Calculators
Voltage Drop Calculator
Calculate voltage drop, minimum wire gauge, maximum length, or maximum current for single-phase and three-phase AC or DC circuits using AWG wire sizes locally in your browser.
Size wires for voltage drop and ampacity.
Enter the system voltage, current, one-way wire length, and AWG gauge to calculate voltage drop in volts and percent for single-phase (2-wire) or three-phase (3-wire) circuits. Choose copper or aluminum conductor material. The calculator uses NEC-based resistance values at 20°C and standard formulas: single-phase Vd = 2 × I × R × L, three-phase Vd = √3 × I × R × L. It can also solve for the minimum AWG to stay within a maximum voltage drop, the maximum wire length for a given gauge and drop limit, or the maximum current for a given run. Results include the actual voltage drop, percentage, and the wire's 60°C ampacity for comparison. This is an electrical planning estimate; always consult NEC tables and local codes for final design. All calculations stay in your browser.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What formula does the voltage drop calculator use?
For single-phase (2-wire) circuits: Vd = 2 × I × R × L. For three-phase (3-wire) line-to-line: Vd = √3 × I × R × L. R is the conductor resistance per foot at 20°C from NEC Chapter 9 Table 8 values, L is the one-way wire length in feet, and I is the load current in amperes. The percentage drop is (Vd ÷ V) × 100.
Which wire gauges are supported?
Standard AWG sizes from 30 AWG through 4/0 (0000) AWG. You can select from the dropdown or type a gauge such as "12", "2/0", or "4/0". The calculator uses NEC Chapter 9 Table 8 DC resistance values at 20°C for solid/stranded copper, scaled for aluminum by a factor of 1.64.
What is the difference between single-phase and three-phase?
Single-phase (or DC two-wire) uses Vd = 2 × I × R × L because current goes out one conductor and returns on the other, so the total conductor length is twice the one-way run. Three-phase line-to-line uses Vd = √3 × I × R × L because the return path is through the other phases and the line-to-line voltage drop uses the √3 factor. Choose the mode that matches your circuit.
Does this account for temperature, power factor, or AC impedance?
No. This uses DC resistance at 20°C from NEC tables. Real AC circuits have additional impedance from reactance, skin effect, and temperature rise, which increase voltage drop, especially for larger conductors and higher temperatures. For precise design, use NEC Chapter 9 Table 9 AC impedance values and apply temperature correction factors. This calculator is a planning estimate; always verify with the NEC and local codes.
What does the ampacity shown mean?
The ampacity is the approximate 60°C (140°F) rating from NEC Table 310.16 for copper and aluminum conductors in a raceway or cable, based on the AWG size. It is shown for comparison with your calculated current or maximum current result. Actual ampacity depends on insulation type, ambient temperature, number of conductors in a raceway, and installation method. Consult the full NEC tables for code-compliant sizing.
Can I use this for DC circuits?
Yes. Select "Single-phase / DC (2-wire)" for DC circuits. The formula Vd = 2 × I × R × L applies directly to two-wire DC runs. For DC, the resistance values are appropriate as-is since there is no skin effect or reactance.
How do I interpret the "minimum wire gauge" result?
When solving for minimum AWG, the calculator finds the largest conductor (lowest AWG number) whose DC resistance keeps the voltage drop at or below your specified limit. If even 4/0 AWG exceeds the limit, it means the run is too long or the current too high for a single conductor at that voltage; consider increasing the voltage, shortening the run, or using parallel conductors.