working out the math?

T

teeg123

Member
Location
Maine
Occupation
Journeyman Electrician
here is the question
You need a branch-circuit conductor for a 40 A non-continuous load in EMT with 8 current-carrying conductors. Conductors are THHN copper (90 °C). Ambient temperature is 40 °C. What is the minimum conductor size required after applying adjustment and correction factors?
I highlighted in blue what I feel is the important information to answer this question.
A-4 AWG
B-6 AWG (correct answer)
C-8AWG
D-10AWG (my answer)

I work through it like this correct me when I go wrong.
1st 40 Amps of non-continuous = 40 Amps
2nd I do temp correction 40Amps*1.15(which is in the 90 degrees for less than 50 degrees) =46Amps
3rd I do the bundle adjustment 46Amps*70%=32.2Amps which in the 90-degree column in table 310.16 shows me #10 AWG after I round up.
so, where am I going wrong and as always thanks in advance for the assistance.
 
Here is why #6 AWG is correct (how to check an answer, not how to determine the answer from scratch):

The 90C #6 Table 310.16 base ampacity is 75A, while for #8 it is 55A. Ambient temperature of 40C for tables based on 30C (such as Table 310.16) gives a correction factor of 0.91 for 90C rated conductors, per Table 310.15(B)(2)(a) (2017 NEC numbering). 8 CCCs gives you an adjustment factor of 0.7.

So the ampacity under these conditions of use for #6 would 75A * 0.91 * 0.7 = 48A, while for #8 it would be 55A * 0.91 * 0.7 = 35A. 35A is less than the 40A non-continuous load, while 48A is more than the load. So #6 is OK, while #8 is not. That makes #6 the smallest size allowed.

Now, you can determine the answer from scratch by reversing the above computation. 40A is the goal, but we are going to reduce the table value by 0.91 * 0.7. That means we need to divide 40A by (0.91 * 0.7) to get the minimum table value. 40A / (0.91 * 0.7) = 62A. So we look at Table 310.16 and find the smallest size that has entry of at least 62A in the 90C column. Which is #6.

Cheers, Wayne
 
@wwhitney walked you through the correct approach to the calculation, I'd like to point out a couple of things that might help develop intuition for what the correct answer will be in the future.

1) Derating for ambient conductors in the raceway always _reduces_ what the conductor can carry. When you run a calculation with derating, as a double check ask yourself "Did this just reduce the size of the conductor?" If you derate for current carrying conductors and somehow come up with a smaller allowed conductor than you started with, then you know you made a mistake.

2) When you adjust for higher than typical temperature, you also increase the size of the conductor. If you run the calculation and the conductors get smaller, either you are dealing with some sort of cold environment or you've made a mistake.

Good luck in your education!

-Jonathan
 
winnie said:
1) Derating for ambient conductors in the raceway always _reduces_ what the conductor can carry. When you run a calculation with derating, as a double check ask yourself "Did this just reduce the size of the conductor?" If you derate for current carrying conductors and somehow come up with a smaller allowed conductor than you started with, then you know you made a mistake.
Click to expand...
The rare exception would be a high ambient temperature less than 30C. I only recommend taking credit for that, if it's a temperature nature determines (e.g. project in Alaska), rather than a temperature maintained by refrigeration or A/C.
 
Thanks very much to everyone for all the help. before this is over, I'm sure I'll have a few more questions.
thanks again to all
 
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