Derating and OCPD Size

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dnem

Senior Member
Location
Ohio
infinity said:
What does the 75 v. 60 degree rating of the CB have to do with this?

smithacetech said:
Are you trying to say you couldnt use the 75 deg chart for derating of the conductor ampacity if the lugs were rated at 60 deg?? You are not derating the lug ampacity just the conductors.

110.14(C) Temperature Limitations. . The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. . Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

You guys are right. . I should have listed 3 separate items. . Let me edit my previous post.

He's mixing 3 things that shouldn't be mixed
1] . 240.4(D)
2] . lug/breaker temp column of T310.16, 240.4(B), 240.6(A)
3] . conductor temp column of T310.16, T310.15(B)(2)(a), 240.4(B), 240.6(A)

That's 3 separate things that give you 3 separate numbers and your OCPD can't be higher than the lowest number of any of the 3.

better ?
David
 
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nyerinfl

Senior Member
Location
Broward Co.
Regardless of the rating of the lugs, you can determine the ampacity through the type of conductor, otherwise there would never be a use of THHN as no equipment is rated 90 degrees. The whole point of 90 degree conductors is for derating, so if you're derating a THHN you derate from 90, not 75. The 75 degree would be the equipments ampacity rating. Assuming THHN, 40A at 45% would give you 18A, w/ a 20A OCPD. Another thing, those street lights should be considered continuous, you need to rate the load at 125%, but at 10A you'd be fine.
 

1793

Senior Member
Location
Louisville, Kentucky
Occupation
Inspector
haskindm said:
The ampacity of the circuit is limited by the lowest temperature rating of any device that goes into making up that circuit. Just like the weakest link in a chain determines the strength of the chain. If I use 90 degree conductors and some terminal in the circuit is rated 60 degrees, then the whole circuit must be limited to the 60 degree rating. See 110.14.

I can follow what you are saying on this but in my opinion you can still do your derating calculations off of the 90 deg table for the conductors. The OCPD would have to be sized to the lowest temp rating anywhere along the circuit, would you agree?
 

dnem

Senior Member
Location
Ohio
Let's forget about the continuous load possibility right now, just to simplify.

He said he had 22 #10s.
Let's speculate that the wire is 90? and the lugs on the breakers are 60? [even tho they're probably 75?]
So the 3 ways to calculate it:

1] . 240.4(D)
#10 is good for 30a.

2] . lug/breaker temp column of T310.16, 240.4(B), 240.6(A)
#10 is 30a for 60? on T310.16
240.4(B) + 240.6(A) leave the ampacity at 30a.

3] . conductor temp column of T310.16, T310.15(B)(2)(a), 240.4(B), 240.6(A)
#10 is 40a for 90? on T310.16
40a x 45% = 18a as required by T310.15(B)(2)(a)
18a moves up to 20a as allowed by 240.4(B) + 240.6(A).

That's 3 separate things that give you 3 separate numbers and your OCPD can't be higher than the lowest number of any of the 3. . And the lowest number of the 3 is 20a.

David
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
1793 said:
I can follow what you are saying on this but in my opinion you can still do your derating calculations off of the 90 deg table for the conductors. The OCPD would have to be sized to the lowest temp rating anywhere along the circuit, would you agree?


Sounds good to me.
 

Michael15956

Senior Member
Location
NE Ohio
I believe that the derating with the said XHHW starts at 75C. See, T310.13 for wet locations.

In this particular case I don't think that any of the said terminations advise applies. Even if the terminations are listed at 60C, the XHHW is being derated as per 310.15(B)(2)(a) and will not be running at their 75C rating.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Michael15956 said:
I believe that the derating with the said XHHW starts at 75C. See, T310.13 for wet locations.

In this particular case I don't think that any of the said terminations advise applies. Even if the terminations are listed at 60C, the XHHW is being derated as per 310.15(B)(2)(a) and will not be running at their 75C rating.


And if it's rated as XHHW-2 you could use the 90 degree ampacity.
 

acrwc10

Master Code Professional
Location
CA
Occupation
Building inspector
lizzie14 said:
I have 22-#10 AWG wires to feed site lighting.

Thanks!


Do all 22 wires count as current carrying ?
If you are running MWBC , it is possible you don't have to be counting all 22. If so that would bring you up from 45% to 50%

Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable

Number of
Current-Carrying
Conductors Percent of Values in Tables 310.16 through 310.19 as Adjusted for Ambient Temperature if Necessary
4?6 80
7?9 70
10?20 50
21?30 45
31?40 40
41 and above 35
 
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