hde_electric
Member
If a main panel Neutral is bonded to Ground why is it dangerous if you don't have the neutral floating in the sub-panel?
Bonding Jumper in Sub Panels
- Thread starter hde_electric
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Guest
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Re: Bonding Jumper in Sub Panels
Here's one thread to chomp on. After you read it post back with any follow-up questions. There are other threads too.
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=7;t=000458;p=1
Here's one thread to chomp on. After you read it post back with any follow-up questions. There are other threads too.
http://www.mikeholt.com/codeforum/ultimatebb.php?ubb=get_topic;f=7;t=000458;p=1
- Location
- Massachusetts
Re: Bonding Jumper in Sub Panels
Imagine that this "sub-panel" is fed with a EMT and that the neutral bar is electrically connected to the enclosure.
Now once you start loading the Neutral bar with current part of that current will flow through the neutral conductor, just like we want.
But part of the current will also flow through the EMT back to the bond at the main panel.
Not only is this not what the EMT and fittings are designed for if you where to put one hand on each side of say a loose coupling you would become a conductor.
There are other ways to get parallel paths for neutral currents if you do not keep the grounding and grounded conductors separate on the load side of the service disconnect.
As far as the NEC is concerned there is no sub-panels, only load side equipment.
Imagine that this "sub-panel" is fed with a EMT and that the neutral bar is electrically connected to the enclosure.
Now once you start loading the Neutral bar with current part of that current will flow through the neutral conductor, just like we want.
But part of the current will also flow through the EMT back to the bond at the main panel.
Not only is this not what the EMT and fittings are designed for if you where to put one hand on each side of say a loose coupling you would become a conductor.
There are other ways to get parallel paths for neutral currents if you do not keep the grounding and grounded conductors separate on the load side of the service disconnect.
As far as the NEC is concerned there is no sub-panels, only load side equipment.
Re: Bonding Jumper in Sub Panels
I believe you want to know why you don't bond the neutral and ground at a sub panel? The purpose of the equipment ground is to ground the equipment cases, it is not to carry normal current. If the neutral and ground are bonded together, they both carry current. On a 480V board with GFI, this can cause false trips.
I believe you want to know why you don't bond the neutral and ground at a sub panel? The purpose of the equipment ground is to ground the equipment cases, it is not to carry normal current. If the neutral and ground are bonded together, they both carry current. On a 480V board with GFI, this can cause false trips.
Ed MacLaren
Senior Member
Re: Bonding Jumper in Sub Panels
The sketch below may help illustrate the problem that the others have referred to.
The grounded (neutral) conductor carries the unbalanced load current in any multi-wire system.
An illegal bonding jumper permits the neutral to be in parallel with the equipment grounding conductor, which is not sized for continuous current.
The equipment grounding conductor would be continuously carrying a percentage of the unbalanced load current. It is often bare, and could be an exposed conduit or cable armor.
Ed
[ October 13, 2003, 06:07 PM: Message edited by: Ed MacLaren ]
The sketch below may help illustrate the problem that the others have referred to.
The grounded (neutral) conductor carries the unbalanced load current in any multi-wire system.
An illegal bonding jumper permits the neutral to be in parallel with the equipment grounding conductor, which is not sized for continuous current.
The equipment grounding conductor would be continuously carrying a percentage of the unbalanced load current. It is often bare, and could be an exposed conduit or cable armor.
Ed
[ October 13, 2003, 06:07 PM: Message edited by: Ed MacLaren ]
- Location
- Massachusetts
Re: Bonding Jumper in Sub Panels
Thanks Ed, as always your drawings make things much clearer.
Thanks Ed, as always your drawings make things much clearer.
Re: Bonding Jumper in Sub Panels
I didn't notice any other reference to " HIGH " neutral to ground voltage.
The current could be high , but the voltage should not be excessively high unless the neutral is also open.
If there is current, there is usually a voltage drop involved.
I didn't notice any other reference to " HIGH " neutral to ground voltage.
The current could be high , but the voltage should not be excessively high unless the neutral is also open.
If there is current, there is usually a voltage drop involved.
karl riley
Senior Member
Re: Bonding Jumper in Sub Panels
As far as danger goes, the main danger (fire) usually cited is "arcing in concealed places". In other words,neutral current on all kinds of metallic paths which were not designed to carry the current safely can result in a loose (sometimes just barely touching) metal-to-metal contact somewhere which can arc and cause a fire.
Karl
As far as danger goes, the main danger (fire) usually cited is "arcing in concealed places". In other words,neutral current on all kinds of metallic paths which were not designed to carry the current safely can result in a loose (sometimes just barely touching) metal-to-metal contact somewhere which can arc and cause a fire.
Karl
karl riley
Senior Member
Re: Bonding Jumper in Sub Panels
And I should add (how could I not?) that this parallel path for the neutral results in "net currents" in the feed as well as on the metal paths. These produce magnetic fields which can cause computer monitors to jitter and various electronic instruments to malfunction. (Electron microscopes are particularly susceptible).
So the electrician who bonds the neutral in a subpanel in a commercial building with computers and stuff may be the cause of a lot of problems for the companies which occupy the building, including having to vacate certain areas (muy expensive).
Karl
And I should add (how could I not?) that this parallel path for the neutral results in "net currents" in the feed as well as on the metal paths. These produce magnetic fields which can cause computer monitors to jitter and various electronic instruments to malfunction. (Electron microscopes are particularly susceptible).
So the electrician who bonds the neutral in a subpanel in a commercial building with computers and stuff may be the cause of a lot of problems for the companies which occupy the building, including having to vacate certain areas (muy expensive).
Karl
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