I used to be under the impression that you don't have to double the length either BUT you do for single phase circuits. Only a three phase doesn't have to be doubled when using that formula. I you see the examples shown in the NEC handbook as I mention in my program, the example shows a differant formula when solving for single phase. I choose to use the "engineering" formula because I have all the information available to me, along with being easier to deal with and done correctly, can be very accurate, which is what I want. Please try my program and see if you get similar results when using single phase wiring. The formula is written out for you.
As another test, you may try the excel sheet on Mikle Holts web site that is also very accurate. I checked all my calcs using this and other methods.
http://www.mikeholt.com/documents/fr...Calculator.xls
The simple math formula can be accurate if all the info is entered correctly.