main panel versus sub panel

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moondog23

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Typical residential single phase 200 amp main panel with 100 amp sub panel on other end of house.

I know the main panel must have the grounded and grounding bus bars connected.

I also know the sub panels must NOT have those two bars connected, and the grounded (neutral) bus bar must be insulated from the metal panel.

My question is, how do I explain the hazard in having the two bars connected at the sub panel?

In other words, I know HOW it should be done on the sub panel, I do not know WHY.

Any help would be appreciated !
 
You don't need to know why other than the code says so. I don't know if anyone can give you a good reason why its OK for a service ground to carry current but a subpanel ground can not (unless it goes to a detached building with no metallic paths in which its OK again).
 
Under normal circumstances, the ground wire is not supposed to carry any current. If you tie the neutral and ground together at the main panel, it's not a problem because that's where the source neutral originates. As long as the neutral is tight, the ground shouldn't carry any current.

Anywhere downstream of the main, though, should have the neutral and ground separated because otherwise the ground wire would carry some (or all) of the neutral current (voltage divider) back to the main panel under normal circumstances. Since ground wires are often bare and exposed, you don't want them carrying current if you can help it.

This is the same idea behind ranges and dryers now requiring four-prong plugs rather than three-prong ones.
 
Thanks!

Thanks!

Sorry, Mark, I do not agree with you. I do think it is important WHY something is in the code. The more you understand WHY the code covers a specific item, the better your installations become because you have clarity as to the intent of the code.

Thanks, Jeff, I really appreciate the response. Makes sense, and easily understood. That is why I enjoy this board - good communication from professionals who strive to improve themselves and their craft.
 
Glad I could help. I've also found knowing the intent behind a rule is helpful when dealing with customers who want things that aren't legal. I can then tell people that "even if _______ weren't a rule, it's a good practice anyway because ..."

I also think it makes you look more professional if you seem to have an understanding of the "why" and don't just come across as a drone who only does what the book says and doesn't know anything beyond that.
 
moondog23 said:
Sorry, Mark, I do not agree with you. I do think it is important WHY something is in the code. The more you understand WHY the code covers a specific item, the better your installations become because you have clarity as to the intent of the code.

Thanks, Jeff, I really appreciate the response. Makes sense, and easily understood. That is why I enjoy this board - good communication from professionals who strive to improve themselves and their craft.
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I do agree we whould know why, as it helps us do our job better. But there are things in the code that no one can seem to explain why it is the way it is. Why are water pipe bonds (not grounds) done per 250.66 instead of 250.122? Why can service equipment carry current but non-service equipment can't? There are probably a whole bunch more...
 
main panel versus sub panel

The reason grounded (neutral) the grounding conductor is connected to the same point at the service is to provide a low impedance path back to the source during a ground fault.
If the grounding conductor was connected to the grounded (neutral) conductor any other place than the service the grounding conductor would be placed in paralleled with the grounded (neutral) conductor. By the paralleling the two conductors current would flow in both conductors energizing any and all non-current carry metal parts, such as metallic enclosures and metallic wiring methods, to which the grounding conductor is required to be connected per the Code.
 
The grounded conductor is a current carrying conductor. If is regrounded past the service, neutral current will flow on the metal parts bonded to that panel. This can cause
Fires
High levels of EMF
shock if some one opens a raceway with neutral current

The NEC only states in 250.6 objectionable current is not allowed, but doesn't indicate what it is.

Karl Riley, one of our moderators and the leading expert on EMF's states that perhaps 80% of our buildings are miswired due to inadvertant neutral to ground connections. He has documentation very high levels of EMFs due to this mistake.
 
I'm not so sure these last two responses answer the original question for dwelling wiring that typically uses non-metallic cabling methods. Certainly, if you run 4 wires in a feeder (one being an equipment ground) or use a metallic conduit to contain the feeder, then you must keep the neutral and grounding wires separate. But in the old days, when people only ran 3 wires in a non-metallic cable to each subpanel and wired them like a service (a combination grounded/grounding conductor), the only hazard I see is the subpanel now has a ground reference that is a few volts higher than the service ground (it follows the voltage drop of the feeder neutral). All grounding wires derived from this panel would be separate non current carrying conductors just like a service panel.

The hazard I can see here would be if the subpanel ends up with a metallic path to an object that also has a metallic path to the first service panel (e.g. you feed a water heater from this subpanel and the cold water pipe was grounded to the service and no dielectric coupling is in the heater). This creates a parallel current path over the pipe and all the bad things Tom mentioned can happen.
 
If the neutral is isolated from the ground at the subpanel, and the neutral return to the service fails, then the circuit is open and someone will fix the failed circuit.

If the neutral is connected to ground at the subpanel and the neutral fails, then the "neutral" current would return to the service panel via the equipment ground, which could overload it, causing overheating.
 
suemarkp said:
This creates a parallel current path over the pipe and all the bad things Tom mentioned can happen.
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Not to mention all kinds of ground-loop problems with audio/video equipment.
 
Bob NH said:
If the neutral is isolated from the ground at the subpanel, and the neutral return to the service fails, then the circuit is open and someone will fix the failed circuit.

If the neutral is connected to ground at the subpanel and the neutral fails, then the "neutral" current would return to the service panel via the equipment ground, which could overload it, causing overheating.
Click to expand...

and, even worse, if BOTH the grounded neutral and the equipment ground fail, ALL of the equipment grounded to the subpanel would be hot when any appliance is on.
 
"As long as the neutral is tight, the ground shouldn't carry any current."

In actuality it will almost always be carrying something...Kirchoff's law. This is why you almost always see some current on the GEC if you put a clamp probe on it.

If someone else in the neighborhood has a loose neutral, the GEC might be carrying a lot.
 
tonyi said:
In actuality it (i.e., the ground wire) will almost always be carrying something...Kirchoff's law. This is why you almost always see some current on the GEC if you put a clamp probe on it.
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There is a bit of confusion here, with regard to which ?ground? is under discussion. David?s comment was about the Equipment Grounding Conductor (EGC). His statements are correct. On the other hand, it is not true that there will always be current in the EGC (if that is what you meant, Tonyi). The EGC is connected to the frame or case of served equipment, and there is normally no connection from an energized wire to that frame or case. Since there is no complete circuit, the EGC will carry no current.

As to whether the Grounding Electrode Conductor (GEC) normally carries some small amount of current, the question is not germane to this discussion.

All that said, let me say three things to David.

First, welcome to the Forum.

Secondly, since you are not an electrician, I am sure you will understand that we could not give you any assistance, should you wish to perform you own electrical installation work. You are free to learn whatever you can from this Forum, but not to seek that kind of assistance.

Third, let me ask that you check the dates of existing threads, before you add your own comments. The last comment on this thread, before you added yours, was over nine months old. I think it is best not to resurrect old threads; any new discussions should take place in new threads.
 
also if your neutral fails and there is not a ground rod, but the water pipe is, then if the plumber is fixing a broken pipe, he could become the path to ground and die. we must protect our fellow tradesmen(and women).
 
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