First let me say that I am not trying to find fault with the book, I think it is a great book and have the greatest respect for mike holt and the people of his organization and the memebers of this forum. I think that possibly the formula for Vd, may not be able to be used under all conditons ( ie. larger amperages and distances ) .
What I am seeing here is that the formula will work under certian conditons and give you a useable answer, but the orignal problem I had, the answer was almost 1/3 cmil that was even able to handle the current with no voltage drop.. 400 cmil vs. 1250 cmil.
It was stated by one of the posters that it didn't take into accont the heat generated by the conductor under full load, and that being only able to calculate the parameters of resistance , amps, length and so on, and you can't blindly use a formula without knowing what you were using it for. Also that this formula would give you the minimum answer and they you would calculate it from there. In my example the answer was so far off you would have been better off starting with the 1250 minimum ampacity for the 574 amps and use the original formula Vd, and the size up as needed.
Unlike the basic ohms law p=I x E can be rearanged to suit your desired answer as in I = P/E , the original Vd formula is not able to be used in this way. And this is the part that was confusing me that the formula rearanged was not really accurate.
I am just trying to fully understand if the formula can be used as an absolute as ohms law can be ( of course ohms law is absoulte with dc and ac resistance) so am I on the right track here or still missing something? thanks for taking the time to help me understand this, and maybe other people too!
sorry, but the voltage drop formula is definitely not an absolute. The K value we use is an average approximation. The "True K" value would have to be calculated on every conductor size, by multiplying the circular mil area of the conductor by its resistance per ft. I know at least one electrical engineer who just shudders at the use of 12.90 for a K value for copper, because it is not an absolute, but varies from one size conductor to another. For practical purposes of estimating voltage drop, the electrical industry has pretty much standardized on the average K values of 12.90 for copper and 21.20 of aluminum.
The voltage drop formula can be rearranged into the 4 transpositions that I previously posted, just like ohms law, and every form is just as accurate as the others.
Just remember that the voltage drop formula is for estimating just that: Voltage drop. It is not used to actually size the conductor. Other factors must be used to size the minimum conductor size required to meet the NEC requirements.
In the example we have looked at a case where we found that 14AWG met the voltage drop requirements with a distance of 100 ft., but other Code requirements would not allow us to use 20A protection on 14AWG, so we had to size it up to meet those requirements in 240.4(D). In the same circuit, extended to 400 ft, the 14AWG was no longer sufficient for the voltage drop, and we had to size the conductor up to 8 AWG, even though it was still protected by a 20A overcurrent protection device. If you failed to use the voltage drop formula to check this, and installed a smaller conductor, you might have experienced too low of a voltage at your load for proper operation.
In your original problem, you found by using the voltage drop formula that the minimum size conductor to keep your voltage drop within the 3% recommendation was 400 KCmils. So you should not be distressed when you actually look up the size of the conductor and find that you need something larger than 400 Kcmils to meet the NEC ampacity requirements of 310.16 [2008 version] because what you have done is confirm that voltage drop will not be a problem if you use anything larger than 400 Kcmil for this problem - meaning the minimum size of conductor that you select using Table 310.16 and appropriate overcurrent protection rules [240.4] is Code compliant and you do NOT have to increase it for voltage drop.
Remember the voltage drop formula is to estimate voltage drop, not to size the conductor. The conductor must be sized based on Code requirements. We are doing 2 distinctly different things here. If you are preparing for a road trip, you need to make sure your car has a fresh oil change. But you also need gasoline. Two separate requirements that both must be met.
