Originally Posted by

**Grouch1980**
Got it. So there's no rule of thumb / formula, other than to actually take a field measurement of the voltage at the point where the new connection is being made, or find as-builts / design drawings and calculate it.

you need 2 v readings

supply and load at the same time (2 guys, do each ph and avg)

take i readings at same time, should be steady, avg 3 phases

feeder Z = (v supply - v load) / i Ohms

max v drop = Z x cond ampacity (x 80% depending on application)

eg

v supply 485

v load 480

i = 100

Z = 5/100 = 0.05 Ohm

if 250 kcmil rated ampacity = 255

v drop = 0.05 x 255 = 13 or 2.7%

if loading is 80% (cond sized 1.25 x load)

v drop = 0.05 x 0.8 x 255 = 10 v or 2.1%

the closer the i is to the rated ampacity the better

>30% of rated ampacity with good measurements yeilds good results

you can also remove power and actually measure a loop by shorting one end

or calc off as-buits

*The difference between genius and stupidity is that genius has its limits.*

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