parallel feeders

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big john

Senior Member
Location
Portland, ME
The power company here terminated thier end of a 4000 amp service not long ago and cut off more than one of their 2 hole compression lugs and reterminated before they were satisfied with the resistance readings they were getting.



JAP>
How are they doing those tests?

In my experience large termination resistance would be basically nonexistent: Often in the teens to single microohms.
 

jap

Senior Member
Occupation
Electrician
The readings they were taking were between the conductor itself and the crimp.
Not sure of the meter they were using or what they considered insufficient, but they redid a few that were hydraulically crimped that they were not pleased with the readings.

JAP>
 

jap

Senior Member
Occupation
Electrician
This particular outdoor gear setup had a 4000 amp Switch on one end and a buss terminal box on the other.(Outdoor)
From there the 4000 amp Feeder buss went inside the plant to a switchgear on a mezzanine.

Gutter between the Outdoor Buss Tap Box and the Switch, Cabled in between with multiple runs of 500mcm copper.

JAP>
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If the conductors are short the contact resistance becomes more important in the balance calculation, with two consequences:
1. Unequal connection resistances will cause imbalance independent of wire length.
2. The higher the *equal* connection resistance, the less critical it is for the wire resistances to match exactly.

And, just for grins, a wire carrying more than its share of current will get hotter than the others, marginally increasing its resistance. At least no thermal runaway as can happen charging parallel batteries.
 

Carultch

Senior Member
Location
Massachusetts
And, just for grins, a wire carrying more than its share of current will get hotter than the others, marginally increasing its resistance. At least no thermal runaway as can happen charging parallel batteries.

True. There is negative feedback that will somewhat curtail the problem. Not enough to fully close the feedback loop for a steady state of balanced currents, but it exists nonetheless.
 

kenman215

Senior Member
Location
albany, ny
You can do the calculations just using the lengths. Since the conductors are of the same size and type, the resistance per foot is the same.
If you use 8 sets of 500 kcmil with longest length being 20' reducing them 6" per run, so that the shortest is 16.5 long and you have a 3000 amp load the three shortest runs will be operating above 380 amps.
The currents for each, assuming the 6" step change starting with the longest (20') would be:
340.8, 349.6, 358.8, 368.5, 378.7, 389.5, 401, and 413.1.

I don't think that there's any need for the calcs on this one, but thanks anyway. I'm going to do what I always do: figure out the longest run, cut all my cables the exact same length, eat up the slack with loops, and tighten them down with my torque wrench...
 

jap

Senior Member
Occupation
Electrician
I don't think that there's any need for the calcs on this one, but thanks anyway. I'm going to do what I always do: figure out the longest run, cut all my cables the exact same length, eat up the slack with loops, and tighten them down with my torque wrench...

Making loops tends to create resistance which defeats what your trying to accomplish...:)

Jap>
 

kenman215

Senior Member
Location
albany, ny
Making loops tends to create resistance which defeats what your trying to accomplish...:)

Jap>

This is true, but is also the lesser of two evils here. The design on TC is crap. Highest phase bussing termination point (A phase) is only 3 1/2' from bottom of an 8' tall cabinet. Typically, we pay the extra cash to get them raised higher in the cab, in which case you have enough height to get rid of some length. But with the lowest bar only 18" up, there's no conceivable way to lose 4' in that distance without the loops, I think.
 
...although the code requires it, paralleled conductors are rarely , if ever, the same length.

What do you mean? Are you saying that electricians usually dont cut each phase to the same length?

I have measured currents on some parallel conductors that I installed and cut to the exact same length. I find significant differences in current distribution. In fact just today I took some readings and got 10% difference on some of them. I dont know if its meter inaccuracy, termination resistance as has been mentioned, or varying impedance for varying reasons.
 

kenman215

Senior Member
Location
albany, ny
What do you mean? Are you saying that electricians usually dont cut each phase to the same length?

I have measured currents on some parallel conductors that I installed and cut to the exact same length. I find significant differences in current distribution. In fact just today I took some readings and got 10% difference on some of them. I dont know if its meter inaccuracy, termination resistance as has been mentioned, or varying impedance for varying reasons.

I think he was saying the same EXACT length, but that being said, I know of other foreman in my company who take pictures of their pretty TC's to show the apprentices "how it's done." But they're not doing what I do on longer service runs, which is to fan out and bring back in what would be the shorter conduit runs outside the building, to make them all closer to the same length.
 

jap

Senior Member
Occupation
Electrician
This is why they invented bus ducts


Sent from my iPhone using Tapatalk

Bus bars in bus duct installation arent exactly the same length either, unless your talking about a straight run,which is generally much more economical to install in wire form.


JAP>
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
But a bus duct isn't in parallel therefore different lengths isn't a issue.

Sent from my iPhone using Tapatalk
A lot of them are....we just removed 10 bus duct runs and replaced them with cable bus. All but the 3 smaller (1600 amp) runs were multi-section bus ducts with parallel runs.
 
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