Branch Voltage Drop Calculator

Example Data Table

These sample values help you verify calculator behavior.

Formula Used

This calculator uses impedance-based voltage drop for planning branch circuits.

• Single-phase AC: Vd = 2 × I × (R × cosφ + X × sinφ) × L
• Three-phase AC: Vd = √3 × I × (R × cosφ + X × sinφ) × L
• DC: Vd = 2 × I × R × L

R and X are per-conductor impedance per unit length, L is one-way length, and the multiplier accounts for the return path (or phase geometry). For parallel conductors, R and X are divided by the parallel count.

How to Use This Calculator

1. Select the system type: DC, single-phase AC, or three-phase AC.
2. Enter supply voltage and one-way run length.
3. Choose conductor material, size, temperature, and parallel count.
4. Select load entry method and provide amps, kW, or kVA.
5. For AC, set the power factor to match your equipment.
6. Press Calculate to show results above the form.
7. Use Download CSV or Download PDF to export.

Notes for Construction Planning

• Many designs aim for ≤3% drop on branch circuits.
• Long runs, smaller conductors, and lower voltage increase drop.
• For final design, verify impedance data with your cable manufacturer.
• Consider ambient conditions, conduit fill, and temperature rise.

Article: Understanding Branch Circuit Voltage Drop

Voltage drop is the reduction in available voltage between the source and the load as current flows through a conductor’s impedance. In branch circuits, excessive drop can dim lighting, reduce motor starting torque, and increase heat at terminations. Checking drop early helps protect performance and reduce rework before conduit routes and panel schedules are finalized.

This calculator estimates voltage drop for DC, single-phase AC, and three-phase AC circuits. For AC, it uses resistance (R) and reactance (X) and resolves the drop using power factor, which represents the phase angle between voltage and current. For DC, it uses resistance only. Resistance is corrected for conductor temperature because conductors often operate above the 20°C reference used in many tables. If you select parallel conductors, the calculator reduces effective R and X by the parallel count to reflect the shared current path.

Percent drop is the most practical way to interpret results. A 3% drop means a 230 V circuit delivers about 223 V at the load under the assumed current. Many projects use ≤3% as a planning benchmark for lighting and receptacle branches, while some equipment may require tighter limits. Always align your target with project specifications and manufacturer instructions. Use the built-in impedance values for planning and confirm final cable data for your selected conductor and installation method.

Example data (field-style check):

Single-phase AC, 230 V, 100 ft one-way, copper 12 AWG, 10 A, power factor 0.90, and 30°C conductor temperature. The calculation typically returns a drop near 3.0 V (about 1.3%). If the same circuit is extended to 250 ft one-way, the drop rises roughly in proportion to length and can approach common planning limits.

• Shorten the route or move the panel closer to the load.
• Increase conductor size (for example, from 12 AWG to 10 AWG).
• Improve power factor for motor loads to reduce reactive drop.
• Use parallel conductors where allowed and properly terminated.

In practice, the most common input mistake is underestimating length. Enter the actual routing distance, including vertical rises and offsets. Bundling, conduit fill, and high ambient temperatures can raise conductor temperature, increasing resistance and voltage drop. Export the CSV or PDF output to share assumptions during coordination and QA/QC reviews. Treat results as a planning estimate and verify the final design against cable manufacturer data and governing standards. Document the chosen conductor size and assumptions for future troubleshooting and commissioning. It also supports clearer handover notes for maintenance teams.

FAQs

1) What length should I enter?

Enter the one-way routing length from source to load. The calculation already accounts for the return path through the multiplier used in the formula.

2) Why does power factor affect AC voltage drop?

Power factor changes the balance between resistive and reactive drop components. Lower power factor increases the reactive portion, which can raise total voltage drop for the same current.

3) Can I use this for three-phase branch circuits?

Yes. Select Three-phase AC and enter line-to-line voltage. The calculator applies the √3 multiplier and uses R and X to estimate the line drop.

4) How do parallel conductors change the result?

Parallel conductors share current, reducing effective resistance and reactance. This calculator divides R and X by the parallel count to estimate the combined impedance.

5) Are the impedance values exact for every cable type?

No. The table uses typical planning values. For final design, confirm resistance and reactance using manufacturer data and the installation method in your project.

6) What is a typical acceptable branch-circuit limit?

Many projects target 3% or less for branch circuits. Some equipment may require tighter limits, so always check your specifications and local requirements.

7) Why does temperature matter so much?

Conductor resistance increases with temperature. Higher resistance increases I×R drop and heat, so warmer conductors can significantly increase voltage drop on long runs.