Ampacity for phase conductors in separate U/G ducts?

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JMeyer2020

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I want to verify the ampacity of an installation that uses 750kcmil 1/C CU 15kV cables running through an underground ductbank. There are two feeds in parallel (a total of six cables). Each 1/C cable is installed in its own separate conduit in the ductbank (a total of six ducts).

Unfortunately, I cannot find a rating for a single 1/C cable in an underground duct anywhere!

The cable's datasheet only gives ampacities for triplex installations. I tried going to the NEC Article 315, but I run into the same problem - Tables 315.60(C)(11) through (14) all describe three current carrying conductors together in one duct/conduit (either three 1/C cables, or a single 3/C cable).

I feel pretty confident that with the conductors being isolated they could safely carry more than 565A at 90C... but I don't know the calculations to prove it. Can anyone help?
 
You really need electrical engineering software for underground duct banks that are carrying significant load. There are a lot of details, including the thermal resistivity of the fill material around the ducts. Often the resulting ampacity is much less than you would expect.
 
Julius Right said:
See IEEE 835/1994 page 355View attachment 2566648
Click to expand...
So the 75 LF and 100 LF refer to overall circuit length? The circuits I'm looking at are a little over 100 LF, so I guess my approximate ampacity is 578 A.

don_resqcapt19 was right - that's not much more than the 565 A given for the triplex installations! At least I can say I looked.

I had not seen IEEE 835 before. I'm glad to know this resource is out there. Thank you very much :)
 
LF=load factor 75% or 100%.The ampacity does not depend on the length. Only voltage drop depends on the length.
 
Julius Right said:
LF=load factor 75% or 100%.The ampacity does not depend on the length. Only voltage drop depends on the length.
Click to expand...
Load factor makes way more sense - I read LF as linear feet! Too many acronyms rolling around in my head.

This is a huge help! Thank you again.
 
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