Sub panel question
- Thread starter concord
- Start date
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- Location
- Massachusetts
Re: Sub panel question
I thought the reason was elevated voltage on the grounding conductors due to voltage drop from the load placed on the combination Grounding, Grounded Conductor?
Bob
I thought the reason was elevated voltage on the grounding conductors due to voltage drop from the load placed on the combination Grounding, Grounded Conductor?
Bob
bphgravity
Senior Member
- Location
- Florida
Re: Sub panel question
It is simply the fact that objectionable current should not ever be on the grounding system considering it is bonded to every metallic and conductive component of the electrical system.
It is true that some installations would be of minor concern, but a metal raceway in metal building to metallic enclosures would be a serious issue.
It is simply the fact that objectionable current should not ever be on the grounding system considering it is bonded to every metallic and conductive component of the electrical system.
It is true that some installations would be of minor concern, but a metal raceway in metal building to metallic enclosures would be a serious issue.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Sub panel question
NOTE TO FORUM MEMBERS: I?m going to use conversational terms here, and make statements that are not precise. But it?s for illustration purposes only, so please don?t feel the need to correct me (unless I am way way off base). Here is an explanation of why the neutral and grounding conductors are connected to each other at the main panel only. The fundamental principal of interest here is that current takes all available paths (not just the one of least resistance) back to the source of power (not to planet Earth).
The ?Normal? path of current is as follows: From the main panel (i.e., the source), via the ?hot leg? of the feeder, to the sub-panel?s bus bars, out the breaker, along the ?hot leg? of the branch circuit, through the load, along the ?cold leg? of the branch circuit, to the neutral bus of the sub-panel, along the ?cold leg? of the feeder, to the neutral bus of the main panel, thus completing the circuit.
Now let?s presume there is a short circuit between a hot wire inside the load to the external case of the load (or to any external metal part of the device). The ?Fault? path of current is as follows: From the main panel (i.e., the source), via the ?hot leg? of the feeder, to the sub-panel?s bus bars, out the breaker, along the ?hot leg? of the branch circuit. Current will continue to flow back to the source as described above. But current will also follow two other paths. ?New Path One? is from the case to the hand of the person who is touching the device, through that person?s body, out both feet, into the floor, along the dirt to the nearest ground rod, up the grounding electrode conductor to the main panel, into the ground bus, via the bonding jumper to the neutral bus, and thus returns to the source. Admittedly this is a high resistance path, but it does not take much current to ruin the person?s day. ?New Path Two? is from the case to the ground wire (i.e., green wire or bare copper wire), along the ground wire of the branch circuit to the ground bus of the sub-panel, along the ground wire of the feeder to the ground bus of the main panel, via the bonding jumper to the neutral bus, and thus returns to the source. This is a very low resistance path, and the current that flows out the circuit breaker will be high enough to cause that breaker to trip, thus terminating the event and protecting the person who is touching the device. This is why there is a ground wire in the first place. But why do we separate the neutral and ground at the sub-panel?
The following would be the ?Normal? path of current, if the neutral and ground were connected at the sub-panel: From the main panel (i.e., the source), via the ?hot leg? of the feeder, to the sub-panel?s bus bars, out the breaker, along the ?hot leg? of the branch circuit, through the load, along the ?cold leg? of the branch circuit, to the neutral bus of the sub-panel. At this point, current separates into two or more paths. The first, as described in the ?Normal? path, goes along the ?cold leg? of the feeder, to the neutral bus of the main panel, thus completing the circuit. The other paths of current are from the neutral bus of the sub-panel, via the (illegal) bonding jumper to the ground bus of the sub-panel, back up the ground wire of every single device in the entire building, to the case (or any external metal parts) of every device in the building, into the hands of anyone who is touching any device in the building, through those persons? bodies, out their feet, into the floor, along the dirt to the nearest ground rod, up the grounding electrode conductor to the main panel, into the ground bus, via the bonding jumper to the neutral bus, and thus returns to the source. See the comment above regarding ruining someone?s day.
NOTE TO FORUM MEMBERS: I?m going to use conversational terms here, and make statements that are not precise. But it?s for illustration purposes only, so please don?t feel the need to correct me (unless I am way way off base). Here is an explanation of why the neutral and grounding conductors are connected to each other at the main panel only. The fundamental principal of interest here is that current takes all available paths (not just the one of least resistance) back to the source of power (not to planet Earth).
The ?Normal? path of current is as follows: From the main panel (i.e., the source), via the ?hot leg? of the feeder, to the sub-panel?s bus bars, out the breaker, along the ?hot leg? of the branch circuit, through the load, along the ?cold leg? of the branch circuit, to the neutral bus of the sub-panel, along the ?cold leg? of the feeder, to the neutral bus of the main panel, thus completing the circuit.
Now let?s presume there is a short circuit between a hot wire inside the load to the external case of the load (or to any external metal part of the device). The ?Fault? path of current is as follows: From the main panel (i.e., the source), via the ?hot leg? of the feeder, to the sub-panel?s bus bars, out the breaker, along the ?hot leg? of the branch circuit. Current will continue to flow back to the source as described above. But current will also follow two other paths. ?New Path One? is from the case to the hand of the person who is touching the device, through that person?s body, out both feet, into the floor, along the dirt to the nearest ground rod, up the grounding electrode conductor to the main panel, into the ground bus, via the bonding jumper to the neutral bus, and thus returns to the source. Admittedly this is a high resistance path, but it does not take much current to ruin the person?s day. ?New Path Two? is from the case to the ground wire (i.e., green wire or bare copper wire), along the ground wire of the branch circuit to the ground bus of the sub-panel, along the ground wire of the feeder to the ground bus of the main panel, via the bonding jumper to the neutral bus, and thus returns to the source. This is a very low resistance path, and the current that flows out the circuit breaker will be high enough to cause that breaker to trip, thus terminating the event and protecting the person who is touching the device. This is why there is a ground wire in the first place. But why do we separate the neutral and ground at the sub-panel?
The following would be the ?Normal? path of current, if the neutral and ground were connected at the sub-panel: From the main panel (i.e., the source), via the ?hot leg? of the feeder, to the sub-panel?s bus bars, out the breaker, along the ?hot leg? of the branch circuit, through the load, along the ?cold leg? of the branch circuit, to the neutral bus of the sub-panel. At this point, current separates into two or more paths. The first, as described in the ?Normal? path, goes along the ?cold leg? of the feeder, to the neutral bus of the main panel, thus completing the circuit. The other paths of current are from the neutral bus of the sub-panel, via the (illegal) bonding jumper to the ground bus of the sub-panel, back up the ground wire of every single device in the entire building, to the case (or any external metal parts) of every device in the building, into the hands of anyone who is touching any device in the building, through those persons? bodies, out their feet, into the floor, along the dirt to the nearest ground rod, up the grounding electrode conductor to the main panel, into the ground bus, via the bonding jumper to the neutral bus, and thus returns to the source. See the comment above regarding ruining someone?s day.
Re: Sub panel question
Charlie: Very good. I am not referring to equipment ground wires with each branch circuit. I am addressing the number of feeder conductors, to a panel, from the service.
A three wire feeder will use the neutral for fault clearing, if the equipment ground wires are connected to the neutral bus in the panel.
I am only referring to a residential occupancy with NM cable as a feeder. I can understand when conductive paths are present, to separate the two grounds.
Charlie: Very good. I am not referring to equipment ground wires with each branch circuit. I am addressing the number of feeder conductors, to a panel, from the service.
A three wire feeder will use the neutral for fault clearing, if the equipment ground wires are connected to the neutral bus in the panel.
I am only referring to a residential occupancy with NM cable as a feeder. I can understand when conductive paths are present, to separate the two grounds.
roger willis
Member
- Location
- Texas
Re: Sub panel question
Concord,
Welcome to the forum.
I won't say anymore, I think Charlie covered it all.
Roger
Concord,
Welcome to the forum.
I won't say anymore, I think Charlie covered it all.
Roger
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Sub panel question
Charlie: Outstanding.
Charlie: Outstanding.
brian john
Senior Member
- Location
- Leesburg, VA
Re: Sub panel question
Very good Charlie, this would be an excellent 1st lesson for any course in electricity.
(not to planet Earth).
The number of electricians that thing the earth is a store house of used electrons scares me sometimes.
Very good Charlie, this would be an excellent 1st lesson for any course in electricity.
(not to planet Earth).
The number of electricians that thing the earth is a store house of used electrons scares me sometimes.
Re: Sub panel question
concord man i got a book here that puts it this way if the egc system were to touch the grounded conductor ( nuetral) system , which is carrying current all the time, there would be a parrallel path for this current to return to the service equiptment bonding point, such as the metallic raceway or the metal armor of an ac cable. this would create the possibility of a potential of voltage to exsist between all of the exposed dead metal parts of electrical equiptment and any other grounded metallic parts within that facility such as metallic pipe and appliances within the plumbing system. that is why u isolate the nuetral and the ground in a sub panel this would create a shock hazard if u touch the metal parts and it could melt the metal parts if the fault was strong enough
concord man i got a book here that puts it this way if the egc system were to touch the grounded conductor ( nuetral) system , which is carrying current all the time, there would be a parrallel path for this current to return to the service equiptment bonding point, such as the metallic raceway or the metal armor of an ac cable. this would create the possibility of a potential of voltage to exsist between all of the exposed dead metal parts of electrical equiptment and any other grounded metallic parts within that facility such as metallic pipe and appliances within the plumbing system. that is why u isolate the nuetral and the ground in a sub panel this would create a shock hazard if u touch the metal parts and it could melt the metal parts if the fault was strong enough
Re: Sub panel question
Charlie b,
Thanks for the explaination, but now I'm going to play the devil's advocate for a moment, because I think we need to dig deeper.
In your last paragraph, you describe what I'll call "illegal" paths because they utilize the illegal bonding jumper in the sub-panel through the ground conductors of the various branch circuits, through some unfortunate people etc, etc, until it gets back to the source. We can't know what the resistance of that portion of the path is after the sub-panel nuetral, but for discussion, let's say it is 25 ohms. I pick that number rather arbitrarily because it is a figure associated with ground rods the actual resistance may be less, but probably is more.
I suggest that in a legal installation without the illegal bonding jumper in the sub-panel there are some "legal" alternate paths that differ from the your "illegal" ones only slightly, as follows: from the neutral bus of the sub panel via the feeder neutral to the neutral bus of the main panel, then via legal main bonding jumper and the EGC of the feeder to the ground bus of the sub-panel and from there they are the same as the "illegal" paths you described.
Suppose the feeder is 60 ft. long, the resistance of the portion of the path through the feeder neutral and the feeder EGC would probably be about .01 ohms depending on the size of feeder and EGC. So, if the resistance of the portion of the "illegal" path from the sub-panel neutral to the source is 25 ohms, the resistance of the portion of the "legal" path from the sub-panel neutral to the source is 25.01 ohms. The current that would flow through the unfortunate person due to the illegal bonding jumper would be nearly the same as what could flow through them due to the "legal" paths that do exist in legal installations.
Am I wrong?
Charlie b,
Thanks for the explaination, but now I'm going to play the devil's advocate for a moment, because I think we need to dig deeper.
In your last paragraph, you describe what I'll call "illegal" paths because they utilize the illegal bonding jumper in the sub-panel through the ground conductors of the various branch circuits, through some unfortunate people etc, etc, until it gets back to the source. We can't know what the resistance of that portion of the path is after the sub-panel nuetral, but for discussion, let's say it is 25 ohms. I pick that number rather arbitrarily because it is a figure associated with ground rods
the actual resistance may be less, but probably is more.I suggest that in a legal installation without the illegal bonding jumper in the sub-panel there are some "legal" alternate paths that differ from the your "illegal" ones only slightly, as follows: from the neutral bus of the sub panel via the feeder neutral to the neutral bus of the main panel, then via legal main bonding jumper and the EGC of the feeder to the ground bus of the sub-panel and from there they are the same as the "illegal" paths you described.
Suppose the feeder is 60 ft. long, the resistance of the portion of the path through the feeder neutral and the feeder EGC would probably be about .01 ohms depending on the size of feeder and EGC. So, if the resistance of the portion of the "illegal" path from the sub-panel neutral to the source is 25 ohms, the resistance of the portion of the "legal" path from the sub-panel neutral to the source is 25.01 ohms. The current that would flow through the unfortunate person due to the illegal bonding jumper would be nearly the same as what could flow through them due to the "legal" paths that do exist in legal installations.
Am I wrong?
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: Sub panel question
Eprice
The only time the NEC allows the rebonding of the neutral-ground is when the subpanel is located in a remote building and there are no metal paths between the two buildings.
the reason this is allowed is because of the resistance of earth will allways be lower than the resistance of the neutral. so if there is a voltage drop across the neutral and the required GE is installed there will be no voltage potential between the grounding and earth. when there is a metal path then there is a possability of shock from this path and the grounding since this path will be at the potential of the main service ground-neutral and the voltage drop of the neutral feeding the sub panel will be between these two paths.
Eprice
The only time the NEC allows the rebonding of the neutral-ground is when the subpanel is located in a remote building and there are no metal paths between the two buildings.
the reason this is allowed is because of the resistance of earth will allways be lower than the resistance of the neutral. so if there is a voltage drop across the neutral and the required GE is installed there will be no voltage potential between the grounding and earth. when there is a metal path then there is a possability of shock from this path and the grounding since this path will be at the potential of the main service ground-neutral and the voltage drop of the neutral feeding the sub panel will be between these two paths.
Re: Sub panel question
1940 Edition NEC, section 2523. No Additional Grounds.
No grounding connection shall be made to the grounded conductor on the load side of the service switch.
This probably originated around 1914, the words are the same today in the 2002 NEC.
This interpretation has also taken on a new spin from when it began. This has gone the same way as the separately derived system,and demonstrates my opposition to the use of "ed" and "ing"
The meaning is that the neutral/ground shall not be connected to earth after the main. Connecting to an equipment ground conductor is only grounding the neutral at both ends back to the service. This is not grounding (earthing) the neutral after the main.
1940 Edition NEC, section 2523. No Additional Grounds.
No grounding connection shall be made to the grounded conductor on the load side of the service switch.
This probably originated around 1914, the words are the same today in the 2002 NEC.
This interpretation has also taken on a new spin from when it began. This has gone the same way as the separately derived system,and demonstrates my opposition to the use of "ed" and "ing"
The meaning is that the neutral/ground shall not be connected to earth after the main. Connecting to an equipment ground conductor is only grounding the neutral at both ends back to the service. This is not grounding (earthing) the neutral after the main.
karl riley
Senior Member
Re: Sub panel question
I think one reason it is difficult for some electricians to understand the effect of connecting neutral to grounding conductors in a subpanel is that they don't see anything different and they don't feel anything different. It becomes a theoretical point to discuss. Bennie evidently theorizes that the neutral current in a NM cable feeder will go obediently back to the main even when "grounded".
Since electricity is invisible we need an instrument to make it visible, to make it real. All you have to do is turn on a gaussmeter and watch the numbers, and then follow the current on all the various metallic pathways, HVAC ducts, wire mesh, pipes, steel girders, grounding conductors, sprinkler pipes, etc. You can't do most of this with a clamp-on ammeter, so if that's all you have, how would you know what's going on?
Even better, for a sense of the reality, is to go to Radio Shack and pick up a cheap telephone listening coil ($5?) and plug it into their inexpensive hobby amplifier and walk around listening to the huge hum as you get near a pipe or conductor carrying diverted neutral. I use earphones with the volume turned down. The sound can be so loud and annoying to the office personnel that I don't want to subject them to it. And the magnetic field on the feed, missing some of its neutral, is just as loud.
Everything Charlie described becomes a loud reality. And once you find and disconnect the N/G connection the noise disappears and peace returns.
If electricians would only carry a small $200 gaussmeter, or a $25 Radio Shack combo, they would know this is reality, not theory.
That's my sermon for today.
Karl
I think one reason it is difficult for some electricians to understand the effect of connecting neutral to grounding conductors in a subpanel is that they don't see anything different and they don't feel anything different. It becomes a theoretical point to discuss. Bennie evidently theorizes that the neutral current in a NM cable feeder will go obediently back to the main even when "grounded".
Since electricity is invisible we need an instrument to make it visible, to make it real. All you have to do is turn on a gaussmeter and watch the numbers, and then follow the current on all the various metallic pathways, HVAC ducts, wire mesh, pipes, steel girders, grounding conductors, sprinkler pipes, etc. You can't do most of this with a clamp-on ammeter, so if that's all you have, how would you know what's going on?
Even better, for a sense of the reality, is to go to Radio Shack and pick up a cheap telephone listening coil ($5?) and plug it into their inexpensive hobby amplifier and walk around listening to the huge hum as you get near a pipe or conductor carrying diverted neutral. I use earphones with the volume turned down. The sound can be so loud and annoying to the office personnel that I don't want to subject them to it. And the magnetic field on the feed, missing some of its neutral, is just as loud.
Everything Charlie described becomes a loud reality. And once you find and disconnect the N/G connection the noise disappears and peace returns.
If electricians would only carry a small $200 gaussmeter, or a $25 Radio Shack combo, they would know this is reality, not theory.
That's my sermon for today.
Karl
Re: Sub panel question
I stand corrected. I am referring to a residential occupancy, with wood framing.
I have a meter main on one side of my garage. The main panel is on the other side. I fed this with SER cable and separated the neutral and ground bus in the panel. There is no sense in doing this.
I stand corrected. I am referring to a residential occupancy, with wood framing.
I have a meter main on one side of my garage. The main panel is on the other side. I fed this with SER cable and separated the neutral and ground bus in the panel. There is no sense in doing this.
karl riley
Senior Member
Re: Sub panel question
Hi Bennie, you really keep a topic alive and keep the opinions coming until we get some kind of clarity. That's valuable.
Though steel buildings with conduit and various systems do give neutral current more paths, much of my work is done in residential wood frame houses. Even here, though, I have traced neutral all through the house. A panel in the house (actually a subpanel but with neutral bonded to the case) allows neutral to travel on the washer's equipment ground, to the copper water pipe supplying the washer, to a gas pipe which happens to be touching the water pipe, to a sheet metal vent pipe for the dryer, etc etc. All this comes together again at the bonding point at the meter outside the house. So wood frame doesn't mean you can't have neutral cavorting around the house in a way that results in areas that may have a high field where the resident doesn't want it.
Karl
Hi Bennie, you really keep a topic alive and keep the opinions coming until we get some kind of clarity. That's valuable.
Though steel buildings with conduit and various systems do give neutral current more paths, much of my work is done in residential wood frame houses. Even here, though, I have traced neutral all through the house. A panel in the house (actually a subpanel but with neutral bonded to the case) allows neutral to travel on the washer's equipment ground, to the copper water pipe supplying the washer, to a gas pipe which happens to be touching the water pipe, to a sheet metal vent pipe for the dryer, etc etc. All this comes together again at the bonding point at the meter outside the house. So wood frame doesn't mean you can't have neutral cavorting around the house in a way that results in areas that may have a high field where the resident doesn't want it.
Karl
Re: Sub panel question
My previous post was a discussion of Charlie's example of the alternate current paths introduced by the illegal bond in the sub-panel. I was pointing out that in a legal system a path does exist from the feeder neutral through the main bonding jumper in the service panel, throught the feeder EGC, through the branch circuit EGC, through connected metal appliance cabinets, through human bodies, through the dirt, through the nearest grounding electrode, and back to the source. This path is nearly identical to the path created by an illegal bond in the sub-panel between neutral and grounding busses, and will differ in resistance only by .01 ohms or so because it must use the feeder EGC to get back to the sub-panel grounding bus.
I brought this up, because I think there needs to be another explaination for the code prohibition of the bond in the sub-panel.
I am aware of this.
My previous post was a discussion of Charlie's example of the alternate current paths introduced by the illegal bond in the sub-panel. I was pointing out that in a legal system a path does exist from the feeder neutral through the main bonding jumper in the service panel, throught the feeder EGC, through the branch circuit EGC, through connected metal appliance cabinets, through human bodies, through the dirt, through the nearest grounding electrode, and back to the source. This path is nearly identical to the path created by an illegal bond in the sub-panel between neutral and grounding busses, and will differ in resistance only by .01 ohms or so because it must use the feeder EGC to get back to the sub-panel grounding bus.
I brought this up, because I think there needs to be another explaination for the code prohibition of the bond in the sub-panel.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Sub panel question
Eprice: Sorry I couldn?t get back to you earlier. It's been a busy couple of days.
The path you described will not carry current in the manner you describe. Let me return to my original statement about the fundamental principal at work here. Once current has left the source (i.e., via the hot leg), it seeks all available paths back to the source (i.e., normally via the neutral or cold leg). But once it has made it there, it does not go any further. I know it will continue to go around the same loop over and over; I don?t mean that. But if current goes from the sub-panel neutral bus to the main panel neutral bus, it?s next move is to the source. That's the end of the circuit. It will not proceed from the main panel neutral to the sub-panel (via the bonding jumper and the feeder EGC). The motive force for current flow (i.e., voltage) will drive current the other way (i.e., back to the source).
If you are taking a round trip flight from Salt Lake City to Chicago, when you arrive back in Salt Lake City you don?t hop a flight to Seattle. You?re home. For current, the source is home.
Eprice: Sorry I couldn?t get back to you earlier. It's been a busy couple of days.
The path you described will not carry current in the manner you describe. Let me return to my original statement about the fundamental principal at work here. Once current has left the source (i.e., via the hot leg), it seeks all available paths back to the source (i.e., normally via the neutral or cold leg). But once it has made it there, it does not go any further. I know it will continue to go around the same loop over and over; I don?t mean that. But if current goes from the sub-panel neutral bus to the main panel neutral bus, it?s next move is to the source. That's the end of the circuit. It will not proceed from the main panel neutral to the sub-panel (via the bonding jumper and the feeder EGC). The motive force for current flow (i.e., voltage) will drive current the other way (i.e., back to the source).
If you are taking a round trip flight from Salt Lake City to Chicago, when you arrive back in Salt Lake City you don?t hop a flight to Seattle. You?re home. For current, the source is home.
brian john
Senior Member
- Location
- Leesburg, VA
Re: Sub panel question
Karl:
With the advent of flexible CT"S readings are possible around most objects. One firm we work with have a 6-foot flexible CT great for measuring current on columns.
Karl:
With the advent of flexible CT"S readings are possible around most objects. One firm we work with have a 6-foot flexible CT great for measuring current on columns.
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