Direct Buried 15 KV Cable, 2 - 3/c Cables in parallel, what separation?

[Jeff Allen]

We have a situation where we have installed 2 runs of 3/c 500 MCM cable, direct buried, in parallel with each other. The cables were installed with approx 24" of separation except for 200 ft out of a 4000 ft run.

The AHJ approved the installation. The conversation is now centered around what derating, if any, should be applied for the 200 ft stretch of cables that are touching each other. There is no discussion of this in the NEC.

 

[Dennis Alwon]

Article 310.60(B)(1) should be helpful-- NEC 2008

 

[charlie b]

I never found that section to be helpful at all, Dennis. It does allow for the possibility of having a higher ampacity for part of the run. But that means nothing, given that all the load flows through the same conductor sizes (i.e., the run does not transition to other conductor sizes outside the 200 foot section), so the lower limit will govern the run anyway.

My question for Jeff would be, what process, table, or calculation did you use, to get an ampacity for the rest of the run (i.e., the remaining 3800 feet)?

Welcome to the forum.

 

[rbalex]

Whether you are using 90C or 105C conductors is a significant factor.

See the definition of ampacity in Article 100.

I see three potential “conditions of use:”

1. Where the conductors are separated

2. Where the conductors are contiguous

3. At the terminations
​

Each of them must be determined individually and then 310.60(B)(1) applies. Assuming the conductors are copper:

Item 1 is basically an application of Table 310.83, “Two Circuits.” (Figure 310.60, Detail 6)

Item 3 is basically an application of Table 310.69, 90C conductor. Most MV connections are both amp and 90C thermal limited, so check their rated ampacity as well [110.14(C)(2)] The Table is pretty benign for your application since it uses a very high base ambient.

Item 2 is just a plain bear - but will very likely be the actual 310.60(B)(1) limiting factor. There are no immediate Table or directly applicable adjustment factors that apply. SO - if at all possible, have a qualified person run a Neher-McGrath calculation. Otherwise, the next best available approach would be applying sound engineering judgment and using the 600V adjustments in 310.15(B)(2) against Table 310.83, “One Circuit” (Figure310.60, Detail 5) recognizing that this will result in a fairly conservative value.

If you are using 105C Conductors they can be used as the "starting point" for either of the Table 310.83 applications.

And my welcome to the forums too