Calculating Wire Sizes

[AlWired]

In attempt to make sure I fully understand how to size wires per NEC requirements, I have written a procedure to go by. I would greatly appreciate input from some of the experts out there. To simplify things, I have assumed none of the execptions to the NEC articles apply. The procedure is shown below.

Thanks in advance.

1. Calculate the minimum conductor size, before the application of any
adjustment or correction factors, to be the sum of 100% of the non-
continuous load plus 125% of the continuous load. See 215.2(A)(1).
2. From the results of step (1) above, choose the conductor size from
Table 310.16 based on the terminal temperature rating in Table 310.16.
Use 60 degree column for conductor ampacities of 100A and less.
Use 75 degree column for conductor ampacities of greater than 100A.
See 110.14(C)(1)(a) & (b).
3. Choose the OCPD based on the results of step (1) above. If the value
from step (1) does not correspond with one of the standard OCPD sizes,
choose the next largest size. See 215.3 & 240.6(A).
4. Make adjustments to ampacity based on conductor insulation rating
found in Table 310.16.
5. Check adjusted ampacity found in step (4) above to OCPD rating found in
step (3) above. The OCPD must be no greater than the next highest
rating above the adjusted ampacity found in step (4) above. If the
OCPD is greater than the next highest rating above the adjusted
ampacity found in step (4) above, the wire size must increased by one size.
6. Repeat step (4) above and check for compatibility with OCPD as done
in step (5) above.
7. Repeat steps (4) and (5) if necessary.

 

[infinity]

2. From the results of step (1) above, choose the conductor size from
Table 310.16 based on the terminal temperature rating in Table 310.16.
Use 60 degree column for conductor ampacities of 100A and less.
Use 75 degree column for conductor ampacities of greater than 100A.
See 110.14(C)(1)(a) & (b).

Conductors with ampacities less than 100 amps can be sized at 75 degrees if all of the terminations have 75 degree ratings so a default to the 60 degree column isn't always correct.

3. Choose the OCPD based on the results of step (1) above. If the value
from step (1) does not correspond with one of the standard OCPD sizes,
choose the next largest size. See 215.3 & 240.6(A).

Going up to the next larger size has limitations, multi-receptacle circuits and larger than 800 amps to mention a few. You might want to include them in your chart.

 

[Dennis Alwon]

2. From the results of step (1) above, choose the conductor size from
Table 310.16 based on the terminal temperature rating in Table 310.16.
Use 60 degree column for conductor ampacities of 100A and less.
Use 75 degree column for conductor ampacities of greater than 100A.
See 110.14(C)(1)(a) & (b).

This is a section of the code that gets misunderstood and I believe it might be worth trying to reword it. If you read 110.14(C) carefully you will see that it says what Trevor stated above. As long as the wire and terminations--lugs, breakers etc, are listed for 75C then you can use 75C. If they were listed for 90 C you could use 90C column. You want find too many situations where 90C will come into play except for derating.

Art. 110.14(C)(1) (a) & (b) shall be used if you cannot determine the temp rating of the terminations, etc.