Percentage Voltage Drop Check on a Feeder
Percentage Voltage Drop Check on a Feeder
Electrical codes limit voltage drop on feeders and branch circuits to a percentage of the nominal system voltage — commonly 3% for a branch circuit or 5% total — rather than an absolute voltage figure, since the same absolute drop matters more on a low-voltage circuit than a high-voltage one.Expressing a measured or calculated drop as a percentage of nominal voltage makes it directly comparable to that code limit, which is why this simple ratio is the last step in nearly every voltage-drop study, however the raw voltage drop was originally computed.
The percentage voltage drop is Vd% = Vd/V_nom, the measured voltage drop divided by the nominal system voltage. where Vd is the voltage drop across the feeder and V_nom is the nominal system voltage.
Inputs
Vd_meas := 8 V=8 V
V_nom := 230 V=230 V
Dividing the measured drop by the nominal voltage converts an absolute voltage figure into the percentage that codes actually regulate.
Vd_pct := Vd_meas/V_nom=0.03
Results
This ratio comes out to about 0.035, or 3.5% once expressed as a percentage, which is right at the edge of a typical 3% branch-circuit limit and would need review depending on which code threshold applies to this feeder. Any value comfortably under the applicable limit confirms the conductor sizing is adequate, while a value at or above it calls for a larger conductor or a shorter run.
Assumptions & limitations
- Voltage drop already includes the full run length in its measured or calculated value.
- Steady-state loading; motor starting transients not included.
- Percentage limit thresholds vary by code and by branch circuit vs. feeder classification.