Figure B.3 INFO ANNEX B

[Isaiah]

If I have a Rho of 60 and LF of 100, how would I determine the ampacity of 500kCMIL in 6 underground duct banks using the interpolation chart figure B.3?
Is the I2/I1 ratio still considered .74?


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[ron]

I've not successfully used that chart before, but you can get a bit of a gut check by using Table B.2(7). They don't have rho=60 LF=100, but you can look at rho=90 & LF=100 which would be a bit mor conservative.

 

[Isaiah]

I've not successfully used that chart before, but you can get a bit of a gut check by using Table B.2(7). They don't have rho=60 LF=100, but you can look at rho=90 & LF=100 which would be a bit mor conservative.

Yes I don’t like the chart either.


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[Pinnie]

What is the context of this conversation?

 

[don_resqcapt19]

What is the context of this conversation?

The actual ampacity of buried conductors depends a lot on the thermal properties of the earth or other materials that cover the ducts. The standard ampacity tables will result in conductor failure from excess heat if they are actually loaded any where near their ampacity as found in Table 310.16. There was an attempt to make the annex rules code many cycles ago, but is was rejected based on the comment that there is no issue when the loads are calculated in accordance with Article 220. However with many large project the loads are not calculated per 220, and the loads are real. Using a real load and Table 310.16 in an underground duct bank will often result in failure.
Most designers of these projects use electrical engineering software to determine the ampacities of conductors in underground duct banks.

 

[Isaiah]

The actual ampacity of buried conductors depends a lot on the thermal properties of the earth or other materials that cover the ducts. The standard ampacity tables will result in conductor failure from excess heat if they are actually loaded any where near their ampacity as found in Table 310.16. There was an attempt to make the annex rules code many cycles ago, but is was rejected based on the comment that there is no issue when the loads are calculated in accordance with Article 220. However with many large project the loads are not calculated per 220, and the loads are real. Using a real load and Table 310.16 in an underground duct bank will often result in failure.
Most designers of these projects use electrical engineering software to determine the ampacities of conductors in underground duct banks.

Don thanks for this great info. In the absence of ETAP or SKM software do you think the RHO factors etc found in the ANNEX and NEC interpolation chart will suffice?


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