Inductive reactance in Parallel feeder

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Nitty

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Installation - 4 parallel runs of 600 KcMil Copper in a short section (<4') of Aluminum Cable Tray between a 2MVA transformer and Switchgear. To accommodate existing transformer taps. The 16) 600 KcMil must enter the top of the transformer through a insulating panel, in an "isolated phase" installation. When parallel conductors are installed in a cable tray NEC 392.8(D) require them to be installed, securely bound in circuit groups in order to prevent current imbalance due to inductive reactance.

Problem; in order to install the 16) KcMil CU in an isolated phase fashion it is impractical to bundle the circuit conductors together on such a short piece of cable tray. The individual conductors have been laid out as single conductors and securly bound to the cable tray to prevent excessive movement during a fault; as follows AAAABBBBCCCCNNNN.

Question; How much of an issue is current imbalance on such a short piece of aluminum cable tray?
 
petersonra said:
what is an "isolated phase" installation?
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Instead of grouping each set of A,B,C,N,G together, you group all the A's with each other, all the B's with each other, etc. It is allowed under certain limited conditions. However, since this install has a single tray, and not four separate conduits, this is not really an isolated phase installation.
 
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Nitty said:
Question; How much of an issue is current imbalance on such a short piece of aluminum cable tray?
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It has been a long time since I have had to do that type of calculation. It is a logarithmic equation. The closer the unsymmetrically spaced conductors are to each other, the lower the inductive reactance, and the less the imbalance. My gut feeling is that the impact on current imbalance will be negligible.

That said, however, I don't think the code allows the installation you described. If a four foot section of tray does not give you the space needed to install in a ABCNABCNABCNABCN pattern, then you need to create a longer tray. Do you have space above the transformer and switchgear that can be used to extend the length of the tray, thus giving you the room you need to rearrange the conductors?
 
Isolated phase installation is when all the A phase conductors are installed in one conduit, B phase in another and so on. In the majority of cases this is a no-no, but there are exceptions. An example of an exception would be a MCC fed from an underground parallel feeder, typically MCC don't give you much room for terminations, so there is an allowance to bring each phase conductor into the MCC in it's own nonmetallic conduit so you don't have a rat's nest in you MCC termination cabinet.

By isolated phase installation I am referring to how the circuit conductors enter the enclosure. All A phase enter the same opening, All B phase the same opening and so on. I believe 300-20(B) allows this with certain conditions. And I know the code only refers to isloated phase installs an instance with non-metallic underground installations. However all the other two, existing taps are exiting the transformer the same way (separate tray installation). The transformer secondary connections are located relatively close to the top of the transformer, which is the reason for the isolated phase installation into the top of the transformer
The length of the tray is not really a problem it is the fact that the two pieces of equipment are close enough together that if you installed it in a bundled ABCN in four bundles you would end up with a big mess as they entered the transformer enclosure. (3/0 would be a different story but we are dealing with 600 KcMil)
392.8(D) address two potential issues 1) Current imbalance due to inductive reactance 2) Excessive movement from fault current

My main question is if current imbalance is really an issue on such a short run of cable?
 
Phil,
The installation is already complete. I did consider buss but because there are already two other taps exiting the top of the transformer (installed the exact same way) this would have made the installation much more complicated. Knowing that the installation wasn't to the letter of the code I felt that adequately securing the cables and using aluminum instead of steel for the cable tray to help limit induced currents that it would meet the intent of the code especially in such a short run.
Thanks,
 
Nitty said:
Phil,
The installation is already complete. I did consider buss but because there are already two other taps exiting the top of the transformer (installed the exact same way) this would have made the installation much more complicated. Knowing that the installation wasn't to the letter of the code I felt that adequately securing the cables and using aluminum instead of steel for the cable tray to help limit induced currents that it would meet the intent of the code especially in such a short run.
Thanks,
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JMNSHO - there is no such thing as the spirit of the code that you can chose to abide by in lieu of what the code actually says.
 
What kind of covers are there on the top & bottom of the tray? Solid?

Since you have a short section (approx. 10'), I'm wondering if it would qualify as an article 366 "auxilary gutter"?

The basic limit on auxilary gutters is they can't extend more than 30' beyond the equipment they supply.
 
Nitty said:
... the installation wasn't to the letter of the code ...

... I felt that adequately securing the cables and using aluminum instead of steel for the cable tray to help limit induced currents that it would meet the intent of the code ...
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nitty -
I think you did fine. The nec does not cover all issues, especially for industrial. There are somethings that are just not a cookie cutter design. We are not talking about wiring Grandpa's cottage - rather wiring Grandma's M1A3 factory.

If there is an AHJ issue, one should discuss this before the fact. I've rarely (maybe never?) been told, "No". As long as it is readily apparent you are not trying to cheap it, or, "slip one by" (hate that phrase), why would they? Who would want a code job instead of one that is right.

If there is no AHJ issue, document the design decisions, put it in the equipment folder, and move on knowing you have a good job. That way, later on, you don't have the appareance hiding anything. You might get told you were wrong, but no one can say you were unsafe or sneaky.

JMO

ice
 
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