Supply side bonding jumpers in tap can/gutter - parallel neutral path?

[tx2step]

I've been looking at 2014 NEC Handbook "Exhibit 250.34". Can anyone with an electronic version post that graphic?

It looks like to me that arrangement creates a parallel neutral path through the nipples that connect between the service gutter and the service disconnect switches/panels?

Am I missing something here? It looks like it would cause a constant partial neutral current to flow through the conduits?

I have a situation similar to this, and am trying to figure out how to properly do the install. My installation isn't with nipples -- it's 2 parallel runs of 2" EMT around 15' in length.

Thanks!

 

[mwm1752]

technically the illistration show the bonding of the metallic nipple only -- the nipple would not be considered a viable path back to the gutter IMO but in real terms cerainly has potential for a parallell path but only between cabnets I believe there is another section in the NEC describing this.

 

[don_resqcapt19]

The code does not have a problem with parallel paths for neutral current on the line side of the service disconnect. In fact the code actually requires some parallel neutral current paths.

 

[tx2step]

The code does not have a problem with parallel paths for neutral current on the line side of the service disconnect. In fact the code actually requires some parallel neutral current paths.

Even if that parallel neutral path is through the EMT? It just doesn't seem correct to knowingly install it so the neutral current continually travels through the EMT or metal conduit like that? It would seem like a good source for arching, or even a voltage potential to ground?

 

[don_resqcapt19]

Even if that parallel neutral path is through the EMT? It just doesn't seem correct to knowingly install it so the neutral current continually travels through the EMT or metal conduit like that? It would seem like a good source for arching, or even a voltage potential to ground?

The only voltage to drive arcing would be the voltage drop on the grounded conductor. Of, course if there is a problem with the grounded conductor there could be a shock hazard on the conduit, but if there is a problem with service grounded conductor there could be a shock hazard on all conductive items that are connected to the electrical grounding system.

Look as some of the other parallel paths...the metal underground water system if it serves more than on building. The shield on the cable TV system. The shield on an underground phone line. All of these things are connected to the grounded conductor in each building that they service and are parallel paths for neutral current...and these connections are required by the code rules.

 

[tx2step]

The only voltage to drive arcing would be the voltage drop on the grounded conductor. Of, course if there is a problem with the grounded conductor there could be a shock hazard on the conduit, but if there is a problem with service grounded conductor there could be a shock hazard on all conductive items that are connected to the electrical grounding system.

Look as some of the other parallel paths...the metal underground water system if it serves more than on building. The shield on the cable TV system. The shield on an underground phone line. All of these things are connected to the grounded conductor in each building that they service and are parallel paths for neutral current...and these connections are required by the code rules.

Hmmmm...I guess that's true. The metal conduit (2 parallel 2" EMT in this case) will be a parallel neutral path, and will carry a proportional share of the neutral current depending on the relative impedance ratio between the neutral (grounded conductor, in this case 2 parallel #3/0 Cu) impedance and the impedance of the metal conduit...right?

 

[don_resqcapt19]

Hmmmm...I guess that's true. The metal conduit (2 parallel 2" EMT in this case) will be a parallel neutral path, and will carry a proportional share of the neutral current depending on the relative impedance ratio between the neutral (grounded conductor, in this case 2 parallel #3/0 Cu) impedance and the impedance of the metal conduit...right?

Yes, the conduit will be a current carrying conductor and the current will divide based on the impedances and the current on the conduit could be equal to or even greater than on the copper conductor.

Often a poor connection on the conduit is cited as a serious problem with heating and arcing because of the poor connection on the conduit. However the the only voltage to drive current across the poor connection is the voltage drop on the parallel path that consists of the actual neutral conductor and the conduit. Assuming that there is not a problem with the neutral conductor, this voltage drop will be very low and unable to drive much current through the poor connection on the conduit. If the current is low there is not an heating or arcing hazard....of course this all changes if there is a problem with the neutral conductor.

 

[tx2step]

Yes, the conduit will be a current carrying conductor and the current will divide based on the impedances and the current on the conduit could be equal to or even greater than on the copper conductor.

Often a poor connection on the conduit is cited as a serious problem with heating and arcing because of the poor connection on the conduit. However the the only voltage to drive current across the poor connection is the voltage drop on the parallel path that consists of the actual neutral conductor and the conduit. Assuming that there is not a problem with the neutral conductor, this voltage drop will be very low and unable to drive much current through the poor connection on the conduit. If the current is low there is not an heating or arcing hazard....of course this all changes if there is a problem with the neutral conductor.

That makes sense...I'm just still trying to wrap my head around the idea of intentionally making the conduit/EMT a parallel current path, since we try hard not to have that condition on the load side. Do you know what the code panel's reasoning is?

On a different question: Is there any requirement that the "service conduits (EMT in this case)" running between the service tap-can and the new panel that contains the new 400A CB "service disconnect" (the main CB for that panel) be bonded on a particular end? Can they (the 2 ea parallel 2" EMT) be bonded to the "grounded service conductor" (neutral) in the tap-can instead of in the new 400A MCB panel?

 

[don_resqcapt19]

That makes sense...I'm just still trying to wrap my head around the idea of intentionally making the conduit/EMT a parallel current path, since we try hard not to have that condition on the load side. Do you know what the code panel's reasoning is?

Because the electrons behave differently based on who owns them:happyno:.

I think it is just that there is no easy way to prevent that on the line side of the service disconnect where the grounded conductor is used both as the grounded current carrying conductor and the grounding conductor.

On a different question: Is there any requirement that the "service conduits (EMT in this case)" running between the service tap-can and the new panel that contains the new 400A CB "service disconnect" (the main CB for that panel) be bonded on a particular end? Can they (the 2 ea parallel 2" EMT) be bonded to the "grounded service conductor" (neutral) in the tap-can instead of in the new 400A MCB panel?

It can be bonded at either end per the NEC, but some utilities require bonding at both ends.