voltage drop calculation

[A.Murphy]

I am confused. When calculating VD on a 3phase 4 wire system feeder, do I calculate for phase to neutral or phase to phase?

 

[bdarnell]

You are calculating for a (assumed) balanced 3-phase load. Hence the difference in the formulae.

 

[charlie b]

If you are asking which voltage to use (for example, 120 or 208), when you figure out the percent VD, the answer is that you use the phase-to-phase voltage (in my example, 208).

 

[jtester]

You can do either, as long as you pay attention to which you do. I prefer to calculate L-N drop since in a balanced 3 phase system there is no neutral current.

The equation that I use is current X distance X z(from Table 9.)/1000.

That is your basic equation E=IR.

To calculate %drop, I divide the above # by 120 for a 120/208 system or 277 for a 277/480 system.

Jim T