ECN Forum Forums Technical Discussion Electrical Theory and Applications Imbalanced Parrallel Conductors

Sorry if this is way off base, but I'm not too familiar with the layout of your 3-ph commercial panels.

Any chance that there's a large area of ferrous metal closer to one or more of the parallel feeders than the others? The inductance could cause an imbalance.

Why is it such a big deal to try and get exactly the same load on each conductor? I don't beleive there is any way to be that exact. Given two pieces of wire that are the same there will be slight differences. Those differences are in its terminations, bends, lengths, etc.. We install parrallel conductors all the time and yes there are always variations in the amount of load that each carries. But there has never been a concern about it.

glenn

PS , I don't think your meter is lying. Its just a fact of parrallel conductors.

One more thing. Sometimes in my business of being a lineman we must install mechanical type jumpers to perform liveline work so that we keep customers happy. It was brought up by someone in the past that when parralleled jumpers are used that they should be installed a certain way to equalize the load.

Hope this makes sense;

wrong way--------1-2----------2-1---------

right way--------1-2----------1-2---------

if done the wrong way then #1 jumper hogs all the load even though the jumpers are exactly the same length.


[This message has been edited by glenn35 (edited 09-25-2001).]

It's ok? happens all the time?

someone call the jaws of life an' help me get both feet outta' my mouth....

Not off base at all. This is what I was alluding to in my post above.

I believe a slight imbalance may be unavoidable, but depending on the magnitude, one of the conductors still may become overloaded.

Thanks for all the feedback guys!

I haven't been back to do any more checking. In fact that may not happen for some time.

I did so some comparitive measurements on a similar set-up. 1200A, 208v, 4 wire service. The main is three parrallel runs of 600mcm CU. None of the service laterals showed an imbalance greater than 5%.

A few things are different;
This is a church/school....the load is not as diverse.
The feeders are longer (100') so a few inches of difference in length would not make less of a percentage change in resistance.

GJ

GJ
The reason I suggested checking the down stream load is I once had a simular problem. A 4000 amp service had 14 runs of 500MCM to it. Kept burning cables. Not a good thing in an ethonal plant. We repulled the 500 MCM making sure all cables were the same length. When we turned the service back on amp readings were scary less than 200 on 2 or more cables over 600 on others with a range of readings in between. The engineers and the rest of us went nuts trying to find the problem.
While the plant was down because of this another crew did some repair of some computer controlled equipment and rebalanced the various loads at some sub panels.
After repulling the 14 runs of 500 MCM again,reterminating and checking every thing else back on the service came. Same problem different cables. Back to tests, discussion-sometime heated- meetings etc.
Other crew comletes their tasks using only enough power to test each individual subpanel.
Management want more tests full power run-- problem disappeared.!!!!!
What caused it is still an arguement something downstream probably causing a harmonic imbalance. What was never agreed to. When we left all 14 500 MCM / phase 3 phase 480 cables were within 4% of each other.
Hope you have better luck and actually find a cause.

Redsy,
your 9-24 post is interesting there...

Dare I ask...
Interesting in a good way or bad?

On Saturday, when we changed the breaker, there was no equipment operating.....only lighting and HVAC. The HID lighting could be causing harmonics.....in fact, thats the first place I would look. But I don't understand how harmonics can affect paralleled feeders.

Can anyone explain this????

GJ