IEEE 835 Load Factor Calculation
IEEE 835 Load Factor Calculation
(OP)
I am using IEEE 835 to determine cable loading and I can not determine how the standard defines load factor or how to calculate.
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
IEEE 835 Load Factor Calculation
|
RE: IEEE 835 Load Factor Calculation
These ampacities were calculated as detailed in the basic ampacity paper, AIEE Paper 57-660, "The Calculation of the Temperature Rise and Load Capability of Cable Systems, by J. H. Neher and M. H. McGrath". See:
htt
Capability-of-Cable-Systems-Neher-and-McGrath
I'd prefer IEC 60287 [for metric u/m].It is less sophisticated. My opinion.
RE: IEEE 835 Load Factor Calculation
Thanks for the link to the Neher and McGrath paper. I had a read but it still does not define how to specifically calculate load factor. In the IEEE table examples it gives a specific load factor but does not state how it was arrived at. I have a load that is very cyclic in nature and want to apply the 75% load factor but can not find justification.
RE: IEEE 835 Load Factor Calculation
RE: IEEE 835 Load Factor Calculation
RE: IEEE 835 Load Factor Calculation
RE: IEEE 835 Load Factor Calculation
RE: IEEE 835 Load Factor Calculation
If you are calculating a monthly loss factor, then use a monthly load factor....etc.
Alan
RE: IEEE 835 Load Factor Calculation
I don't think this is the intent of load factor when applied to the IEEE tables. For instance, if I load a cable to 100% for a month, and then do not load it for the other 11 months, and calculate a yearly load factor, it would come out to be 8.333%. If I applied a low load factor to the cable when sizing I would obviously damage it during the month long loading.
Thus comes the question, what time frame did the authors of IEEE use when developing 100% and 75% load factor tables.
I think the answer lies in the a century old paper referenced by Neher and McGrath but would suspect 24 hours a reasonable time.
The IEEE white book has a formula for load factor which matches 7anoter4 definition.
RE: IEEE 835 Load Factor Calculation
Keep in mind that the LF in the Neher-McGrath equation is loss factor not load factor. LF is determined by the approximate equation LF = 0.3·lf + 0.7·lf², where lf is the load factor.
RE: IEEE 835 Load Factor Calculation
The period can be as small or large as you need to model properly. If it is only used for 1 month a year, then calculate the monthly load factor....or weekly....or daily as necessary. It really depends on why you need to calculate a load factor to start with.
If a cable is ever going to be loaded to 100%, then size it for no less than that.
Alan
RE: IEEE 835 Load Factor Calculation
That is the quandry, if I use the 75% load factor, my 100% ampacity is much higher, thus I need to be satisfied I am not overloading the cable.
The load is a wind farm with a projected capacity factor of 40%, using the 24 hour time frame and collecting real time data from existing sites with a similiar wind regime the highest recorded load factor recorded has been 68%. With the large amount of cable in these facilities there can be significant $$$ savings realized by not oversizing.
Thanks for your help.
RE: IEEE 835 Load Factor Calculation
I would never undersize a cable once I know the loading associated with it. Heat is the worst enemy of insulation and overloading on a regular basis will only shorten its useful life.
I would also think that your electrical codes would govern the installation.
Alan