Why voltage drops along a wire
No conductor is a perfect path; every length of wire has some resistance, and pushing current through that resistance consumes voltage along the way (Ohm’s law, V = I × R). The longer the run, the higher the current, or the thinner the wire, the more voltage is lost before it reaches the load.
The drop is counted over the whole circuit. A single-phase run uses a factor of 2 because the current travels out to the load and back along the return conductor, doubling the wire it passes through. A balanced three-phase circuit uses √3 (about 1.732) instead, reflecting how the phases share the return.
Reading the results
The voltage drop is how many volts are lost in the conductors, and the voltage at load is what is actually left for the equipment. The percentage drop expresses the loss relative to the source voltage, which is the figure electrical codes care about. The chart splits the source voltage into the part that reaches the load and the part lost in the wire.
Keeping drop in check
Excessive drop dims lights, weakens motors and wastes energy as heat in the wiring. A widely used guideline is no more than 3% on a branch circuit and 5% from the service to the load.
- Use a thicker conductor (lower Ω/kft) — the single most effective fix for a long run.
- Shorten the run or relocate the load closer to the source where possible.
- Raise the system voltage for the same power so less current flows.
- For very long runs, copper’s lower resistance can beat aluminium of the same gauge.
Assumptions and limits
This estimate uses a simple resistance-per-1000-feet value, so it captures DC resistance well but ignores AC effects such as conductor reactance, power factor, conductor temperature rise and bundling, which matter on large feeders. For code-compliant design on critical or high-current circuits, verify against the conductor tables and methods in your local electrical code.
Formula
Vdrop = factor·length·current·(Ω/kft)/1000 (factor = 2 single-phase, √3 three-phase)Frequently asked questions
- What is an acceptable voltage drop?
- A common guideline is no more than 3% on a branch circuit and 5% total from service to load.