Conductors in parallel

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tpeterson

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I have a contractor installing 8-parallel 500 MCM runs to a 3000 Amp switch gear. The run is only about 40 feet long but the way they put their conduits in there are some ungrounded conductor that are 4' shorter than others of the same phase. My question is in section 310.4(B)(1) it says they shall be the same length, what does that mean? I have worked as an electrician for years and have had foreman that laid the conductors out on the ground and cut them to the exact length needed and i have had foreman that just pulled them in and terminated them so they looked good. what is the acceptable industury tolerance? In this case referenced the difference in ungrounded conductors on the same phase is about 10%.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
tpeterson said:
My question is in section 310.4(B)(1) it says they shall be the same length, what does that mean?
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About the same thing as "nearest the point of entrance" for services.

It means "the same length," with no allowance or tolerance specified.


How close to the conductors' rated ampacities will they realistically be loaded?
 
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charlietuna

Senior Member
There's a reason for the requirement in 310.4 and it is very important. And i really didn't believe it makes much difference until we built an emergency 4000 amp vault stub. If you want the amperage to balance--they better be very close to the same legnth! Especially when you get up close to the amperage rating of the conductor!!! It made a believer out of me!
 
don_resqcapt19

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
If everything other than the length is exactly the same and the lengths are 36, 36, 37, 37, 38, 38, 40 and 40, and the load is 3000 amps, there would be 392.64 amps on the 36' ones, 382.02 on the 37' ones, 371.97 on the 38' ones and 353.37 on the 40' ones.
If the real load is 80% of the 3000 amp rating the highest loaded cable would be at 314.11 amps.
 
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charlietuna

Senior Member
From an experience we had, i believe this current shift between conductors is GREATLY effected by overcurrent conditions. We had a 2000 amp bus duct blow up and to get the building back "on line" we cored a hole thru the slab and installed jumpers(open air) from the main switchgear and bypassed the damaged bus section. We had to guess at the expected load and went with three sets of 500 mcm cu. all cut at the same legnths. Early in the morning the total load was around 700 amps and the currents were fairly close between each group. Unexpectantly, the afternoon total current was up around 1300 amps and one group was hogging the load up to 550 amps, while the other two were around 380 amps. Then as the total load subsided in the evening the currents seemed to be evenly shared.
 
drbond24

drbond24

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Location
Barboursville, West Virginia
LarryFine said:
"Who are you, and what did you do with the body?"
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harrison_ford_fugitive.jpg

It wasn't me! It was the one-armed man!​
 
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