I ran into a situation while working at my church and I ran across a sub-panel wired with a 3 wire (#2 seu cable) and all the neutrals and grounds were connected together and bonded to the cabinet. I know a 4 wire should have been ran but was this an acceptable installation prior to 2008? I'm guessing it was done in the late 80's or 90's.
Sub-panel bonding
- Thread starter frofro19
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
This was never code compliant for a sub-panel within the same structure.
Maybe I'm thinking of the detached structure. What would make the difference on being detached or not?
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
- Location
- Roanoke, VA.
- Occupation
- Sorta retired........
I believe as long as there wasn't a metallic connection (water pipe) between the two a 3 wire feeder was allowed. I could be wrong.
- Occupation
- Licensed Electrician
If there are no metallic paths between the structures then there is no alternate path for objectionable current to flow.
That ended in 2005.
That ended in 2005.
But that was only for a separate structure, correct? My question is why was it allowed back then for a detached building but wasn't allowed for a sub-panel within a building?
- Occupation
- Licensed Electrician
Because you don't want neutral current flowing on metal paths inside the building thats not part of the wiring. We like to keep the electrons that are making the lights and the toaster oven on the wires that have black and white insulation and off of the metal for beams and water pipes, etc
There would be possible voltage differential between subpanel EGCs and main panel EGCs due to voltage drop on the subpanel feeder neutral.
Imagine touching 2 grounded appliances each fed from a different panel, and receiving a slight shock.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
It has never been allowed within the same structure. Prior to 2008 NEC it was treated like a service for detached structures. Someone could add a metallic path in the future to an existing structure creating an issue.
This is where I'm getting confused. This particular sub-panel is 5 feet in front of a very old 3 phase 400 amp disconnect in a church basement. There's a lot of issues in the disconnect (cu and al wires under the same lugs ) and multiple wires under lugs where they have added on during the years.
In the sub-panel, isn't the current trying to get back to the source (transformer) and if the grounds and neutrals are connected together, wouldn't they travel on the neutral over to the disconnect and not travel back the opposed way to whatever the sub-panel feeder wires are feeding? At the disconnect they are connected and travel back to the source, why doesn't the same happen at the sub-panel and the current travel back on the neutral conductor and not spread out in all directions that are opposite of the source? Am I just overthinking everything? Could a EGC be added to the seu cable and then separate the grounds and neutrals in the sub-panel?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
All the metal parts would have the potential to carry some neutral current if the neutral is bonded to the enclosure so at the panel the neutrals and EGC's are isolated from each other. No you cannot add an EGC to SEU cable.
All the metal parts would have the potential to carry some neutral current if the neutral is bonded to the enclosure.
Isn't this what happens at the disconnect? Does the EGC have to be in the cable itself?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Not sure what the first question means. Yes.
Does all metal parts at the disconnect have the potential to carry some neutral current where the grounds and neutrals are connected.
qcroanoke
Sometimes I don't know if I'm the boxer or the bag
- Location
- Roanoke, VA.
- Occupation
- Sorta retired........
By code Yes. in the cable or In the raceway.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Yes.
In the US we run a bonding conductor (called the Equipment Grounding Conductor, EGC) that is separate from the grounded conductor (neutral) after the service disconnect(s). i.e. they are bonded together in the service disconnect(s) and thereafter kept separate on the load side. It has been required to run the separate EGC within the same structure since at least the 1950s, I believe.
The exception for detached structures before 2005 allowed the circuit to the detached structure to be treated the same as a service, i.e. bring only hots and neutral to the building, then ground the neutral and bond the EGCs at the first disconnect. That is no longer allowed except for existing situations that were installed compliant before 2005.
Why is it not a issue (neutrals and grounds connected together) at the service disconnect but and issue at the sub-panel? (Potential to carry neutral current)
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
- Occupation
- Electrician
Never been allowed but is seen all the time. Imagine a building wired with BX cable and the BX is in contact with a water pipe (example a electric water heater wired with BX)
The WH is wired from the sub panel with BX. The neutral between the sub panel and the main panel get compromised by corrosion or a bad connection. Now you have all the neutral current flowing on the water heater BX and water pipes carrying neutral current and it starts a fire.
The WH is wired from the sub panel with BX. The neutral between the sub panel and the main panel get compromised by corrosion or a bad connection. Now you have all the neutral current flowing on the water heater BX and water pipes carrying neutral current and it starts a fire.
As long as the N-G bond is only in one place, there are no parallel paths to carry objectionable current.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
It's really just the convention in this country and much of the world, but essentially under the NEC jurisdiction they are always kept separate in the circuit wiring, while on the utility network one conductor (the grounded conductor) serves both purposes. The service is where they are bonded together because that's where the two networks meet so that's where the EGC splits off from the grounded conductor under the NEC. (They are also bonded together at the source of a separately derived system.)
The intention under the NEC is that no current flows on EGCs and other exposed metal within buildings, under normal conditions. The EGC has to be connected to the grounded conductor at one point in order to serve its purpose, but it should only be connected at one point to avoid the EGC becoming a parallel path for current under normal operation.
The system we use is described on this Wikipedia page on earthing systems, it is the TN-C-S system. The protective devices and rules we use (breakers, GFCIs, EGC sizes, etc.) are designed around this system. In countries with different earthing systems they need different kinds of protective devices.