Wire Derating

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dabatman

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I have a question on conductor sizing. I am using table 400.5(B) and a G-GC Type cable. My load is 104.2A and is all non-continuous. I also have 4 current carrying conductors with a 4wire+gnd cable.

1) Do I only have to derate for the extra current-carrying conductor for 90degC using the .8 factor?

2) Or do I still have to use have to derate from the 75degC for the extra current-carrying conductor?

I am trying to get down to use a #2 cable. 0.8*152=121.6A from 90degC column and 0.8*133=106.4A. Is this restricted by table 310.16 for 115A?

Just trying to get my numbers.
 
Engineers don't do homework. We leave that to engineering students! :)

I don't know how to handle this situation. I never deal with flexible cords myself. It seems that the rules in table 400.5(B) do not account for the possibility of a 4 conductor (plus EGC) flexible cable. I would also like to ask what the application is, and specifically why you are counting all four conductors as current-carrying.
 
Looking at the text of 400.5 (it?s always a good idea to look at the text, before trying to interpret the table), I see that it describes the exact situation you are discussing. It tells you to start with the 3-conductor rating, and multiply by the derating factor in table 400.5. But I also see how your question comes into play. Table 400.5(B) gives three possible temperature ratings for a cable, and doesn?t tell you which temperature rating applies to which cable type. In Table 310.16, you can see which cable types are rated 60C, which are rated 75C, and which are rated 90C. But I do not know the temperature rating of the G-GC cable you are discussing.

So let me throw another question back at you. Can you buy type G-GC cable with any of the three temperature ratings listed in the table? If you can buy (or from your perspective, if you can specify) the G-GC with 90C rating, then you can use the 90C column as the basis for derating.
 
charlie b said:
Engineers don't do homework. We leave that to engineering students! :)

I don't know how to handle this situation. I never deal with flexible cords myself. It seems that the rules in table 400.5(B) do not account for the possibility of a 4 conductor (plus EGC) flexible cable. I would also like to ask what the application is, and specifically why you are counting all four conductors as current-carrying.
Click to expand...



I apologize, I never checked his profile and jumped to the wrong conclusion
 
dabatman said:
I have a question on conductor sizing. ...

... Is this restricted by table 310.16 for 115A?

Just trying to get my numbers.
Click to expand...
As long as your terminal temperature ratings at either end are 75?C, the minimum size conductor is limited to that which the Table 310.16 75?C column value for the conductor size used is greater then the minimum circuit ampacity. You are correct in that the minimum size of conductor would be #2 for a 104A non-continuous load.

For temperature correction and derating you use the appropriate value from Table 400.5(B) to start, then temperature correct per Table 310.16 and derate per Table 400.5. The resulting ampacity value for the cable conductor size, type, and usage also must be greater then the minimum circuit ampacity.

Not knowing all the necessary installation criteria makes it a bit hard to nail down from here... but it appears you would be okay using #2 75?C-rated cable as long as the ambient temperature conditions don't go above 30?C. Otherwise, you will have to go with the 90?C-rated version of the cable or increase the conductor size...
 
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