3-phase power question

  • #1
Hey guys, I'm an Electrical Engineering major @ McNeese State University in Lake Charles, Louisiana. I only have 60 hours toward my B.S., so forgive me if my questions are too stupid.

I'm currently working on my internship at the Port of Lake Charles. The Port has recently fired its old board of directors because of shady legal dealings and poor management, and we're having to pick up the pieces they left behind, including abysmal record keeping and few blueprints to speak of, so I'll give you as much information as I can come up with. I think I've figured out most of the basic stuff already, but enough background. . .

I'm working on a bus bar replacement project. We have right now 2 runs of 3-phase 1500 amp bus bar (3 current carrying bars and 1 ground) that draw no more than 400 amps @ 480 V under full load. The bars are 700 ft. long and fed at one end by 3 #750 MCM in 4" rigid metal conduit with 1 ground wire (not sure of ground wire's size, can find out if necessary).

We're looking at replacing the existing bus bar with either more 1500 amp bar with the single feed point on the end, or possibly going with 700 amp bar and using 2 feed points to compensate for lower ampacity.

We would need to run another 700 ft. of conduit from the substation to the local disconnect box to the second feed point. My question is two-fold:

1) To run 3 more power feeds, do they have to be rated for 400 A/480 V each all the way out to 700 ft., or since the total load is 400 A, would each wire be rated for 133 A instead (400/3 wires = 133.33)?

2) If we have one feed point at one end, and another feed point at the other end, and we need 400 amps of net current at each end of bar, does each feed point need to be pumping in 400 amps, or does each feed point need to be 200 amps?

I hope I didn't mangle anything too bad, thanks in advance.
 

Answers and Replies

  • #2
anorlunda
Staff Emeritus
Insights Author
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You haven't explained enough about the loads to answer your question. Since you're using bars rather than cables, it might be that you tap off load at intermediate points.

p.s. Lake Charles is a great place for Cajun food.
 

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