Derating assistance for 105°C cable using 75°C ambient temp correction factors

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Petergg98

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New Hampshire
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Electrical
I have a cable that needs 200 Amps minimum 105°C (ambient temp is around 180°C). Chart 310.15(B)(1)(2) only lists correction factors up to 66-70°C. What would my next step be?
 
I'm using a cable rated for 105°C, I'm trying to determine the size but it needs to have minimum 200 Amp rating, the ambient temperature inside the junction box the cable will be located is around 82°C(180°F). Our terminals are rated for 75°C, But the correction factor chart only goes to 66-70°C for 75°C temp rating of conductor
 
Petergg98 said:
What would my next step be?
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Ampacity is determined by allowable temperature rise. The basic table 310.16 is based on 30C ambient, so the 60C, 75C, and 90C columns are really 30C, 45C, and 60C allowable temperature rise. Then Equation 310.15(B) tells you how to adjust for a lower temperature rise.

If you have a 105C conductor in an 82C ambient, the allowable temperature rise is 23C. So you could use the 60C column ampacity, which is based on 30C temperature rise, and multiply by sqrt(23/30) as per Equation 310.15(B).

If you prefer, you could see 690.31, which is in the solar section, as it covers 105C ampacities based on 30C ambient and provides derating factors up to 105C. The physics is the same regardless of application. Unless 690.31 includes some unspecified application specific additional factors, the result you get should be the same as my first suggestion, up to the various roundings (ampacities in Table 310.15(B) are rounded to a multiple of 5A; correction factor temperatures are binned by ranges of 5C, etc).

Cheers, Wayne
 
If I understand Wayne, I think I might not agree with him. (which is scary because I am more likely wrong.)

First, you can only use 690.31 if you are using the cable for a PV system. For building power and lighting, you don't have such a thing as 105ºC wire. So, that makes the question, "What wire TYPE do you have? It has to be one of the wire type list in tables 310.16 to 310.21. Then you use tables 310.15 (B)(1) or (B)(2) the column based on the temperature rating listed in the above tables.


So, unless I am wrong, we would need better information from you. The application and the wire type.
 
Strathead said:
First, you can only use 690.31 if you are using the cable for a PV system.
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That's true, but the numbers provided in 690.31 should be accurate on the physics of calculating ampacity. So if you have an NEC allowance to use 105C wire in a non-PV application, using the numbers from 690.31 should get you the correct answer.

Strathead said:
For building power and lighting, you don't have such a thing as 105ºC wire.
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I don't think that's quite true, it's just pretty uncommon. Looking at 2023 NEC Table 310.4(1) there are several wire types listed with a maximum operating temperature of 150C, 200C, or 250C. They all reference footnote 2 or are limited as "Only for leads within apparatus or within raceways connected to apparatus, or as open wiring (nickel or nickel-coated copper only)"

Footnote 2 says "Higher temperature rated constructions shall be permitted where design conditions require maximum conductor operating temperatures above 90°C (194°F)." Seems to me that you can argue that 82C ambient temperature requires conductor operating temperatures above 90C, as an 8C temperature rise is pretty anemic. [Well, the correction factor for a 90C conductor at 82C is sqrt(8/60) = 0.365, and it's a bit of a judgement call as to whether that's low enough that using 105C wire is "required".]

So as long as the OP is using 105C wire of one of those types listed in 2023 NEC Table 310.4(1) that reference Footnote 2, computing the ampacity as I described appears kosher to me. It's a little surprising that Footnote 2 provides the allowance without any guidance about how to calculate the ampacity.

Cheers, Wayne
 
wwhitney said:
I don't think that's quite true, it's just pretty uncommon.

Cheers, Wayne
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This is where I am coming from. The conductor types listed in 31.4(A) and subsequently tables 310.16 through 310.21 are the only conductors allowed for building power and lighting. None of those carry a rating of 105ºC per the NEC, so the OP isn't using 105 wire. Note 3 that you reference is merely telling us that there are only special applications where Z or ZW can be used at 150º. Not an unidentified 105 rated wire.

BTW while we don't have 105 rated wire, type MTW carries a 105 rating in Canada.
 
Strathead said:
This is where I am coming from. The conductor types listed in 31.4(A) and subsequently tables 310.16 through 310.21 are the only conductors allowed for building power and lighting. None of those carry a rating of 105ºC per the NEC, so the OP isn't using 105 wire.
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I see your point is that Table 310.4(1) doesn't have any allowable insulation types that max out at 105C, entries jump from 90C to 150C. And that it is unlikely that anyone is making wire with an insulation type that could be rated 150C and marking it just 105C. So if the OP wants to go with a higher temperature rating than 90C because the 0.365 temperature correction for 90C wire at 82C is prohibitive for their application, they will end up with a 150C rated wire rather than a 105C rated wire.

In which case with 150C rated wire you can just use 2023 NEC Table 310.18's 150C column (if the conductors are in a raceway or cable) along with Table 310.15(B)(1)(2) correction factors.

Thanks, Wayne
 
Strathead said:
Note 3 that you reference is merely telling us that there are only special applications where Z or ZW can be used
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Thank you for clarifying NEC idiot tables 310.16 - 310.21, not intended for me, or reasonable people to understand.

Looks like OP may require medium voltage cable with higher-temp ratings, and a transition upsize for the 75C terminations.
 
ramsy said:
Thank you for clarifying NEC idiot tables 310.16 - 310.21, not intended for me, or reasonable people to understand.

Looks like OP may require medium voltage cable with higher-temp ratings, and a transition upsize for the 75C terminations.
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They really aren't that bad once someone walks you through it. I am pretty sure you can find a video by Mike on the web site to do that. The thing less likely to just "get" is that the wire types are the only wire types you can use unless you were to get some sort of AHJ approval for deviation.
 
ramsy said:
Looks like OP may require medium voltage cable with higher-temp ratings, and a transition upsize for the 75C terminations.
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You don't have to go to medium voltage to get a temperature rating above 90C.

Depending on the application, you don't even have to go beyond 90C conductors. E.g. if you have a 15A load, and you need to run the conductors through 82C ambient, you can just use a 90C conductor with an ampacity of 15/0.365 = 41A (if no more than 3 CCCs) E.g. #8 Cu or #8 Al.

Cheers, Wayne
 
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