Minimum conductor size formula

[rinezry]

All of you are probably familiar with the formula S = I x sqrt(t)/k used to determine the minimum conductor size in mm2. What factor do you put in to get it in American size (AWG/kcmil)?

I tried looking at the NEC but couldn't find anything, and the only way I can think of is to use a metric/American conversion table. Hope someone can shed some light into this. Cheers.

 

[jghrist]

To convert mm² to in², divide by 25.4²

To convert in² to kcmil, multiply by 4000/pi

To convert in² to AWG is a bit more complicated:

AWG = 36 - ln(400·sqrt(A/pi))·39/ln(92)

where A is area in in² and AWG is the AWG# for positive results, #1/0 for a result of zero, #2/0 for a result of -1, #3/0 for a result of -2, etc.

 

[dpc]

Just a caution: If you are actually trying to size conductors to comply with the NEC, you will have to look up the ratings in the appropriate Table. The ratings, particularly in smaller sizes, do not always follow a mathematical relationship between cross-sectional area and current rating.

 

[rinezry]

Thanks for that jgchrist/dpc.