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main and sub equal

#1
02-19-06, 06:09 PM
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Join Date: Nov 2005
Posts: 135
main and sub equal

Okay, this is just a hypothetical observation/inquiry.
If you were to equip a sub panel without the EGC as
one of the feeder conductors, only using the two hots
and the one neutral, wouldn't the sub panel be just like
the main panel? And therefore, wouldn't bonding the ground
and the neutral bus bars work, since there would be no parallel
path(EGC) for the return current to the power source?
The sub panel would be fed with two hots and one
neutral, just like the main panel. And, it would have a ground
wire from the bus bar to a grounding electrode, so that
if the sub panel box were to be energized, the current would
go to the grounding rod(s).

And, if I'm not totally mistaken, or off my rocker, I could swear that I read something similar to this scenario in the 2005 NEC. If I'm wrong, don't worry about making me feel dumb, or bursting my bubble. It's okay!!

#2
02-19-06, 06:15 PM
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Join Date: Feb 2004
Posts: 536
Without going into electrical theory 101 I'll put it simply.Yes it will work.....Will it be safe....NO! And no i'm relatively certain this is not in the 05' NEC

#3
02-19-06, 06:23 PM
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Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
The problem with this setup is that the ground for the sub panel is no longer at the same voltage as the ground in the main panel. Because of the voltage drop in the cable, the sub panel hot wires are at a slightly lower voltage than the hot wires in the main panel. At the same time, the neutral wires in the sub panel are at a slightly higher voltage than the neutral wires in the main panel. By bonding the neutral and ground at the sub panel, the ground wire is at this slightly different voltage than the ground wires from the main panel.

This by itself is not a big deal. However, should you join a ground wire from the main panel and a ground wire from the sub panel (which you would have to do if a circuit from each panel passes through a common junction box) you now have current flowing on the ground wires. Also, should you touch a ground wire from the sub panel and something grounded, such as a water faucet, you provide a path for the current through your body. This is not a good thing.

You are allowed, in certain situations to run a three wire cable to a sub panel, but the sub panel must be in a detached structure. And there must be no other ground path between the buildings, such as a water pipe or a phone line, etc.

#4
02-19-06, 06:31 PM
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Join Date: Nov 2005
Posts: 135
The main panel is on the side of the house, and the sub panel is in a detached garage. There are NO water pipes or phone lines running between the two structures. So, is it then theoretically okay?

#5
02-19-06, 06:31 PM
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Originally Posted by Willg54
And, it would have a ground
wire from the bus bar to a grounding electrode, so that
if the sub panel box were to be energized, the current would
go to the grounding rod(s).
If you simply ran a ground wire to a ground rod nothing would happen. Ground rods are NOT designed, intended or adequate to clear a fault. The voltage would simply enter the ground. There would be no path back to open a breaker.

In the event of a broken or compromised neutral you would have no protection.

#6
02-19-06, 06:32 PM
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Join Date: Jan 2006
Location: PA
Posts: 1,909
Originally Posted by Willg54
If you were to equip a sub panel without the EGC as
one of the feeder conductors, only using the two hots
and the one neutral, wouldn't the sub panel be just like
the main panel?
No.

And therefore, wouldn't bonding the ground and the neutral bus bars work, since there would be no parallel path(EGC) for the return current to the power source?
You are in error. Of course there is another path.

it would have a ground wire from the bus bar to a grounding electrode, so that if the sub panel box were to be energized, the current would go to the grounding rod(s).
First of all, that is a parallel path right there.

Secondly, if the panel is energized, it goes to the neutral and back to the transformer. It does not go to the ground rods.

if I'm not totally mistaken, or off my rocker, I could swear that I read something similar to this scenario in the 2005 NEC.

Actually, the NEC permits this for separated buildings which have no metallic pathway between them. This is nothing new. Eventually such buildings end up with another metallic pathway between them, and you have a problem it you used only three wires.

It is so much simpler if you just use four wires.

#7
02-19-06, 06:38 PM
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Join Date: Jan 2006
Location: PA
Posts: 1,909
Originally Posted by Willg54
The main panel is on the side of the house, and the sub panel is in a detached garage. There are NO water pipes or phone lines running between the two structures. So, is it then theoretically okay?
This week. But you will run a communication circuit or create some metallic pathway next week.

The other problem is that it creates stray voltage in the earth. So you generally won't see the shortcut taken where there are chickens or livestock.

#8
02-19-06, 06:44 PM
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Join Date: Nov 2005
Posts: 135
Well, good thing it was only a question, and I haven't already done that, HUH?!? Now more people will know about the downfalls of wanting to do this.

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