Ok, I have positive, negative and zero sequence impedance for this ratio. I was not sure if should calculate all three individually and use the max/min values for my equipment, or if there was equation I forgetting that would take all three into account.If you have the positive sequence impedance as a complex number, you can find the X/R by expressing the impedance in R + jX form and then just divide X/R. As Jim says, it isn't too critical for arc-flash calculations, but is more important for equipment duty calculations. The X/R for line-to-ground faults is more complicated but that really won't have any impact on arc-flash calcs based on IEEE 1584.
What kind of assumptions should I make? Do you have any recommended materials for me to refer to in the future?The X/R ratio on the primary side of a utility owned transformer will not have much impact on your secondary side values.
Try running your analysis with several assumptions and compare the results at your service entrance equipment. I remember using starting values of 8 for a POCO and 20 for a generator.
What guidance does your power analysis software give you? They usually have some reasonable default values, which they explain in their 'help' writeups.What kind of assumptions should I make? Do you have any recommended materials for me to refer to in the future?
Ok, Thank you.For arc-flash purposes, the X/R ratio is based on the positive sequence resistance and reactance. You can ignore the other sequence impedances since IEEE 1584 calculations are based on balanced three-phase faults.