De-rating

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Can someone please explain this question to me and how they arrived at there answer? (I know the answer but, not sure how the system calculated it) thanks.

What is the ampacity of a 300 kcmil XHHW conductor installed in a raceway outside on a rooftop on 3” blocks with a total of 6 additional current carrying conductors and a ambient temp of 92F.
 
horsegoer said:
assume entire run is on roof and none within building?
also you say additional. so a total of 9 ccc?
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This question is written exactly how it’s asked on a 2020 code course. We can assume entire run is outside and why is it asking for ampacity of “a” “conductor”. So what 7 conductors total or 9?
 
92°F with 7-9 current carrying would give you a multiplier of .672 of the 90° ampacity (215 Cu. , `174 AL)

(Incorrect see Post #11)
 
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What kind of raceway and if it is exposed to sun shine?
 
augie47 said:
92°F with 7-9 current carrying would give you a multiplier of .672 of the 90° ampacity (215 Cu. , `174 AL)
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What Augie said, x 0.8 for that portion of continuous loads, x 0.8 for that portion of motor loads.
 
191.52 A You need to use the 75° C column to start off with because XHHW conductors are installed in a wet location. 70% of that ampacity for 7-9 current-carrying conductors and 96% for the ambient temp. of 92° F.
300 MCM @ 75° C = 285 A
285 x .7 = 199.5 A
199.5 A x .96 = 191.52 A

I'm not seeing a rooftop adder because the bottom of the conduit is higher than 7/8" from the roof.
 
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My post was incorrect as Rick noted. I am so used to XHHW-2 that I used the 90° column. As he noted. XHHW (no -2) is a 75° conductor in a wet location such as a rooftop changing the multiplier to .658. 300 KMCIL Cu =188 AL= 151.
 
Rick 0920 said:
I would like to see how that worked out. I wonder if they were using the temp. adder for rooftops in the 2014 NEC?
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I would to. It doesn’t show you how they came up with that answer and the reason why I posted on the forum because all my answers didn’t match any of there’s.
 
ramsy said:
What Augie said, x 0.8 for that portion of continuous loads, x 0.8 for that portion of motor loads.
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In re continuous loads, that 0.8 factor is not applied concurrently with the ampacity correction and adjustment factors.

If you think of the checks as being separately at the terminations and along the run, then the 125% continuous factor only applies to the terminations check. The adjusted and corrected ampacity is already a continuous rating.

Cheers, Wayne
 
wwhitney said:
In re continuous loads, that 0.8 factor is not applied concurrently with the ampacity correction and adjustment factors.

If you think of the checks as being separately at the terminations and along the run, then the 125% continuous factor only applies to the terminations check. The adjusted and corrected ampacity is already a continuous rating.

Cheers, Wayne
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Is it just an assumption that there are continuous loads and motor loads. I was just basing the calculation on what I read.
 
Let's say it is about an open top ladder cable tray.
According to 392.80(A)(1)(c) you have 2 possibilities:
if the cable tray width is 12" that means the clearance between cables is not one cable diameter [minimum 14.44"] then ampacity of 1 cable of 300 mm XHHW will be 65% of Table 310.15(B)(17)[500 if 90oC terminals or 445 if it is only 75oC].
2) if cable tray width it is 18" then the ampacity may be 100% of Table 310.15(B)(17)
 
wwhitney said:
In re continuous loads, that 0.8 factor is not applied concurrently with the ampacity correction and adjustment factors.

If you think of the checks as being separately at the terminations and along the run, then the 125% continuous factor only applies to the terminations check. The adjusted and corrected ampacity is already a continuous rating.

Cheers, Wayne
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Thanks for bringing this up.

I agree these test questions along with installers typically omit adjustments for motor load branch circuits per 220.18, continuous loads per 210.19(A)(1), fuses per 210.20(A), & feeders per 215.2(A).

Each of these code sections repeatedly instruct us that "conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors."

Yet real-world motors and lighting loads requiring conductors sized 125% of load, above and beyond 310.15 de-rating adjustments, are missing this additional 125%, caught only by thermal probes that find over-heating at the terminations.

Test questions and installers will never get this right, until NFPA-70 brings all conductor-adjustment requirements together under section 310.15.
 
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