should be bonded but that is allowed to be bonded by the EGC of the circuit likely to energize it so the ground in the humidifier circuit would do it.
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Or, obviously, the furnace ground.
This obviously begs the question as to whether it's continuous throughout the ductwork (as per the OP's original posting), but I don't see evidence of inspectors being anal over it (see previous posting).
--
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
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: Hello: : : Checked the hot air ductwork from a forced hot air furnace, and find that it : is not grounded. : : Probably because there is a non-metallic "flex-joint" between the furnace : and the main outgoing duct. : : What does the Code say regarding this ? : : Should I just run a ground wire between the furnace shell (which is : grounded, probably via the : the AC power ground) and the Duct ? : : This should work well, I guess, as most furnaces that don't have a : non-metallic joint are probably being grounded, in effect, in this manner. : : What about the electrical Code, though ? : : B.
NEC, far as I know, doesn't require you to ground that in most cases. The requirement for grounding is for anything that has electricity somehow connected to it, that could cause the frame etc. to become live. Ductwork normally isn't going to have anytyhing electrical connected to it unless a remotly located humidifier or such is being added.
If there were a requirement to ground the ductwork, you'd be seeing wired connections between pieces and even to the registers themselves, as metal touching metal can NOT be considered a functional connection for safety purposes. So, no, it's not necessary to ground it.
That assumes, of course, that there has been no electrical device or equpment added IN the ductwork at some location - then there might, not will be, a need for grounding it in some very few cases.
HTH, Pop
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