Uneven current ratings.

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rhamblin

Senior Member
During a recent Infrared thermography scan of our plant, we came across some unusual readings from one of our main Switchgear units.

At our plant, we have a 24,900 volt phase to phase/14,400 volt phase to ground transformer dropping voltage down to 480V 3-phase. After this transformer, 11 conduits (parallel runs of 500 MCM) run into a Surge device cabinet.

IMG_1146.jpg IMG_1148.jpg IMG_1147.jpg

From this Surge protective cabinet, another 11 parallel runs of 500 MCM feed to the 480 volt switchgear.

After finding the hot spot we decided to put a amp clamp on the hot wire, it was around 315 amps. So then we decided to measure a few of the other wires on the same phase. We couldn't check all of them because of the bundling of the wires. Here are some of the measurements that we encountered on these A phase parallel wires:

  • 165 amps
  • 137 amps
  • 69 amps
  • 105 amps
  • 138 amps
  • 111 amps

Now for my question, if all the wires are in parallel, shouldn't they have pretty close to identical resistances and therefore identical currents?

I don't think it would matter, but apparently the Surge protection devices in this panel, have been "burned out" since before I started 6 years ago. I'm looking for any information here. In my mind we have a lot of loose connections or bad crimps that is causing more current to be dispersed on some of the conductors.
 

Ingenieur

Senior Member
Location
Earth
how long is the run?
the only one Inwould check is the 69

there are variables
purity of the Cu
how the ends were prepared, cleaned, compression force, etc
tightness of bolted connections

should be the same in a perfect world
+/- 15% around the avg would not concern me
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
...
...
Now for my question, if all the wires are in parallel, shouldn't they have pretty close to identical resistances and therefore identical currents?
As long as the wires are really routed to result in a very close path length, you would expect the resistances to be substantially equal.
However, especially if the wires are short (under, say, 20 feet), the biggest part of the resistance will be the contact resistance at the terminals at both ends.
Since the overall resistance is so low even a small difference in contact resistance can lead to a large current imbalance.

Note also that current is not dispersed along the way. Instead the voltage drop increases along the way, leading to reduced current to allow the total voltage drops to be equal across all the parallel wires. :)
 

rhamblin

Senior Member
how long is the run?
the only one Inwould check is the 69

there are variables
purity of the Cu
how the ends were prepared, cleaned, compression force, etc
tightness of bolted connections

should be the same in a perfect world
+/- 15% around the avg would not concern me
The conduit length is 3 feet, so conductors are maybe 10-15’.
 

kwired

Electron manager
Location
NE Nebraska
The shorter the overall length, the bigger a minor difference is in relation to the whole picture.

6 inch difference in length of one conductor in a ten foot run is going to have much larger impact on resistance/current division through the parallel set then a six inch difference in an individual conductor of a 100 foot run.
 
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