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Old 07-10-2008, 10:58 PM   #16
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Ground rod or not?


yes, i stand corrected. i wasn't aware it had changed. but it is true. Now you must take a ground and nuetral, seperate them, and still drive ground rods if its detatched
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Old 07-11-2008, 07:14 AM   #17
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Ground rod or not?


Hello Billy

If you are still confused as to why we bond 3 wire neutral and ground and we don't bond neutral and ground on 4 wire feeders let us know and we will explain.
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Old 07-11-2008, 07:29 AM   #18
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Ground rod or not?


It is still very confusing because we read so many different interpretations of how to do sub panels. You posted the following on 5/8/08 and it does not mention 3 or 4 wire feeds or detached building. so it is confusing. If a 3 wire needs to be bonded and 4 wire isolated I get it. If not I am still lost.
As previously mentioned the neutral and ground are bonded at the main panel only. You seem to not quite understand why this is required and it is probably because you are confused between ground and neutral. the white taped neutral in your panel is the service neutral or commonly called the service grounded conductor. Grounded means intentionally connected to earth which is given a Zero (potential). So any voltage measurements from an ungrounded (hot) wire to the service neutral will be some voltage above zero. Typically 120 volts in US residential dwellings.
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Old 07-11-2008, 09:38 AM   #19
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Ground rod or not?


By the way Stubbie I read all your postings if I have time and I know you are a master. Thanks for all the time you spend helping us try to understand.
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Old 07-11-2008, 09:55 AM   #20
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Ground rod or not?


Very good

Ok lets put it in a different perspective 3 wire feeders need bonding of neutral and ground at a sub-panel and 4 wire feeders do not. Forget about detached or attached.
Now consider a sub-panel and the path that current takes to get back to the transformer. Neutral current, another way of saying current on the grounded legs of a branch circuit (usually the white wires) flows to the neutral bar after it flows through the connected load on a branch circuit. From there it flows on the feeder neutral wire to the main panel neutral bar and then flows out on the service or utility neutral to the center tap of the transformer serving your home. Notice that I did not mention it flows to ground, because it does not, it flows to the source (transformer). These wires are current carriers (conductors).
Now consider the equipment ground in a 3 wire connected sub panel. It is not considered a current carrying wire unless a fault to ground occurs. If this type of event happens fault current travels on the equipment ground (bare wire in NM-b or romex) of the branch circuit to the neutral bar of the sub-panel. It then has to get back to the source (transformer) or the sub-panel breaker on the branch circuit where the fault exists will not trip out and clear the fault. Fault current on the equipment ground must use the feeder neutral wire to get back to the main panel and then to the transformer as no other path is available to it. If it can't get back to the center tap of the transformer then your branch breaker will not trip out and all metal likely to be energized from the fault will become energized to line voltage.
It is because the feeder neutral is the only path available for both fault current and neutral or grounded leg current that we must bond the two in the sub-panel because only one path exists back to the main disconnect panel where we always bond neutral and ground because the only path back to the transformer is the service or utility neutral. The service to your house provided by the utility is always 3 wire and we can never change this... so bonding of neutral and ground always occurs at the main disconnect panel.
So 3 wire feeders only have one path that current can utilize to return to the transformer and that is the grounded conductor or neutral wire in the feeders. So we have to connect neutral and ground at the main panel and sub-panels in order for both to utilize this path back to the transformer.

The draw back to this is the possibility of an open neutral event on the feeder to the sub-panel that will break this one and only path back to the transformer. If this occurs then fault currents if a fault should occur will not be able to get to the transformer and the fault cannot clear creating an electrocution hazard. I attached a diagram to show this below.

On 4 wire feeders we must not bond neutral and ground at the sub-panel because if we do then neutral current from our branch circuits will utilize both the feeder neutral and the feeder equipment ground to get back to the main panel and then to the source. Neutral current will take any path available to it to get to the transformer. We do not want any current on the equipment ground wire of branch circuits or feeders other than momentary fault currents to trip a breaker.

So now consider this 4 wire feeder.... We do not connect or bond neutral and ground at the sub-panel. We provide a termination bar for the neutral wires or grounded legs which carry current in normal operation of the electrical system. We then provide a termination bar for the equipment grounds but we do not connect or bond it to the termination bar for the neutrals. Notice that a neutral bar is always on insulated standoffs from the metal of the sub-panel but the ground bar is bonded to the metal. In order to bond the neutral bar it to the equipment ground we must either install the main bonding jumper (usually a green screw) or a jumper to the metal can bonding it to the equipment ground bar via the metal of the sub-panel enclosure or some other listed connection.

Now consider the ground fault path. When a fault occurs the fault current flows to the ground bar. The equipment ground wire of the feeder from the main panel to the sub-panel is also connected to the ground bar in the sub-panel. The fault current flows on the feeder equipment ground back to the main panel where neutral and ground are bonded and then uses the service neutral (because of the bonding) to get back to the source. Remember no other path for neutral current is available line side of the main panel disconnect.

So on 4 wire feeders there is the possibility of 2 paths that neutral current may use to get back to the source. We do not want it on but one path. So we remove any possibility for it to use the equipment ground of the feeder to get back to the source by not bonding the neutral bar to the equipment ground.

However with 4 wire feeders if we have an open neural event we have provided a path for fault current to use other than the neutral of the sub-panel feeder and we can still get a fault to clear.
Attached Thumbnails
Ground rod or not?-open-neutral-4-wire-feeder.jpg   Ground rod or not?-open-neutral.jpg  

Last edited by Stubbie; 07-15-2008 at 01:10 AM.
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Old 07-11-2008, 10:24 AM   #21
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Ground rod or not?


Hi again Billy

I located this graphic from Mike Holt to show the objectionable current if we were to bond at the subpanel or any panel served with 4 wires. Also be aware that metal conduit between panels may serve as the equipment ground instead of an individual wire as shown in Mikes graphic.

Another very important aspect is that with 4 wire feeders if we have an open neutral and because of the incorrect bonding as shown below the equipment ground of the feeder will begin carrying all the neutral current returning to the source from the sub-panel. This will mask the typical symptoms of an open neutral and we will never know that we have a potentially dangerous problem.


Last edited by Stubbie; 07-11-2008 at 11:29 AM.
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Old 07-14-2008, 04:58 PM   #22
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Ground rod or not?


Hi Stubbie,
I believe it finally makes sense. I will continue studying the drawings. I want to thank (for all of us that uses the forum) you and all the pros and knowledgeable people on here that help. We know it takes valuable time away from your day to educate us. Again Thanks a lot.
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Old 07-14-2008, 06:38 PM   #23
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Ground rod or not?


We should stop to think that not every location has adapted the 2008 NEC.
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Old 07-14-2008, 06:44 PM   #24
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Ground rod or not?


Quote:
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We should stop to think that not every location has adapted the 2008 NEC.

Yes, but I would rather just stop 3 wire feeders to outbuildings...
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Old 07-14-2008, 09:05 PM   #25
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Ground rod or not?


Quote:
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Yes, but I would rather just stop 3 wire feeders to outbuildings...
The code making panel is with you. I never saw any problem with it myself. I wonder when we will see 4 wire services on 240/120 resi.
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Old 07-14-2008, 10:19 PM   #26
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Ground rod or not?


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I wonder when we will see 4 wire services on 240/120 resi.
Never will.
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Old 07-15-2008, 08:38 AM   #27
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Ground rod or not?


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Never will.
That's what I said about garage and barn feeders.
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Old 07-15-2008, 10:51 AM   #28
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Ground rod or not?


Well if they ever do run 4 I bet they will be aluminum and not copper.....nooo.....they NEVER will be copper...anybody want to take that bet.....
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Old 07-15-2008, 11:23 AM   #29
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Ground rod or not?


Quote:
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Well if they ever do run 4 I bet they will be aluminum and not copper.....nooo.....they NEVER will be copper...anybody want to take that bet.....
In our area we are seeing copper neutrals on the overhead drops, due to squirrels. They will chew the Al in half, but won't touch the copper.
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Old 07-15-2008, 11:24 AM   #30
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Ground rod or not?


What is it they say....never say never.....
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