Cable and Terminations limit

phoenix68

Member
Location
US
Occupation
Student
I am using an underground duct bank, which has terminations above ground surface and rated at 75C.
Is it possible to have the temperature of cables inside duct more than 75C as cable are rated to 90C, and since the temperature of cables above ground is below 75C due to direct heat dissipation through air.
 

bwat

EE
Location
Western PA
Occupation
EE
I was hoping somebody else would jump in with specific experience on fighting this battle before. I can only offer an opinion.

My basic position is that it's ok as long as it doesn't directly violate NEC if you wouldn't have been using SKM. Meaning, if NEC methods would have determined that the temperature of the conductors was sufficient for the ampacity at 75C, then you're OK. But if NEC methods would have shown the cable needed to be increased, but then you are using SKM to utilize the "engineering supervision" clauses and go beyond what NEC may have allowed, then I think you're into a grey area and somebody has to make the call.

In the real world, if I felt the length of conductor above ground was sufficient to claim below 75C ratings, I would be ok with it. I'd want it to be well under 75C though. Alternatively, and why I have this opinion, you could put in a short 90C rated splice as soon as the conductors left the ductbank, and then you certainly only have a 75C connection from this splice to your original termination, even if it was short. Would a splice and 10 feet of cable really be better than no splice and continuous 50 feet of cable that went from the original 75C termination into the duct bank? I wouldn't think so.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
The physical conductor at the termination needs to be the size of a 75C insulated one.
The installed conductor insulation temperature rating can not be exceeded.

Software, like SKM, is simply a tool. Inputting numbers and hitting the 'go button' is not engineering.
IMHO the software companies are not engineering experts.
 

bwat

EE
Location
Western PA
Occupation
EE
Software, like SKM, is simply a tool. Inputting numbers and hitting the 'go button' is not engineering.
IMHO the software companies are not engineering experts.
Yes of course. I would say the tool has to be used by an engineer if we're calling it engineering. But I don't think anybody questions the validity of the software when using it to calculate available short circuit current or arc flash analysis. Why wouldn't cable ampacity be similar?

Like I said, this has been a grey area for me so I was hoping someone could jump in with a definitive answer. I had a feeling something like SKM was used which is why I asked the question in post #2. My coworkers and I have internally discussed this before and went around in circles.
 

phoenix68

Member
Location
US
Occupation
Student
The physical conductor at the termination needs to be the size of a 75C insulated one.
The installed conductor insulation temperature rating can not be exceeded.

Software, like SKM, is simply a tool. Inputting numbers and hitting the 'go button' is not engineering.
IMHO the software companies are not engineering experts.
NEC itself states that use the derating tables, or perform calculations under engineering supervision. If calculations in software are under engineering supervision, why would it create problems for certifications/testing?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Engineer
If calculations in software are under engineering supervision, why would it create problems for certifications/testing?
Who is the qualified engineer supervising the results of the calculations and were they involved in making any assumptions for the input? The software is simply a tool, often their examples use values to prove their point.
 

phoenix68

Member
Location
US
Occupation
Student
Who is the qualified engineer supervising the results of the calculations and were they involved in making any assumptions for the input? The software is simply a tool, often their examples use values to prove their point.
The Sr. Electrical Engineer has used the values that were found in the field. No assumptions were made. I myself have verified the data that generally SKM would have taken by default.

Also I agree that it is merely a tool. But given our unique input values, we would get results as would be ideally observed in field. That as per my understanding should suffice.
 
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