Hi to all of you.

I make so few posts on this site, but Mike Holts’ computation formulae and opinions on voltage drop compels me to this comment, so please bear with me as this is only a matter of physics and math.

At the point of your test, just use the formula that your instructor or test wants you to use and nothing more as it is impossible to argue with “city hall” and win. If you are really interested in this subject, then after regurgitating their formula for the purpose of the test you might want to look into these comments.

Voltage loss is sensitive to ambient temperature, conductor and insulation types, distance, conduit type, ambient conditions, if buried then depth and soil conditions, power factor, harmonics, and a lot more. A brief Google search on Neher-McGrath will shed some light on all the factors and conditions that are associated with the R variable in V =IR (which is correct but your formula is far too simple to be of any value). I don’t think that a complete understanding of these formulae is necessary at this point, but an understanding of their existence and concept of use will be very useful.

The IEEE has developed standards that are globally respected for voltage drop computations. Understanding and using the IEEE standard 141 exact formula will produce and voltage drop value that will be the same as a site measurement. This is the only formula that can boost this level of accuracy.

Volts is an electrical design software program that uses Neher-McGrath and IEEE formulae in its voltage drop computations. It can be found and downloaded from this website’s bookstore and used freely for 10-days. Try entering a number of different devices under different environmental conditions and you will see the impact on the voltage drop.