For the inspectors out there.

[kwired]

The method I use actually does that, but uses wire length and not the actual resistance and without actually calculating the voltage drop. The resistance is directly proportional to the resistance so the results are the same without using the actual resistance.

Did you by chance mean 'length' in place of one of the uses of 'resistance'

 

[don_resqcapt19]

Did you by chance mean 'length' in place of one of the uses of 'resistance'

yea

 

[iceworm]

I don't know how to link to a thread. Maybe like this. http://forums.mikeholt.com/showthread.php?t=146339

This one is from two month ago. It has 27 posts concerning the math. Did you happen to read it?

ice

 

[don_resqcapt19]

I don't know how to link to a thread. Maybe like this. http://forums.mikeholt.com/showthread.php?t=146339

This one is from two month ago. It has 27 posts concerning the math. Did you happen to read it?

ice

Yes, but I don't see a conflict between that and what I have posted here. The math works out the same both ways, I think.

 

[iceworm]

Yes, but I don't see a conflict between that and what I have posted here. ....

Yes, I'm fine with what you said. The "you" should have been "youall' or maybe even "all youall". The comment was for everyone posting. Seemed a shame to waste 27 on-point posts (Okay, maybe only 15 on-point posts)

ice

 

[texie]

The code gives no leeway. The code rule, as written, makes the use of parallel conductors a code violation as it is not physically possible to make the conductors the same length.

From a practical matter, a couple of percent difference in length will not likely cause any real world issues. There are other things, such as the physical arrangement of the conductors, that also change the impedance of the parallel paths. The goal is to keep the current as equally divided between the parallel conductors as possible. The current will divide in inverse proportion to the lengths...the shortest length (lowest impedance) will have the most current and the longest length (highest impedance) will have the least amount of current.
If you would have a 600 amp circuit with 3 sets of 3/0s and the lengths were 10', 11' and 12', the currents would be 218.8, 198.9 and 182.3 amps. Note that in this case you would exceed the rated conductor ampacity of the 10' run.
If the lengths were 100', 102.5, and 105' the currents would be 204.9, 199.9 and 195.2 amps. Again the shortest conductor would be operating above its rated ampacity. These numbers assume that the only change in the impedance of the paths is the change in conductor length.

For the record, I concede that you are correct now that I had a moment to work out the math. I worked it other ways to convince myself. It is somewhat sobering to see the effect. It just seems counter intuitive on large conductors, but the math doesn't lie. Maybe the lesson here is to verify and allow for uneven currents when choosing conductor sizes for paralleling.