3 wire feeder to separate building?

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Also what are the exact dangers if it's a 3 wire system ( 2 hots and a ground) if the separate building has the grounds and neutrals bonded together in the sub panel and also has ground rods at sub panel?
 
frofro19 said:
Also what are the exact dangers if it's a 3 wire system ( 2 hots and a ground) if the separate building has the grounds and neutrals bonded together in the sub panel and also has ground rods at sub panel?
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There is no real danger if there is not a metallic parallel path between the two structures.
 
I know this is a old post but I really never understood that. I know that if a #2 AL is feeding a 100 amp sub panel that supplies the entire load of a dwelling it's acceptable because of the diversity. And if you feed a 100 amp sub panel from a main 100 amp panel then the feeders doesn't need to be any larger than the service conductors. But you still could possibly have 99 amps on the #2 AL feeding the sub panel.
winnie said:
The electrode in the ground does 'ground' the subpanel. However the soil is not an effective fault current path.

If this subpanel had ground and neutral bonded (which _is_ allowed in some very limited situations) then the neutral would provide the fault current path.

An EGC must be run with the circuit it protects, so at a minimum the triplex needs to be dug up to install the required and missing EGC. At which point I personally would go with the conduit install.

#2 Al is NEC compliant for a 100A residential service, but not a 100A feeder. Lots of people will miss/pass this one and allow #2 for a 100A feeder in a residential situation. The one situation where this is compliant if it is a 100A feeder coming off of a 100A service, since the feeder conductors do not need to be larger than the service conductors.

-Jon
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What would be some limited situations and wouldn't it be easier to just bond the grounds and neutrals together rather than digging up the entire triplex especially if it's a rather long distance and if it would meet some special circumstance?
 
1) A residential service calculated (using article 220) at less than 100A, built with a 100A main breaker _might_ use 100A of current for a short time. It might even exceed 100A for a very short time.

But the amount of time spent at high currents is so short that the NEC permits such a service to be built with 83A conductors. Residences often have very high transient demands but low average demand.

Now imagine a feeder from this hypothetical service...the load on this feeder must be less than or equal to the total service load, so the feeder conductors don't need to be larger than the service conductors.

2) All services are done with the grounded conductor also functioning as the bonding conductor. This causes problems whenever there are metallic conductive paths between different buildings supplied by the same utility neutral. Basically current can flow into one building by its service drop, through loads, thence via things like underground piping to the other building and then back to the transformer.

If an installation is under the NEC, using a neutral as a bonding conductor is only permitted for detached structures with absolutely no shared grounded metal, no metal piping, no communication grounds, etc. The NEC is intended to prevent current flow on these 'parallel paths'.

Jon
 
frofro19 said:
I know this is a old post but I really never understood that. I know that if a #2 AL is feeding a 100 amp sub panel that supplies the entire load of a dwelling it's acceptable because of the diversity. And if you feed a 100 amp sub panel from a main 100 amp panel then the feeders doesn't need to be any larger than the service conductors. But you still could possibly have 99 amps on the #2 AL feeding the sub panel.
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Nothing will keep the load under 100 amps other than what is connected or not connected to the panel. Load calculations have quite a bit of leeway built into them so historically the 83% conductor size is adequate.

frofro19 said:
What would be some limited situations and wouldn't it be easier to just bond the grounds and neutrals together rather than digging up the entire triplex especially if it's a rather long distance and if it would meet some special circumstance?
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There is no special circumstance in the NEC. You might convince the AHJ that you have one.
 
Also, understand what an EGC does. We know what happens with a line-to-neutral fault occurs: the OCPD opens.

The purpose of an EGC is to mimic a line-to-neutral fault when a line-to-exposed-conductive-surfaces fault occurs.

Otherwise, those energized exposed conductive surfaces become touch hazards. A ground rod will not trip an OCPD.
 
winnie said:
1) A residential service calculated (using article 220) at less than 100A, built with a 100A main breaker _might_ use 100A of current for a short time. It might even exceed 100A for a very short time.

But the amount of time spent at high currents is so short that the NEC permits such a service to be built with 83A conductors. Residences often have very high transient demands but low average demand.

Now imagine a feeder from this hypothetical service...the load on this feeder must be less than or equal to the total service load, so the feeder conductors don't need to be larger than the service conductors.

2) All services are done with the grounded conductor also functioning as the bonding conductor. This causes problems whenever there are metallic conductive paths between different buildings supplied by the same utility neutral. Basically current can flow into one building by its service drop, through loads, thence via things like underground piping to the other building and then back to the transformer.

If an installation is under the NEC, using a neutral as a bonding conductor is only permitted for detached structures with absolutely no shared grounded metal, no metal piping, no communication grounds, etc. The NEC is intended to prevent current flow on these 'parallel paths'.

Jon
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The project I worked on over a year ago was a wedding venue barn with a 400 amp service with a 200 amp panel directly behind the meter base. The owner wanted to install a 200 amp disconnect for a future barn/shed. I installed the disconnect and went back months later and the owner, which works for a poco, had trenched and installed a 3 conductor underground wire. I didn't terminate the wire into the disconnect for several reasons. First and foremost, there is no future building on the other end of the wire yet. It's probably over 100 yards away from the disconnect. And secondly, he buried the wrong wire. I refused to terminate the wire as it was not sufficient being only 3 conductors. Haven't heard anything back yet and I assume there's no future barn yet. If there's no other metal conductors in any way to connect the buildings 100 yards away, could the 3 wire system be connected in the disconnect safely without any issues?
 
frofro19 said:
The project I worked on over a year ago was a wedding venue barn with a 400 amp service with a 200 amp panel directly behind the meter base. The owner wanted to install a 200 amp disconnect for a future barn/shed. I installed the disconnect and went back months later and the owner, which works for a poco, had trenched and installed a 3 conductor underground wire. I didn't terminate the wire into the disconnect for several reasons. First and foremost, there is no future building on the other end of the wire yet. It's probably over 100 yards away from the disconnect. And secondly, he buried the wrong wire. I refused to terminate the wire as it was not sufficient being only 3 conductors. Haven't heard anything back yet and I assume there's no future barn yet. If there's no other metal conductors in any way to connect the buildings 100 yards away, could the 3 wire system be connected in the disconnect safely without any issues?
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You did the correct thing, and I was mistaken.

A '3 wire feeder' to a detached structure was permitted until the 2008 code. Lots of people (including me) thought the exception was still there.

It is not totally unsafe (utility services under the NESC rather than the NEC still use this type of connection) but it isn't code compliant.

-Jon
 
LarryFine said:
By insisting that a permit and inspection be included, or hire someone else.

When he said his work would pass an inspection, I would say, "You'll get paid when it does."
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If he's in Monroe County they require inspections. But boy are they lacking in content. That's typical in Indiana.
Don't know which county he's in. But some don't have inspectors, no inspections, none, except the health dept. inspects the sewer tank.
I recently wired two pools in counties that have no inspections. I told the owners, lucky they found me, Because it's going to be done right.
Don't forget Indiana does not have a state license for electrical.
 
frofro19 said:
Safely as a feeder or service lateral?
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As service conductors, 3-wire is both safe and compliant. As feeder conductors, 3-wire is most likely safe in most conditions.

Neutral-current voltage drop (rise) can cause voltages between the electrical ground and nearby earth and earthed surfaces.

The concern with other metallic pathways is they virtually assure voltage differences by providing a solid ground reference.
 
LarryFine said:
As service conductors, 3-wire is both safe and compliant. As feeder conductors, 3-wire is most likely safe in most conditions.

Neutral-current voltage drop (rise) can cause voltages between the electrical ground and nearby earth and earthed surfaces.

The concern with other metallic pathways is they virtually assure voltage differences by providing a solid ground reference.
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This here. Even services can have neutral to ground voltage. You can drive several ground rods and it won't necessarily fix anything. The problem is the MGN system used by the POCO. If neutral is carrying current there will be a voltage drop on it. With MGN you can still have rise in volts that has nothing to do with your supply system but is rather caused by primary side neutral current but is imposed on all conductors (primary and secondary) that are bonded together to make the MGN.
 
LarryFine said:
As service conductors, 3-wire is both safe and compliant. As feeder conductors, 3-wire is most likely safe in most conditions.

Neutral-current voltage drop (rise) can cause voltages between the electrical ground and nearby earth and earthed surfaces.

The concern with other metallic pathways is they virtually assure voltage differences by providing a solid ground reference.
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The biggest danger at the second building is if the feeder neutral becomes open. Every thing connected to the electrical grounding system at the second building will become energized to earth. Yes the same thing happens if a service neutral would become open, but the NEC does not have any control on the line side of the service point.
 
don_resqcapt19 said:
The biggest danger at the second building is if the feeder neutral becomes open. Every thing connected to the electrical grounding system at the second building will become energized to earth. Yes the same thing happens if a service neutral would become open, but the NEC does not have any control on the line side of the service point.
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So in most situations would ok?
 
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