ECN Forum Forums General Topics of the Trade General Discussion Area Too much grounding?

I see where you're going with this now.

If we have 100A already flowing on the neutral at a sub-panel, then a neutral-ground short occurs at the receptacle on the short cable, then you have a parallel path again.

Some of the existing 100A neutral current will be diverted down the neutral of the cable and back along the grounding conductor, then via the ground connection from sub-panel to main panel to get back to the main neutral.

It's not going to be quite 50A in this case, because the resistance in the parallel path will be a little higher than before due to the length of cable to the receptacle.

It might be interesting to work out just how much of the current would take that path for different lengths, but it could certainly cause problems.

It will be much less then 50% in this case. The main path for the grounded conductor current is the feeder neutral to the panel. The secondary path would be via the feeder equipment grounding conductor, branch circuit grounded conductor and the branch circuit equipment grounding conductor. While the resistance for this path may be small it will be much larger then the main path. It is the relative relationship between the resistance's of the paths that determine the current flow. The secondary path could easily have 10 to 20 times more resistance then the main path. A #12 has 10 times the resistance of a #4 and 20 times the resistance of a 1/0. If the path has 10 times the resistance, then only 1/10 of the total current will flow in that path.
Don

Here goes:

50 meters of 3 x 25 sq. mm. copper cable.
100 Amps.

The resistivity of copper is 0.017 ohm mm/m
Hence, the resistance in each conductor is: 0.017 x 50 / 25 = 0.034 ohm

Just to check that the voltage drop is below the recommended 4% (= 9,2V at 230V)

2 x resistance x current = 2 x 0.034 x 100 = 6.8V ==> OK! Within spec.

At load side of the above cable:

3 meters of 3 x 2.5 sq. mm. copper to receptable.

Path length if neutral-ground short occurs at receptable: 2 x 3 = 6 meters
Resistance of this path: 0.017 x 6 / 2.5 = 0.0408 Ohm

The resulting resistances are:
Neutral conductor directly: 0.034
Ground condutor via receptable: 0.034 + 0.0408 = 0.0748 Ohm

Relative resistance: 0.0748/0.034 = 2.2

This means that the neutral conductor will carry 2.2 times as much current as the ground conductor. If we do the math it works out to a current of 100/(2.2 + 1) = 31A in the ground conductor and the cable to the receptable.

This is on top of the current the cable would be normally be carrying. If the cable is already carrying 16A, the currents in the different conductors would be:

Hot 16A
Ground 39A
Neutral 39A

{Corrections to above values made.}

Are my calculation above correct? Please check!


[This message has been edited by C-H (edited 10-06-2002).]

C-H,
What about the feeder equipment grounding conductor back to the power source? If this is at a panel where there is a main bonding jumper, there is no current flow on the branch circuit grounding conductor with a grounded to grounding short at the receptacle, other then the division of the grounded conductor current for the receptacle load itself. If there is a feeder equipment grounding conductor, then you have to add in that resistance. If we assume that the feeder equipment grounding conductor is the same size as the circuit conducotrs, which is not normal, then the relative realation ship is 5.4. This number will be even higher when the equipment grounding conductor is smaller than the circuit conductors.
Don

Don

I agree with this, but haven't you forgotten to take the length of the conductors into account? If the branch circuit has the same length as the feeder, there is probably no problem.


I take "main bonding jumper" to mean a connection between ground and neutral? If so, I fully agree with the above.


You've got a point in that the grounding wire is normally smaller than the other conductors for large cables. However, in the case of 3-phase systems the neutral conductor is often downsized too. Therefore I'm not sure the downsizing of the grounding wire will have a significant impact.

What puzzles me is where you got the figure 5.4 from?

C-H,
The first path is from the service main bonding jumper to the panel via the feeder grounded conductor to the neutral bus in the sub-panel. The second path is from the neutral bus in the sub-panel to the receptale via the branch circuit grounded conductor, to the branch circuit equipment grounding conductor, to the panel grounding bus and back to the main bonding jumper via the feeder equipment grounding bus. This feeder equipment grouning conductor is the additional 0.034 that added into th fault path.
Don