Cables temperature vs load calculation

[iwire]

I can't find much information on these except ETAP has a software that can do some calculation based on steady-state temp cal using Neher-Mcgrath -1957 http://www.electriciancalculators.com/ampacity/nm1.pdf

and IEC-60287 - 2006

Thermal analysis based on a few factors -

1) conduit/direct buried

2) soil condition

3) cable construction

4) loads on the cable - but assuming full loaded system should be under certain temp based on manufacturing specification

Do you know where I can find information how to run some calculation?

 

[knight1fox3]

Are you referring to a de-rating calculation? If so, the NEC has de-rate tables based on ambient temperature and number of current-carrying conductors in raceway/cable/conduit.

 

[iwire]

Are you referring to a de-rating calculation? If so, the NEC has de-rate tables based on ambient temperature and number of current-carrying conductors in raceway/cable/conduit.

Nope. I know the deratin portion but to calculate the actual or at least in theory the temperature rises according to load due to various factors

 

[Ken PE 3.1]

Sounds like that would get complicated very fast.

 

[iwire]

Sounds like that would get complicated very fast.

it does..But there is a software out there that can do the calculation but I want to understand how

http://etap.com/cable-systems/underground-thermal-analysis.htm

 

[Ken PE 3.1]

You want to understand the complexities of a computer algorithm?

I just go with the "it works" mentality.

 

[Flyer_PE]

The ETAP manual should have some information as to how they are performing their calculation.

 

[iwire]

You want to understand the complexities of a computer algorithm?

I just go with the "it works" mentality.

Not the computer but the calculation itself..I can't more information than that Mcgrath paper

 

[knight1fox3]

To me it sounds like there are quite a few dynamic elements that you wouldn't necessarily be able to "predict". As such, the software and corresponding algorithm makes certain assumptions that's built into their calculations. And might even be built into a database where depending on the conditions you select, the assumed values are selected accordingly. You might try Flyer's advice and check the manual to see what information they have. Otherwise it might get pretty technical to the point where you would have to speak with an ETAP developer/software eng.

 

[iwire]

cheers. I will keep checking.

I believed if you look at the McGrath paper, it does has some calculation.

 

[Kovz]

I ran into this issue when I came across some undersized cables embedded in conduit underground. I did some heavy reading on the Neher-Mcgrath method to try and do the calculations by hand, but it was very complicated and in my circumstance, I had a lot of unknowns.

I ultimately ended up using one of our sister companies that had E-Tap to do some consulting for us and run the calculations. We only have SKM PowerTools at my office.

Check out Neher-Mcgrath's website and do some reading on that. Maybe it will help you understand the variables in the formula a little better.

http://www.neher-mcgrath.com/41/

http://www.electrician2.com/articles/ampacity.htm

 

[iwire]

I ran into this issue when I came across some undersized cables embedded in conduit underground. I did some heavy reading on the Neher-Mcgrath method to try and do the calculations by hand, but it was very complicated and in my circumstance, I had a lot of unknowns.

I ultimately ended up using one of our sister companies that had E-Tap to do some consulting for us and run the calculations. We only have SKM PowerTools at my office.

Check out Neher-Mcgrath's website and do some reading on that. Maybe it will help you understand the variables in the formula a little better.

http://www.neher-mcgrath.com/41/

http://www.electrician2.com/articles/ampacity.htm

thanks for the links..I will check it out.

 

[jackson70]

[SIZE=11pt]iwire, the simplest solution would be to just insert the required calculations in and viola, you’ll be good to go. If you want to understand the algorithm you’ll have to read the paper that basically explains that. I’m not sure if the people who wrote the software would eventually tell anyone about the paper that they have based their calculations on. [/SIZE]

Regards,

[SIZE=11pt]J. Jackson[/SIZE]

[SIZE=11pt]http://blog.7pcb.com[/SIZE]

 

[iwire]

[SIZE=11pt]iwire, the simplest solution would be to just insert the required calculations in and viola, you’ll be good to go. If you want to understand the algorithm you’ll have to read the paper that basically explains that. I’m not sure if the people who wrote the software would eventually tell anyone about the paper that they have based their calculations on. [/SIZE]

Regards,

[SIZE=11pt]J. Jackson[/SIZE]

[SIZE=11pt]http://blog.7pcb.com[/SIZE]

Ya, thats what i am finding out ...

 

[KundaliniZero]

ETAP do a more x

 

[KundaliniZero]

ETAP has a algorith closed because of copyright.

But you can do calculations with a pen and sheet. I am almost sure i saw it two years ago investigating.

Algorithm is related with loses in a wire. As we know loses are related with joule effect i.e. heat is proporcional to I ^2 R where R is resistence and I nominal current.

I don t remember how they modeling the temperature between wires but it can be taking models how heat move from one wire to another, maybe i am wrong but i thing the formule had a logarithm.

I will continue investigating. I have tables to avoid ETAP and i am sure they use matrixs in the code to model multiples wires.