Parallel Contuctors trimmed by Power Company

[milwaukeesteve]

I just though I would share this experience.
I had a church that needed a new service due to damage from a garbage truck. I came in with a new 800Amp service. Since it was for a damaged service, I just replaced the main switch and everything upstream to the weather head. However, the physical size of the new switch did not allow my parallel conduits to enter both on the top or both on the bottom. I had to take one conduit out the top and one out the bottom. They both met at the LB's going outside, and ran neatly up the side of the building to the Head.
When the condutors got installed, due to the placements of the conduits and bending radius inside the switch, I was left with several different lengthed conductors sticking out of the 2 weather heads. I kept the parallels the same length, but they looked different lengths from the outside.

The Power Company came and CUT all the conductors to the same length. He said it was standard practice to cut all the conductors to same length. I told him that that is fine on single conductors, but these were parallel. We had to figure out which end came from which, and the cut off the same amount on a cut conductor's parallel. The one conductor ended up being only 6 inches long.

It took me a day to run these parallel runs myself, I did not want to do it again, but what is worse, is that if I had not noticed the cut ends laying on the ground, I would have had parallel conductors that truly weren't parallel, because all the current would have been on the shortest of the 2.


Just a 'heads up' when it comes to an installation like this. Yes I would have liked to make it a nice pretty installation, with all of my conductors the same length, but what I had to work with didn't allow for that.

 

[charlie b]

. . . I would have had parallel conductors that truly weren't parallel, because all the current would have been on the shortest of the 2.

Not all. I suspect the ratio would have been no worse than 60%/40%, and 55%/45% would have been more likely. It depends on the difference in lengths of the two parallel runs, as a percentage of the overall length. Given that most buildings never use their service transformer to the limit of its ratings, this particular building might have operated safely for decades, and the error might never have made any difference.

Nevertheless, it is an interesting story, and it reminds us not to take anything for granted. Thanks for sharing it.

 

[infinity]

I'd guess that the moral of the story is that the POCO will always cut them the same length so it's up to the EC to make sure that his installation takes that into account. Thanks for passing along the story.

 

[George Stolz]

If a set of parallel conductors are very short, say 5' from start to stop, and one of the conductors is a foot shorter than the other, what would happen?

I can picture it over a distance, but a very short run I can't picture.

 

[don_resqcapt19]

George,
If you have 2 conductors and one is 4' and the other is 5', 44% of the total current will flow in the 5' cable and 56% in the 4' cable.
Don

 

[tallgirl]

If a set of parallel conductors are very short, say 5' from start to stop, and one of the conductors is a foot shorter than the other, what would happen?

I can picture it over a distance, but a very short run I can't picture.

It's worse the shorter the distance is for the same difference in conductor length.

So, if you had a pair of parallel conductors, 50' and 49', the difference in current for each conductor is less than the example you gave of 5' and 4'. As Don explained, in the 4' and 5' example, 5/9ths of the current (56%) flows on the 4' conductor and 4/9ths (44%) flows on the 5' conductor. But with longer lengths and the same difference in lengths, the different in current per conductor decreases. For 49' and 50', the values are 50/99ths (50.5%) for the 49' conductor and 49/99ths (49.5%) for the 50' conductor.

 

[George Stolz]

Thanks.

 

[milwaukeesteve]

Thank you guys (and girl) for your engineering help.

 

[winnie]

Steve,

You don't mention, but what sort of conduit did you use? If you have two different lengths of steel conduit, then even if the conductors are of the same length, the conduit would provide different magnetic interaction with the conductors, and thus the two runs would have different impedance.

I am not sure if this is significant for installations where a complete set of service conductors (all phases and neutral) are located in each conduit, since the net current flow should be balanced and zero, thus negating any magnetic effects. It might only make a difference during ground fault conditions, where one set of service conductors would carry far more current than the other.

-Jon