Table 310.11- derating

[sberger3]

I have not found a post related to this question:

I have already upsized the conductors for voltage drop to #8. Do I need to derate the conductors also because I will have 4 CCC in the coduit? I so, how do I derate then if they are already upsized? Thank you in advance.

 

[infinity]

Did you mean 310.15(B)(2) not 310.11?

 

[charlie b]

What matters is the ampacity that is required, based on a calculation of the intended load, and the ampacity that is available, based on the size of the conductor, its insulation system's rating (i.e., 60C, 75C, or 90C), and any applied derating factors. For example, if your #8 happens to be type THHN (90C insulation rating), then you start with a 90C ampacity of 55 amps. Then you apply the 80% derating factor (due to 4 CCCs), and that drops the ampacity to 44 amps. That is less than the 75C rating of the same wire, so you conclude that your ampacity is 44. Now you ask yourself if that is adequate for your load. If, for example, your load was 30 amps, so a #10 might have been sufficient, but then you upsized to #8 because of VD concerns, then you now have a 44 amp wire serving a 30 amp load, so you are fine. Please note that you also have to upsize the EGC, but that was not part of your question.

 

[rcwilson]

#8 awg, 75C (THWN) = 50 amps. Derate x 80% for more than 4 conductors = 40 Amps.

Since the conductors were upsized for voltage drop, they must have been either #10 or #12 to start, with an ampacity of 30 or 20 Amps. Even with derating there is no problem.

If you are using 90C rated conductors (THHN, RHW-2), you can start the derating at 55 Amps from the 90C column. 55 x 80% = 44 amps.

 

[sberger3]

Table B.310.11

Table B.310.11

NEC 70 2008, Table B.310.11- Adjustment Factors for More than Three current carrying conductors in a raceway.
Thank you,

 

[infinity]

NEC 70 2008, Table B.310.11- Adjustment Factors for More than Three current carrying conductors in a raceway.
Thank you,

I can't find that table in my code book.

I'm assuming that you're asking if you need to "double derate" since you're up-sizing for voltage drop and CCC's in a raceway? The answer is no.

 

[charlie b]

I can't find that table in my code book.

It's in the back, page 70-702 in the soft bound edition. And it is probably not the table that is needed for the application under discussion. You don't want to go into "load diversity," unless you have a PE to sign and seal a calculation that demonstrates the value you choose for load diversity. Table 310.15(B)(2)(a) is generally what we use.

 

[sberger3]

If, for example, your load was 30 amps, so a #10 might have been sufficient, but then you upsized to #8 because of VD concerns, then you now have a 44 amp wire serving a 30 amp load, so you are fine. Please note that you also have to upsize the EGC, but that was not part of your question.[/SIZE][/FONT][/QUOTE]

Thanks for the example, the real world application clarified very well. My load is only (3) amps, but the distance is 950 feet. So if I am understanding everyone, the derating would not come into play until my load approached 32amps (60deg C @ 40amps X 80%=32amps).
(ps- my EGC is upsized also).
Thank you,

 

[infinity]

The two issues you've mentioned are voltage drop and derating. Derating is required by the NEC, voltage drop considerations are a design issue (with a few exceptions) and not under the purview of the NEC. So from a code perspective you only need to be concerned with the issue of derating. If using larger conductors to satisfy the derating requirements also takes care of the VD issues then you're good.


Do you have several 3 amp loads? Just wondering how did you end up with 4 CCC's?

 

[sberger3]

Do you have several 3 amp loads? Just wondering how did you end up with 4 CCC's?[/QUOTE]


yes, I have (2) circuits: one (3)amp load and another (1) amp load all at 480v. There is where the (4) CCC comes into play.
Thank you,

 

[infinity]

It's in the back, page 70-702 in the soft bound edition. And it is probably not the table that is needed for the application under discussion. You don't want to go into "load diversity," unless you have a PE to sign and seal a calculation that demonstrates the value you choose for load diversity. Table 310.15(B)(2)(a) is generally what we use.

No wonder I didn't see that. Only you professional load diversity types would even know to look there.

 

[LarryFine]

Do you have several 3 amp loads? Just wondering how did you end up with 4 CCC's?

yes, I have (2) circuits: one (3)amp load and another (1) amp load all at 480v. There is where the (4) CCC comes into play.

Why not combine the two loads into a single 2- or 3-wire load? Even if you opt for a small sub-panel at the load end, it will cost less than a fourth conductor, and will eliminate the CCC issue.