How to read the result
The large percentage is the estimated voltage loss relative to the source voltage. Voltage drop shows the same loss in volts, while the 3% target shows the voltage budget represented by that planning threshold. Comparing both values tells you not only whether the run crossed the target, but by how much.
Available ampacity is a separate preliminary check: it estimates how much current the selected conductor can carry under the modeled conditions. A conductor can satisfy ampacity and still have excessive voltage drop on a long run, so neither result replaces the other. Effective impedance is the per-1,000-foot electrical opposition used in the AC calculation.
A practical guide
How voltage drop works
Every conductor opposes current flow. That opposition turns part of the source voltage into loss along the run. The calculation becomes useful when you understand the four inputs doing most of the work: current, route length, conductor impedance, and the circuit multiplier.
Table values are expressed per 1,000 feet. The multiplier accounts for the complete current path.
Enter one-way route length
Measure the routed path from source to load—not a straight line across the drawing. The calculator closes the circuit mathematically: two-wire AC and DC use a multiplier of 2, while three-phase AC uses √3. Do not double the distance before entering it.
Impedance depends on the installation
For AC, conductor resistance combines with reactance and power factor to produce effective impedance. Conductor material, size, and raceway can all change that value. For DC, the model uses conductor resistance without AC reactance or power factor.
Percentage depends on system voltage
The same voltage loss matters more on a lower-voltage system. A 3 V loss is 2.5% of 120 V but only 1.25% of 240 V. Percentage drop is calculated as volts lost divided by source voltage, multiplied by 100.
What to check when the result is high
- Verify the route. Confirm the one-way length follows the intended pathway and includes real bends, offsets, and routing constraints.
- Verify the current. Use the expected design load for the question you are answering; breaker rating and actual load are not automatically the same input.
- Review the assumptions. Phase, material, raceway, and power factor can change an AC result. Make sure the defaults describe the installation.
- Compare conductor options. A larger conductor generally reduces impedance, but also affects material cost, raceway fill, termination compatibility, and installation effort.
What changes between AC and DC?
AC voltage-drop estimates can include conductor resistance, raceway reactance, power factor, and the selected phase arrangement. This calculator uses modeled AC impedance values from NEC 2023 Chapter 9, Table 9 (opens in a new tab).
DC uses direct-current resistance from Chapter 9, Table 8 and a two-wire multiplier. Selecting DC disables the AC-only controls so the result stays on a consistent calculation basis.
Know the modeled scope
- Lengths are one-way route lengths in feet.
- Supported conductors are copper and aluminum from 14 AWG through 600 kcmil.
- AC raceway choices are EMT and PVC Schedule 40; other wiring methods are not modeled.
- DC uses Table 8 resistance at 75°C and does not apply AC power factor or raceway reactance.
- Terminal, equipment, parallel-conductor, service, feeder, harmonic, and jurisdiction-specific conditions may require additional review.
Use this page to learn the relationships and compare preliminary options. Confirm project inputs, adopted code, equipment requirements, and field conditions before construction.
From one check to the whole plan
Move from a single voltage-drop check to a connected home-run plan.
Trace home runs on the drawing and keep measured length connected to preliminary conductor, voltage-drop, raceway-fill, and material checks.