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Old 05-28-2009, 10:58 PM   #16
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Romex as ground conductor?


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Old 05-28-2009, 11:46 PM   #17
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Romex as ground conductor?


At risk of hijacking this post, why is lightning rod cable braided?

I think that KC may be on to something! Its been found that electrons in a conductor travel in the 'skin' rather than in the core.
In this case, is it possible that a back EMF could be generated in adjacent conductors, resulting in resistance to current flow?
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Old 05-29-2009, 01:58 AM   #18
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Romex as ground conductor?


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OK, I'm ducking...
Honestly it sounds like a functional approach to solve the problem with budget considerations taken into account. I see no real reason to duck...Not a bad idea.
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Old 05-29-2009, 07:19 AM   #19
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Romex as ground conductor?


This is an interesting question. My concern would be that the ground would fail at the point where the three wires branched to their individual conductors. I would be concerned that the current would burn through the individual wires when they were subjected to the full current of the lighting strike. I am not an electrical engineer, but this seems a valid concern.
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Old 05-29-2009, 09:00 AM   #20
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Romex as ground conductor?


Quote:
Originally Posted by Wildie View Post
At risk of hijacking this post, why is lightning rod cable braided?

I think that KC may be on to something! Its been found that electrons in a conductor travel in the 'skin' rather than in the core.
In this case, is it possible that a back EMF could be generated in adjacent conductors, resulting in resistance to current flow?

It is a fat braid because of skin effect like you said. At DC, there is effectively no skin effect. At 60hz skin effect is only an issue with very large conductors (significantly larger than 4/0)... at say 10khz, skin effect dominates and current is only carried in the very outer part of a conductor.

The rapid rise in current is effectively a "high frequency"... And skin effect happens primarily at higher frequencies.

So even though it's technically a DC flow, you don't want to present an instantaneous impedance to the rapid rise in current, so you need a lot of surface area to carry the current on the skin.
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Old 05-29-2009, 09:02 AM   #21
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Romex as ground conductor?


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Originally Posted by Bigplanz View Post
This is an interesting question. My concern would be that the ground would fail at the point where the three wires branched to their individual conductors. I would be concerned that the current would burn through the individual wires when they were subjected to the full current of the lighting strike. I am not an electrical engineer, but this seems a valid concern.
I don't think that #6 could handle the full current of a direct lightning strike.
Perhaps 4/0 would do better.

If the antenna were to take a direct hit, it would be destroyed. I don't know what would happen after that. If the #6 wire melted, there would be an arc between the point it melted, and the closest ground, possibly through the wall of the house to an electrical cable or a water or sewer pipe.

I had a CB antenna on my house a very long time ago, and only ran a #10 ground wire from the mast to an 8ft ground rod below it. This ground rod was not tied to the SE ground.
I never had an issue with lightning, but I did disconnect the radio when storms were in the area.

I read somewhere, that the primary purpose of lightning protection (grounding) for antenna is to discharge dangerous atmospheric charges before they become lightning strikes.

I don't know whether this theory holds water though, considering how often the Empire State building is hit each year.

FW

Edit: I may be able to pick up some #4 THHN at a decent price.
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Old 05-29-2009, 09:28 AM   #22
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Romex as ground conductor?


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I don't think that #6 could handle the full current of a direct lightning strike.
Perhaps 4/0 would do better.
I don't think anything you can buy at the supply house can handle the full current of a lightning strike.

The words "sacrificial wave guide" come to mind... you are lucky if the copper vaporizes to provide a nice conductive ion path. The point is to avoid fire by giving it that chance to establish a path rather than running inside the house and arcing across water pipes/etc.
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Old 05-29-2009, 10:25 AM   #23
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Romex as ground conductor?


I wish that I had gone to the supply house instead of HD yesterday.
The guy at the supply quoted me 0.30/ft for #6 THHN, and HD is selling it at 0.80/ft?
Seems like something wrong. Maybe the prices pasted to the reel rack were in the wrong place, or the reels were out of place? Or, maybe the guy who helped me read it wrong. I think I saw a price of 0.40/ft for an item, but he told me it was something else.

Anyway, I am already bidding on 100ft of #4 THHN on Ebay, which will cost me more per foot (after I add shipping cost) than the #6 locally but it's #4, not #6, so a comparable cost, except that I will have 50ft left over. Guess that could come in handy sometime.
Or, maybe I'll be outbid and just go to the supplier.
Last time I ever go to Home Depot for "specialty" electrical. For Romex, HD is great, but as someone said earlier, they basically take a loss on certain items to get you into the store and buy the other stuff at ripoff prices.
I'll bet that $7 bronze ground clamp will only cost me $4 at the supply house.

FW
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Old 05-29-2009, 12:52 PM   #24
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Romex as ground conductor?


Quote:
Originally Posted by Gigs View Post
It is a fat braid because of skin effect like you said. At DC, there is effectively no skin effect. At 60hz skin effect is only an issue with very large conductors (significantly larger than 4/0)... at say 10khz, skin effect dominates and current is only carried in the very outer part of a conductor.

The rapid rise in current is effectively a "high frequency"... And skin effect happens primarily at higher frequencies.

So even though it's technically a DC flow, you don't want to present an instantaneous impedance to the rapid rise in current, so you need a lot of surface area to carry the current on the skin.
Soiunds reasonable to me, however I'll leave the theory to you, young fellers! Too much info my aged neurons.
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Old 05-29-2009, 01:14 PM   #25
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Romex as ground conductor?


I understand the skin effect, having been an amateur radio op for many years. Most people don't think of the skin effect for transients like lightning.
I worked many years in electronics, with high speed switching (logic) circuits, and had to think a lot about high frequency effects, since switching takes place in nano seconds, which translates to Ghz.

I don't know what the rise time, or dI/dT caused by a lightning strike is, but I would think it is very high.
By this thinking, the braid would be the best conductor. Maybe a 2" wide braid?
Now we're talking expensive!

I'm starting to wonder whether I should drive a second ground rod, I think it has to be at least 6ft away from the first one, and bond them as per code.
This would hopefully decrease the resistance to ground, and improve my protection, overall.

Maybe increasing the size of the ground conductor from the SE panel to the cold water pipe would also be desirable.
But now we're really talking about a project. To work on the SE panel in this area, I would have to get a temporary shut off by POCO. I would not do this type of work on a live panel.

Sorry... I'm getting ahead of myself. I'll just relax, have a few beers, and leave the antenna project for another day...

FW
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Old 05-29-2009, 03:32 PM   #26
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Romex as ground conductor?


Thats why I always tell people to check on wire prices at the supply house. 30 cents a foot vs 80 cents. Copper prices change everyday, supply house's adjust the price accordingly.

As far as the NM. I agree with 220/221. Strip the cable completely. No insulation at all. Put one end in a vise and at the other end start twisting the bundle with lineman's. Make your own bare braided cable. If you have a propane torch you can braise the ends.

I would use the bare #6 at 30 cents a foot though.
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Old 05-29-2009, 04:12 PM   #27
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Romex as ground conductor?


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Originally Posted by J. V. View Post
Thats why I always tell people to check on wire prices at the supply house. 30 cents a foot vs 80 cents. Copper prices change everyday, supply house's adjust the price accordingly.

As far as the NM. I agree with 220/221. Strip the cable completely. No insulation at all. Put one end in a vise and at the other end start twisting the bundle with lineman's. Make your own bare braided cable. If you have a propane torch you can braise the ends.

I would use the bare #6 at 30 cents a foot though.
That price is for THHN, not bare copper.

I don't think I'm going to make my own braid using a lineman's plier. Twisting 50ft of wire that way would be nearly impossible, or take 1/2 a lifetime<g>

FW
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Old 05-29-2009, 04:47 PM   #28
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Romex as ground conductor?


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Originally Posted by KE2KB View Post
Sorry... I'm getting ahead of myself. I'll just relax, have a few beers, and leave the antenna project for another day...

FW
No, you got it backwards. What you need to do is have about 6 or 8 beers, then start messing with the service.
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Old 05-29-2009, 04:52 PM   #29
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Romex as ground conductor?


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Originally Posted by KE2KB View Post
I think HD's price of 0.80/ft is robbery.

FW
If you have Menard's stores in your area, check them out. Bare #6 copper, cut to length, was 44 cents a foot a few weeks ago when I bought some for a ground rod.
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Old 05-29-2009, 05:00 PM   #30
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Romex as ground conductor?


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I don't think that #6 could handle the full current of a direct lightning strike.....
Actually, it can. And the maximum amount of current that can be dissipated by a ground rod can be safely transmitted over #6 wire.

That is why the Code never requires anything larger than a #6 when connecting your main service to driven ground rod (or "made electrodes").

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