Calculator Inputs
Use the responsive form below. It shows three columns on large screens, two on medium, and one on small screens.
Example Data Table
These examples show how the calculator can be used across branch, feeder, and DC runs.
| Scenario | System | Material | Size | Length | Current | Voltage | Estimated Drop | Drop % |
|---|---|---|---|---|---|---|---|---|
| Branch circuit | 1Ø AC | Copper | 8 AWG | 90 ft | 35 A | 120 V | 4.75 V | 3.96% |
| Feeder example | 3Ø AC | Aluminum | 3/0 AWG | 220 ft | 150 A | 480 V | 7.62 V | 1.59% |
| DC equipment run | DC | Copper | 2 AWG | 60 ft | 110 A | 48 V | 2.56 V | 5.34% |
Formula Used
Single-phase: Vd = 2 × I × L × (R × cosφ + X × sinφ) / 1000
Three-phase: Vd = 1.732 × I × L × (R × cosφ + X × sinφ) / 1000
Vd = 2 × I × L × R / 1000
Single-phase or DC: CM = (2 × K × I × L) / Vd
Three-phase: CM = (1.732 × K × I × L) / Vd
Where Vd is voltage drop in volts, I is load current, L is one-way length in feet, R is resistance in ohms per 1000 feet, X is estimated reactance in ohms per 1000 feet, and CM is circular mil area.
The calculator uses K = 12.9 for copper and K = 21.2 for aluminum. It also applies a temperature adjustment to resistance and an optional steel-raceway reactance factor.
How to Use This Calculator
- Choose AC or DC, then select phase type if AC is used.
- Pick conductor material, conductor size, and number of parallel conductors.
- Enter source voltage, load current, and one-way run length.
- For AC systems, set the power factor and raceway estimate.
- Enter conductor temperature and your preferred design limit percentage.
- Choose whether ampacity should use a continuous-load basis.
- Press the calculate button to show the result above the form.
- Review the suggested size, graph, and export the result as CSV or PDF.
Design Notes
This tool is intended for engineering estimates and early design review. NEC informational guidance commonly points designers toward about 3% maximum drop on branch circuits and about 5% combined drop on feeder plus branch circuits.
Final conductor selection should also consider conductor insulation type, actual installation conditions, termination ratings, ambient corrections, adjustment factors, overcurrent protection, and local code review.
Frequently Asked Questions
1) Is voltage drop a strict pass-fail rule?
Usually it is treated as a design recommendation, not a universal hard limit. Many designers aim for about 3% branch-circuit drop and 5% combined feeder plus branch drop.
2) Why does the calculator ask for one-way length?
Voltage-drop formulas already account for the return path with the correct multiplier. Entering one-way length avoids double counting and keeps the form consistent for AC and DC runs.
3) Why does power factor matter for AC circuits?
AC voltage drop depends on both resistance and reactance. Lower power factor raises the reactance contribution, so the total drop can increase even when current stays the same.
4) Does conductor temperature affect the result?
Yes. Higher conductor temperature increases resistance. That raises voltage drop, which is why this calculator adjusts resistance from a 75°C reference value.
5) What do parallel conductors change?
Parallel conductors reduce effective resistance and reactance per phase path. They also increase available ampacity, which may help both voltage-drop performance and conductor sizing checks.
6) Why is there a suggested minimum size?
The suggestion searches the listed conductor table for the smallest size that meets both the chosen voltage-drop target and the ampacity reference based on your entered load.
7) Can I use this for aluminum conductors?
Yes. The calculator includes aluminum resistance, K constant, and ampacity reference values. It lets you compare aluminum against copper under the same operating conditions.
8) Is the graph useful for planning?
Yes. It shows how drop percentage changes with run length. That makes it easier to see when a design moves beyond your selected target and when upsizing becomes reasonable.