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Author Information Topic:   Why don't you bond the neutral at a sub panel?
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Name: Joel Crawford
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posted August 12, 2002 at 10:53 PM       Edit/Delete Message
so why is it not allowed?? just curious. thanks.

also what's the equation for voltage drop?

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posted August 12, 2002 at 11:20 PM       Edit/Delete Message
If the neutral were bonded to the enclosures at both ends of a feeder (both panels), and the panels were joined be a another conductive path, such as an equipment grounding conductor (EGC), the EGC would be in parallel with the neutral conductor.

Can you see what hazard is caused by this connection error?

For your voltage drop question, try http://www.mikeholt.com/studies/vd1.htm

Ed

[This message has been edited by edmac@attcanada.ca (edited August 12, 2002).]

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posted August 12, 2002 at 11:20 PM       Edit/Delete Message
Frequently Asked Question: Why do the grounds and neutrals need to be separated in a subpanel? What happens if they aren't?

Answer: Though the neutral doesn't have significant voltage, it does carry current. Remember, it's current that kills, not voltage. In a 2-wire circuit, the neutral carries just as much current as the hot conductor. If the neutral and ground are connected in a subpanel, that current will travel on other paths, such as bare ground wires, equipment enclosures, and metal piping systems, on its way back to the service panel. One problem created by this condition is possible shock hazards, the severity of which depends on the locations of the equipment and the person touching the enclosure or piping system. Another problem is magnetic fields that do not cancel themselves out. Since the return current has multiple paths, the current remaining in the neutral will not counterbalance the current in the hot wire. The resulting imbalance creates a magnetic field that can interfere with sensitive electronic equipment. In a metal conduit system, the imbalance will induce current into the conduit, which could cause the conduit to overheat.

Q: Given that this is a fairly common condition, why don't we hear about problems, and why do some electricians not understand the problem?

A: For the feeder from one panel to another, the neutral only carries the imbalanced load between the two hot legs. Most of the time, the amount of current on the neutral is very low. However, in a situation where a single 120-volt appliance is in use, and there are few other loads in operation, the current on the equipment grounding conductors and other paths could be quite high, resulting in the problems noted above. Other conditions could cause the load imbalance to be quite high, if for example, all the lights in use at a given time just happened to be originating from the same phase conductor.

The National Electrical Code (NEC) has prohibited re-grounding the neutral after the service since the 1923 edition. Exceptions have been made for dryers and ranges that use the neutral as a grounding means, and for separate buildings. However, those loopholes have been effectively closed in the 1996 and 1999 editions of the NEC. The references in the 1999 edition of the NEC are found in 250-24(a)(5), 250-142(b), and 384-20.

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posted August 12, 2002 at 11:51 PM       Edit/Delete Message
Few years ago I investigated an elctrical failure in an office supplied from 100A subpanel.
Some day lights started to dim and flickering, computers and copiers refused to work. In addition, a women was almost fatally shocked in kitchenette, trying to open metal door of microwave with the other hand on metal sink.
What happened.
Contact of aluminum neutral wire was deteriorated (oxidated) possibly originally not properly clamped. But on subpanel, neutral was bonded to enclosure (VIOLATION!). Unbalanced neutral was conducted trough steel enclosure, EMT (loose tihted by locknuts), main panel enclosure and main bonding jumper to service neutral. Nobody saw any abnormalities. Few years later locknut bonding deteriorated and "welding" sparks around locknuts was inside the wall (fortunatelly sparks did not ignite wooden stoods). But some day "return path" had so high resistance (oxidation) that voltage for lighting and appliances was unbalanced. Voltage between "electric ground" and real ground (water faucet) was uncontrolled (may be 50V or more). Hard touch of two obiects on different potential (with wet hands) could be fatal.
DO NOT BOND neutral to ground down the service. Equipment grounding conductor should not be part of current carrying circuit, because is formed with poor conductive materials (steel) and could be quickly deteriorated conducting unbalanced current.
Deteriorated NEUTRAL can be easy detected and repaired but fire and shock hazard is minimized because all it happens INSIDE raceways and enclosures, not OUTSIDE (as sparks on connectors, couplings etc.).
Andre

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posted August 13, 2002 at 07:04 PM       Edit/Delete Message
i understand the problems associated with it now, but why couldn't all these bad things happen at a service panel also?

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posted August 13, 2002 at 08:26 PM       Edit/Delete Message
The reason that this wont happen at the service panel is that the service entrance does not generally contain a EGC in paralell with the grounded conductor.

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posted August 13, 2002 at 09:16 PM       Edit/Delete Message
could you define "parellel". i want to be on the same page.

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posted August 13, 2002 at 09:47 PM       Edit/Delete Message
parallel: electrically connected at both ends, a portion of the current will flow thru each path, the amount depends on the resistance.

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posted August 14, 2002 at 10:51 AM       Edit/Delete Message
I think that the principal reason that this won’t happen at the Service Panel is roughly like saying that you have reached the finish line, so it’s time to stop running. Here’s what I mean: One of the best known, indeed almost beloved phrases in our profession is that “electricity takes the shortest path to ground.” It is also utter nonsense. Electricity takes all paths, not just the shortest (or the path of least resistance). Also, electricity isn’t trying to find its way to planet Earth, it is trying to find its way back to the source. The transformer that feeds the Service Panel is the source, and the Service Panel is the first thing that the transformer feeds, once you get into the building. The Service Panel is the both the starting line and the finish line for current flow within the building.

What we want is for current to leave the Service Panel’s hot leg (i.e., the breaker), to travel to and through the load, and to make its way back via the neutral, without having the option of taking any other path. If we connect the grounded conductor (i.e., neutral) to the equipment grounding conductor (i.e., green wire) anywhere downstream of the Service Panel (e.g., bonding them inside a sub-panel), then the current will have two paths to take: the neutral and the EGC. And it will take both paths. The neutral is out of reach of human hands; the EGC is not, since it connects to external metal parts. Therefore, if a human touches the metal parts, the current will have three paths back to its source: the neutral, the EGC, and the human. And it will take all three paths.

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posted August 14, 2002 at 06:54 PM       Edit/Delete Message
I like it, Charlie. A great way to explain it.

Ed

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posted August 14, 2002 at 10:13 PM       Edit/Delete Message
Charlie that was really good, thanks.

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posted August 15, 2002 at 06:11 AM       Edit/Delete Message
Charlie,

The common adage of "electricity will always try to get back to earth/ground" is something I've rallied against for a long time. I'm sure that this "fact" is responsible for a lot of the confusion thgat we see over the subject of grounding.

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posted August 15, 2002 at 08:00 AM       Edit/Delete Message
The problem with the idea that electricity is always trying to get back to ground is made worse by the code use of the words "equipment grounding conductor". This strongly implies that the important connection is to earth, but that is not true. The word grounding should, in most parts of Article 250 be replaced with bonding. That would help clear up the confusion. The word grounding would then be used only for direct connections to ground. The title of the Article would be changed to "Grounding and Bonding". I think the Canadian code uses "equipment bonding conductor" in place of equipment grounding conductor.
Don

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posted August 15, 2002 at 11:02 AM       Edit/Delete Message
I am confused about when you run 3 wire to a detached garage, no metallic non-current conductors, that you connect the grounded conductor bar and the ground bar together. Ground rod and GEC installed at garage.

You have now introduced another ground potential the the system. Seems it would be better to have grounded conductor bar isolated from the case?

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posted August 15, 2002 at 11:11 AM       Edit/Delete Message
If you do not have them bonded in that situation(no EGC run with feeder to seperate building), you could not get a breaker to operate on a line to ground fault because there is no return path to the source. You treat it just like you would the main service.

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posted August 15, 2002 at 11:35 AM       Edit/Delete Message
so at a transformer, the neutral is not grounded to earth??? equipment grounding conductor is misleading.

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posted August 15, 2002 at 12:16 PM       Edit/Delete Message
There are at least four separate things happening here, and it’s best (though not easiest) to treat them separately.

First, ground rods give lightning a path to planet Earth. Yes, this can be reconciled with my “It is also utter nonsense” statement above, but I’ll not attempt it alone. Try this link: http://www.ipclp.com/html/how.html

Secondly, ground rods establish a reference potential for the power system. You want to have any “grounded” or “grounding” or “bonding” anything with which your hand might accidentally come into contact to be as close as possible to the same potential as the thing on which you are standing. I think that is why we install grounding electrodes at each separate building. When you are in your garage, you don’t really care what the reference potential might be in your kitchen. When you are in your kitchen, you don’t really care what the reference potential might be in your garage.

(The third and fourth things both relate to metal surfaces within the reach of human hands. In no case do we want a human hand to contact something that is carrying current at the time. But if it does happen, we want to protect the human.)

Third, for equipment that is working properly, we don’t want to create a parallel path for the normal flow of current (i.e., neutral and EGC – see my discussion above).

Fourth, for equipment that fails in such a way as to establish a connection between its current-carrying conductors and its case (and thereby to the human hand that is holding the case), we recognize the fact that current will now have three paths to take: the neutral, the EGC, and the human. How do we protect the human? By having a low-resistance path back to the source (i.e., the EGC). This means that most of the current will take that path, and less current will take the path through the human. It also means that the source will be supplying vast amounts of current – from the breaker to the equipment, through the fault point to the case, and through the EGC back to the source. This high current will quickly trip the breaker and terminate the event. If everything is sized and built in accordance with the code, the human may not feel a thing, but will only notice that the equipment has been turned off.

Now back to your question. The answer lies in the fourth item discussed above. If you run an EGC to the garage (reference NEC 250.32(b)(1)), you don’t bond neutral and ground at the garage panel. In this case, the current from a ground fault goes through the EGC within the garage back to the sub-panel in the garage, and via the continuation of the EGC back to the main panel in the house. It will trip the circuit breaker feeding the equipment. It might also trip the garage feeder in the house. On the other hand, if you don’t run an EGC to the garage (reference NEC 250.32(b)(2)), you do bond neutral and ground at the garage panel. (The ground rod has nothing to do with this discussion.) In this case, the current from a ground fault goes through the EGC within the garage back to the garage panel, and then via the garage feeder’s neutral leg back to the main panel in the house. Here again, it will trip the circuit breaker feeding the equipment. It might also trip the garage feeder in the house. Whichever method you pick, the human is still protected in the two ways described above.

Paul is right. If you don't have the EGC back to the house and if you don't bond at the garage panel, then the current path back to the source will include the garage's ground rod, planet Earth, and the house's ground rod. Draw a picture, and you'll see what I mean. The planet has enough resistance that the fault current will be too low to trip the breaker.

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posted August 15, 2002 at 01:39 PM       Edit/Delete Message
Joel,
In my opinon, the most important connections are the equipment bonding conductor and the main bonding jumper. This is the path that current must flow on to open the OCPD when there is a fault. The connection to earth plays little or no part in this process. While the grounding electrode system helps maintain an equal potential between the bonded electrical equipment and earth under normal conditions, this is not true under fault conditions. The non-current carrying parts of the bonded electrical system will be raised above earth by the amount of voltage drop in the fault return path. The greatest votage will appear on the items that ate bonded to the equipment that has the fault. The rest of the bonded electrcial equipment will be have a votlage to earth equal to the voltage drop in the main bonding jumper and the grounded conductor. Remember that fault currents can be very high and that the voltage drop can often exceed 50 volts. This high voltage to ground will only exist for the time it takes the OCPD to clear the fault. The better the fault clearing path, the more current that flows, means that the OCPD will clear the fault quicker.
Don

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posted August 15, 2002 at 05:29 PM       Edit/Delete Message
equipment bonding conductor = neutral??
main bonding jumper = neutral to enclosure??

correct or not?

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posted August 15, 2002 at 07:29 PM       Edit/Delete Message
Joel,
The equipment bonding conductor does not equal the neutral. The neutral is current carrying conductor. The bonding conductor only carries current under fault conditions. It is connected to the neutral (grounded conductor) by the main bonding jumper. The main bonding jumper connects the grounded conductor to the bonding (grounding) conductors. There is also a connection to the enclosures.
Don

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posted August 15, 2002 at 10:44 PM       Edit/Delete Message
Joel, perhaps this sketch of a typical service will help, along with the excellent comments above.

Ed

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posted August 16, 2002 at 12:06 AM       Edit/Delete Message
once again ed, you've come to the rescue. excellent drawing.

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posted August 16, 2002 at 11:07 AM       Edit/Delete Message
Is a ground rod required at the garage? Also if the garage is a metal structure, does it have to be bonde to the ECG, back to the main panel?

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posted August 16, 2002 at 11:09 AM       Edit/Delete Message
Is a ground rod required at the garage? Also if the garage is a metal structure, does it have to be bonde to the ECG, back to the main panel?

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posted August 20, 2002 at 08:40 AM       Edit/Delete Message
If the garage is detatched, you must have a grounding electrode system established at the structure, seperate from the one at the main service. What type of metal building is it? 250-104(c)covers this. If you bond the metal, it will go to the ground bar in the sub-panel, and ultimately back to the main service panel, if there is an EGC run to the garage.

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posted August 21, 2002 at 11:22 AM       Edit/Delete Message
Originally the code section stated the ground/neutral conductor shall not be connected to a ground electrode on the load side. This makes sense, to not create two or more earth connection points.
This changed to not being connected to a ground conductor. Ground conductors are often bare or conduit. The parallel load current would create an arc at loose joints, plus generate a lot of interference EMF.
There is no practical reason for a cable supplied panel, such as a residential system, for the neutral and ground being separated, or the ground conductor even being present. Other than to sell more copper or aluminum.

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