I think the calculation method is well explained in Ch.2- "References " and Ch.3-"Technical feature of the tables" namely Sub.ch. 3.5 "Method of calculation". Also in Appendix A are described some examples.
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:
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.
Thanks that gets me heading in the right direction. The next question is will the time window used affect the calculation and ultimately cable heating.? Hourly, daily, weekly, monthly, annually? The cable obviously takes a period of time to heat, Neher and McGrath must have taken into account a specific defintion of load factor
If I well understood the sentences around the loss factor [LF] equation [3] from the Neher and McGrath above mentioned paper, the load factor [lf] is not the rms average but the linear average of the load within a day [integral (I*dt) for 24 hours divided by 24].The peak value it is also the maximum average within an hour in one day.
Load factor, as I have always understood, by definition is the ratio of the average load divided by the maximum for a given period. The period can be any that you choose for your particular situation.
If you are calculating a monthly loss factor, then use a monthly load factor....etc.
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.
I don't have the paper, but Rating of Electric Power Cables by George J. Anders, 1997, references "Procedures for calculating the temperature rise of pipe cable and buried cables for sinusoidal and rectangular loss cycles," J.H. Neher, AIEE Tran., vol. 72, part 3, pp. 541-545 as the source for the Neher-McGrath cyclical adjustment factors.
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.
That is the reason you have the option to choose your period....but irregardless of the time frame you choose, load factor is still average/max load for the period.
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.
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.
There are always dollars to be saved by cutting corners up front, but that can bite you long term.
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.
