ser cable

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Sman

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Section 338 says #10 or smaller shall be derated to 60 degrees. Does this mean #8 and bigger can use the 75 degree rating. This is for ser cables used as branch circuits in residential. Lots of EC's are running these in texas these days
 
Depending on your Code cycle (I think I was '11 that limited all sizes to0 60°).
A lot of our contractors switched to SE cable from NM to take advantage o the 75° rating..
You do need to take care that them terminations are not limited to 60°.
 
electrofelon said:
Does anyone know why the code gods/knuckleheads won't allow 75 for NM, at least in larger sizes or when used as feeders?
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Because of 334.80. I wish I could give you a more intelligent answer. It may have something to do with the material composition of the outer jacket not being able to stand up to the higher temperature.
 
Sman said:
Section 338 says #10 or smaller shall be derated to 60 degrees. Does this mean #8 and bigger can use the 75 degree rating. This is for ser cables used as branch circuits in residential. Lots of EC's are running these in texas these days
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That is correct. The only ser cables that require the 60C would be ser cables #10 and smaller. I have not seen ser that small in many many years and in fact I have never installed it.
 
I have said this before but it is only a guess. IMO, the 60C rating for nm was implemented because it is too hard to deal with ambient temperature de-rating. Nobody I know has ever been told that since their Nm cable is in an attic you must de-rate the wiring. This issue would be hard to deal with so the cmp just restricted Nm to 60C.

Again this is a guess
 
My understanding of derating nm is because multiple wires where run into light boxes,then switch loops ran to switches . With 3 or 4 sets of nm cables plus the heat from light fixtures,wires where melting down. Doesn't make sense to derate bigger wires to appliances though
 

338.10(B)(4) SE Cable. Installation Methods for Branch Circuits and Feeders.

This 2 AWG aluminum Type SE cable has XHHW-2 (90°C) conductor insulation and is installed in an exterior wall that will be insulated after rough inspection is passed. The cable is fed from a breaker with 75°C rated terminals and supplies an apartment unit subpanel that has 75°C rated main lug...
www.electricallicenserenewal.com
Here is a link to some good read on your question.
I took it your on 2017 or 2020
 
electrofelon said:
Right, but why is that there? I believe the conductors are specifically actually rated 90.
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The letter "B" in NM-B cable (Romex) means that the conductors inside the nonmetallic sheath are rated for 90 degrees C. Art. 334.80 says we can use the 90 C. rating for ampacity correction or adjustment calculations, provided that final ampacity doesn't exceed that of a 60 C. conductor. Dennis brought up a question above that had confused me for years. Looking at Table 310.15 (B)(1), A conductor with a temperature rating of 60 C. (140 F.), can not be installed in an ambient of 60 C. (140 F.) It would have ZERO ampacity. I know I've been in attics here in Florida during the summer and I can assure you the temperature was 140 F. or above. But, by using the 90 C. column which is permissible for correction factors, a #12 conductor rated at 90 C. has an ampacity of 30 amperes. In an ambient temperature of 140 F. (attic, for example) Table 310.15 (B)(1) has us using a correction factor of .71
30 amperes x .71 = 21.3 derated ampacity Art. 334.80 states "the final calculated ampacity does not exceed that of a 60 C. (140 F.) rated conductor.
This is why we can install NM-B in an attic, but our OCPD can't exceed 20 amperes (#12 @ 60 C.)
 
Rick 0920 said:
The letter "B" in NM-B cable (Romex) means that the conductors inside the nonmetallic sheath are rated for 90 degrees C. Art. 334.80 says we can use the 90 C. rating for ampacity correction or adjustment calculations, provided that final ampacity doesn't exceed that of a 60 C. conductor. Dennis brought up a question above that had confused me for years. Looking at Table 310.15 (B)(1), A conductor with a temperature rating of 60 C. (140 F.), can not be installed in an ambient of 60 C. (140 F.) It would have ZERO ampacity. I know I've been in attics here in Florida during the summer and I can assure you the temperature was 140 F. or above. But, by using the 90 C. column which is permissible for correction factors, a #12 conductor rated at 90 C. has an ampacity of 30 amperes. In an ambient temperature of 140 F. (attic, for example) Table 310.15 (B)(1) has us using a correction factor of .71
30 amperes x .71 = 21.3 derated ampacity Art. 334.80 states "the final calculated ampacity does not exceed that of a 60 C. (140 F.) rated conductor.
This is why we can install NM-B in an attic, but our OCPD can't exceed 20 amperes (#12 @ 60 C.)
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Good explanation, thank you.
Would be nice to have an exception. Rated at 90 C unless installed in Florida attic space then rated for 60 C :- )
I know if it was able to be used anywhere except attics at 90 that people would use it in attic space at 90 also, BUT people misuse other things as well knowingly or not knowing.
Just doesn't make sense. what if I was wiring in a basement or in Alaska, or a basement in Alaska? There should be some type of exception for this.
2 cents
 
ritelec said:
Good explanation, thank you.
Would be nice to have an exception. Rated at 90 C unless installed in Florida attic space then rated for 60 C :- )
I know if it was able to be used anywhere except attics at 90 that people would use it in attic space at 90 also, BUT people misuse other things as well knowingly or not knowing.
Just doesn't make sense. what if I was wiring in a basement or in Alaska, or a basement in Alaska? There should be some type of exception for this.
2 cents
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It doesn't matter where on the planet you are using NM-B cable. You MUST consider it to be a 60 C. rated conductor when it comes to sizing the OCPD. The code section allows you to use the 90 C. numbers for adjustment or correction only. When I started wiring houses years ago, the conductors themselves had TW insulation on them, which is a 60 C. insulation. With today's code, a 60 C. insulated conductor could not be installed in ambient temperature of 140 F.
 
ritelec said:
....what if I was wiring in a basement or in Alaska, or a basement in Alaska? There should be some type of exception for this.
2 cents
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It's called ambient temperature adjustment, and it works both ways.

The ampacity chart we use is based on 77-86⁰ farenheit. If you go higher, you're supposed to reduce the ampacity.

But if your ambient temperature is between 69-77⁰ you can add 8% to the ampacity.

If your ambient temperature is below 50⁰ you get to add 29%20220413_220100.jpg
 
How does one know what temperature to actually use as ambient in a given installation?

Even directly measuring doesn't mean you checked when the conditions were the hottest.
 
LarryFine said:
How does one know what temperature to actually use as ambient in a given installation?

Even directly measuring doesn't mean you checked when the conditions were the hottest.
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Look up climatic designs for the U. S. on the ASHRAE website. You should be able to navigate through some pages and find the ambient temperatures for different regions of the country. ASHRAE has been an industry standard for many years.
 
James L said:
It's called ambient temperature adjustment, and it works both ways.

The ampacity chart we use is based on 77-86⁰ farenheit. If you go higher, you're supposed to reduce the ampacity.

But if your ambient temperature is between 69-77⁰ you can add 8% to the ampacity.

If your ambient temperature is below 50⁰ you get to add 29%View attachment 2560227
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With regards to NM cable its still a little different. 334.80 does say "........provided the final calculated ampacity does not exceed that of a 60 C. (140 F) rated conductor" So even if your ampacity is higher because of a much cooler ambient temperature, you would still have to use the 60 C column of Table 310.16 for the selection of OCPD's. Excellent point though about the cooler temperatures.
 
Rick 0920 said:
With regards to NM cable its still a little different. 334.80 does say "........provided the final calculated ampacity does not exceed that of a 60 C. (140 F) rated conductor" So even if your ampacity is higher because of a much cooler ambient temperature, you would still have to use the 60 C column of Table 310.16 for the selection of OCPD's. Excellent point though about the cooler temperatures.
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I see what you're saying, but those ampacities aren't static. They're all based on a certain ambient temperature - 30⁰c (range of 77-86⁰F is specified, too).

So in other words, if you have an ambient temperature of 80⁰F, you have to keep the load under 20 amps on a #12 NM to keep the conductor temp under 140⁰

But if you have an ambient temp under 50⁰F then a load of 25.8 amp will still keep that same conductor under 140⁰F
 
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