wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
In trying to understand how wire reactance affects voltage drop in AC circuits, I entered the following input to the Southwire calculator https://www.southwire.com/calculator-vdrop
AC, Single Phase, Power Factor 1
Copper, 250 MCM, 1 set
1000' run, 100V, 10A
The results it spat out:
0.054 Ohms Resistance per kft
0.041 Ohms Reactance per kft
1.44% Voltage Drop
As I understand it, if Vs is the system voltage, Vw is the voltage change over the wire, and Vl is the voltage seen at the load, then as vectors/complex numbers, Vs = Vw + Vl. And voltage drop would be |Vs| - |Vl|.
With a two-way distance of 2000', the wire resistance will be 0.108 ohms, and the wire reactance will be 0.082 ohms. If we say Vs = (100,0), then with power factor 1, the current is (10,0). So Vw = (1.08, 0.82). And Vl = Vs - Vw = (98.92, -0.82). Then |Vl| = sqrt(98.922 + 0.822) = 98.92. For a voltage drop of 1.08V, or 1.08%, not 1.44%. [Not sure if it is significant, but 1.44/1.08 = 4/3.]
Am I missing something in my approach, or is there an error in the Southwire calculator? If power factor 1 means the current is in phase with Vl, not Vs, that could make a small change, but as they are only 0.47 degrees apart with the alternate meaning, I can't see that explaining the difference.
Cheers,
Wayne
AC, Single Phase, Power Factor 1
Copper, 250 MCM, 1 set
1000' run, 100V, 10A
The results it spat out:
0.054 Ohms Resistance per kft
0.041 Ohms Reactance per kft
1.44% Voltage Drop
As I understand it, if Vs is the system voltage, Vw is the voltage change over the wire, and Vl is the voltage seen at the load, then as vectors/complex numbers, Vs = Vw + Vl. And voltage drop would be |Vs| - |Vl|.
With a two-way distance of 2000', the wire resistance will be 0.108 ohms, and the wire reactance will be 0.082 ohms. If we say Vs = (100,0), then with power factor 1, the current is (10,0). So Vw = (1.08, 0.82). And Vl = Vs - Vw = (98.92, -0.82). Then |Vl| = sqrt(98.922 + 0.822) = 98.92. For a voltage drop of 1.08V, or 1.08%, not 1.44%. [Not sure if it is significant, but 1.44/1.08 = 4/3.]
Am I missing something in my approach, or is there an error in the Southwire calculator? If power factor 1 means the current is in phase with Vl, not Vs, that could make a small change, but as they are only 0.47 degrees apart with the alternate meaning, I can't see that explaining the difference.
Cheers,
Wayne
