Actually, I think any use of a ?load factor? is impractical. That is, in part, because the NEC does not get around to defining it. But if I apply the definition from one of my old textbooks, I see that it essentially means the ratio of average to peak. Consider a factory that runs at 100% power for 12 hours a day, then shuts down completely (0% power) for the other 12 hours. The peak is 100%, the average is 50%, so the ?Load Factor? is 50/100, or 50%. If the factory runs at 100% for 12 hours and 50% for 12 hours, the ?Load Factor? is 75%. I think it is hard to get a real situation in which a calculated LF value of 50%, as used in the table, can be justified.
The rest of your question can be answered by recognizing this entire paragraph is an explanation of how to interpolate. The ?F? value is what you multiply time the tabulated value of ?ampacity at 60 RHO and 50% LF,? in order to get the ampacity for your situation, given that your situation has a RHO other than 60, and an LF other than 50%. The value of I2/I1 is the ratio of two values taken directly from the table, and is simply a part of the interpolation process.
