Correct Use of Ampacity Table -- Not 310.16

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kaltbier

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I have encountered an outdoor 1500-kVA step-down transformer, 13.8-kV delta to 0.48-kV wye. They are running the secondary conductors (535-kcmil DLO) in tray to an indoor 2000-A circuit breaker, which is part of an MCC assembly.

First, the wye-point of the secondary (neutral) is being run to a high-impedance resistor to keep the fault current to less than 5 amperes. The conductor is a 1/0 AWG DLO single conductor. The equipment bonding conductor is also 1/0 AWG DLO single conductor. Each phase is a square of 4 bundled 535-kcmil conductors spaced equally in a 6-inch fiberglass tray, 18 inches wide. The neutral is placed between the A-B phases and the EBC is placed between the B-C phases.

Q1. Aren't the phases supposed to be triplexed together (A-B-C) or squared together (A-B-C-N), and then paralleled to gain the necessary ampacity. They have it mounted and spaced as in a power-line configuration.

Q2. If the mounting is okay, how does one determine the spacing and placement of conductors?

Q3. One of the Engineers used Table 310.20 (single conductors in free air), 90-degree C column to justify the ampacity of these conductors. Doesn't 110.14(C)(1) limit the sizing to the 60 C and 75 C columns? I have only used Table 310.16 and I am not familiar with the useage of these other tables.

Q4. Are there heating issues involved where these conductors enter steel enclosures? All of these conductors pass through the indoor MCC steel enclosure wall via steel bushings and CG glands, and pass through the transformer enclosure through steel bushings (Myers hubs) and TMCX glands. They are arranged in an orderly manner, but they are as close together as the bushing will allow.

Thx
 
Quite a mouthful.............

Lets start at the top and see if this can't be widdled down a little bit at a time.

First, why is DLO cable being used? Could you expand on the environment in which this installation will exists.
 
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Why DLO?

Why DLO?

The one who thought of DLO has long believed that he can size cable according to the 90 C column of the ampacity tables. He thinks locomotive wire is the answer to most of his ampacity "oops's", whether they are enclosed in multiconductor cables, or in this case, single conductors. I have twice acquainted him with 110.14(C), but it may take several incidents to cure him.

Environment: I am on a production platform a few miles off the coast of Cabinda, Angola, Africa.
 
Can't speak for the OP, or the installers.....but I/we often use DLO cable for its flexibility, or its 2KV rating. FWIW

Having all leads of one phase bundled together & going through separate bushings (per phase) is not a good thing.....you will experiance magnetic flux and heat problems. At least one wire of each phase should go through each bushing or entrance (zero sequence)
 
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After further review-

After further review-

Facts:

1. 1500KVA Xfmr @ 480V has maximum ampacity of 1800A.
2. 2000A Main Breaker; assume it has and adjustable trip device settable to 1800A.
3. (4) // sets of 535 KCMIL DLO cable, with ampacity of 730A based on installation of single conductor, in free air, 30 deg C ambient, and 90 deg C conductor temp, per NEC 310.17.
4. (4) // #1/0 AWG used as neutral conductor (assumed (4) // )
5. (4) // #1/0 AWG used for earthing conductor through high Z resistor.
6. Installed in cable tray (assumed to be open top ladder type) w/ each phase of conductor 4 x A, 4 x B, and 4 x C each grouped together, spaced 6 inches apart in an 18 inch wide tray.

Answer:

Q1 - Refer to NEC 392, more specifically, 392.8(D) and (E).

Q2 - The installation described is in non-compliance with the NEC. Refer to 392.10 (A) (2) for installation of the conductors.

Q3 - Refer to NEC 392.11 (B). This is where you have a couple of choices for installation. Depending on your poison, will depend on what ampacity you can use. Remember that 90 de C can be used for derating, but the full load current at the terminations can't exceed 75 deg C unless the breaker and transformer terminals are so marked.

Q4 - You will have issues where the conductors go into the enclosure, with the method that is currently being used, e.g. same phases bundled together. If the conductors are bundled correctly as required to meet NEC 392, you will not have any issues where they go through the enclosure or cable gland.
 
Kaltbier,

250.122, the EGC to be sized for the main c.b. for each run.

1600a MCB needs a 4/0 for each group

2000a MCB needs a 250kcmil for each group
 
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