Need answers

[Profwhit]

Help, I am new to the forum. I may not be in the right place. I am reading the "Illustrated Guide to Exam Preparation" book and have run across several items that I need some clarification on. for example, in the Feeder Conductor Sizing Example, on page 194 at the top right side of the page. In this example, you have the "Corrected Ampacity = 255A x 0.9d1 x 0.80. The answer is given to be 211A. When I worked the math the answer comes out to be 185.6A. When I used the value 290A for the ampacity of the wire, and then used the correct correction factors, I came out with the 211 amps that is given in the "correct ampacity" line. Should the value of 255 in the example be 290 instead?

 

[charlie b]

Most of us do not have a copy of that book handy. You will have to post the entire question, in order for me to be able to give you any assistance.

 

[infinity]

If the conductor is 250 kcmil copper with a 90° C insulation (such as THHN) then you can use the 90° C ampacity (290 amps) for the calculation. This was a mistake in the book. Check out the corrections here:

https://www.mikeholt.com/documents/catalog/14_EP/14_EP_Correx_Directory.pdf

 

[Carultch]

Help, I am new to the forum. I may not be in the right place. I am reading the "Illustrated Guide to Exam Preparation" book and have run across several items that I need some clarification on. for example, in the Feeder Conductor Sizing Example, on page 194 at the top right side of the page. In this example, you have the "Corrected Ampacity = 255A x 0.9d1 x 0.80. The answer is given to be 211A. When I worked the math the answer comes out to be 185.6A. When I used the value 290A for the ampacity of the wire, and then used the correct correction factors, I came out with the 211 amps that is given in the "correct ampacity" line. Should the value of 255 in the example be 290 instead?

Check out the last part of 110.14(C):
"Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both".

This means that even if your terminations are limited to 75C, you still can use the 90C column ampacities for your conditions of use "derate" calculations. In any case, prior to any conditions of use "derate" calculations, you still have to meet the sizing requirements for terminations and equipment temperature ratings.

Usually terminations are sized to the 75C column, and conductor ampacity starts from the 90C column. Occasionally you may have to use the 60C column for 100A and less. And it is very rare that you can use the 90C column for terminations.