Derating of cable in duct banks

[mshields]

I've got a 2500A 480V feeder from an outside customer owned pad mounted transformer. The feeder will be via a duct bank for about 20 feet and then in open air the rest of the 80 feet or so to the switchboard.

Do I need to derate the cable as a result of the length that run encased in a concrete duct bank?

Thanks,

Mike

 

[chris kennedy]

I've got a 2500A 480V feeder from an outside customer owned pad mounted transformer. The feeder will be via a duct bank for about 20 feet and then in open air the rest of the 80 feet or so to the switchboard.

Do I need to derate the cable as a result of the length that run encased in a concrete duct bank?

Thanks,

Mike

Your install falls under 300.5, if it was a 300.50 install then it would be a different story.

 

[mshields]

Can you expand on that a bit

Can you expand on that a bit

Chris,

I'm afraid I don't quite follow. 300.50 is for over 600V so I understand that this does not apply, but I don't see anything in 300.5 that answers my question about derating?

Much appreciated

Mike

 

[charlie b]

I think you answer lies in the Article 100 definition of "ampacity," where is speaks of the conditions of use. The conditions of an underground run are different than the above ground run, and you have to use the smaller of the two ampacities. So it's not a matter of derating per se, but rather one of determining which portion of the run has the smaller ampacity. My money is on the underground run.

But there is a twist that may or may assist. How did you come up with 2500 amps? Is that the result of a load calculation, or is it simply the rating of the switchboard that happened to be purchased already? The Article 220 load calculation process has a great deal of inherent conservatism. Some have said, and the State Inspectors where I live are among them, that you can use Table 310.16 values for ampacity for underground duct banks, provided only that the value of "required ampacity" was the result of a load calculation.

 

[chris kennedy]

Do I need to derate the cable as a result of the length that run encased in a concrete duct bank?

Short answer is no, assuming you are using 6 sets of 600's. Here is an install similar to yours without the concrete. 2500A 480y/277 with 6 sets of 600's in 4" PVC.

But there is a twist that may or may assist. How did you come up with 2500 amps? Is that the result of a load calculation, or is it simply the rating of the switchboard that happened to be purchased already? The Article 220 load calculation process has a great deal of inherent conservatism. Some have said, and the State Inspectors where I live are among them, that you can use Table 310.16 values for ampacity for underground duct banks, provided only that the value of "required ampacity" was the result of a load calculation.

I'm not following the above. How else would one arrive at 2500A?

 

[SEO]

Off the subject but the electrician in the picture sure is involved in a lot of interesting projects.:smile:

 

[chris kennedy]

Off the subject but the electrician in the picture sure is involved in a lot of interesting projects.:smile:

We have had the good fortune of having very interesting work the last few years. Certainly not but boring.:smile:

 

[charlie b]

I'm not following the above. How else would one arrive at 2500A?

Rennovation of an existing building, with existing gear, new transformer in different location than old, and existing loads determined via 30 day measurement, just for one possibility.

 

[chris kennedy]

Certainly not but boring.:smile:

Mad English skills.:-?

Rennovation of an existing building, with existing gear, new transformer in different location than old, and existing loads determined via 30 day measurement, just for one possibility.

Of coarse, thank you.

 

[mull982]

I've got a 2500A 480V feeder from an outside customer owned pad mounted transformer. The feeder will be via a duct bank for about 20 feet and then in open air the rest of the 80 feet or so to the switchboard.

Do I need to derate the cable as a result of the length that run encased in a concrete duct bank?

Thanks,

Mike

I belive that the underground de-rating only applies if a certain percentage of the total run is underground.

 

[bsh]

The cables in this feeder generate heat, all cables that carry current generate heat . That heat needs to be dissipated. The cable routed above ground can have the heat removed by the air passing by the cable/conduit. The heat produced by the cable in the ground must rely on the soil in the ground to dissipate this heat. Soil acts like an insulation blanket and does not allow this heat to dissipte easily. As a result the each cable in an underground duct can carry less current.
The NEC gives some cable ampacity tables in Appendix B. Appendix B is only included for information. These are based on 3, 6, or 9 electrical ducts. If you look at these you can see that a similar cable listed in table 316 will not carry the same amount of current when put in the ground. IEEE Standard 390 (brown book) chapter 13 goes into a deeper discussion of this. Each conductor you add reduces the amount of current the other conductors can carry.