Calculator Form
Formula Used
The calculator uses an impedance voltage drop method for AC circuits.
Voltage Drop = M × I × D × ((R × PF) + (X × sin(arccos(PF)))) ÷ 1000
For DC circuits, reactance is ignored and power factor is treated as 1.
DC Voltage Drop = 2 × I × D × R ÷ 1000
M is 2 for single-phase or DC. M is 1.732 for three-phase. I is current. D is one-way distance in feet. R is resistance per 1000 feet. X is estimated reactance per 1000 feet.
How to Use This Calculator
- Select the circuit type.
- Enter system voltage and load current.
- Add the one-way circuit length.
- Choose conductor material and wire size.
- Set power factor, temperature, and raceway type.
- Enter your allowed voltage drop percent.
- Press Calculate.
- Download CSV or PDF for records.
Example Data Table
| Phase | Voltage | Current | Distance | Material | Wire | Estimated Drop |
|---|---|---|---|---|---|---|
| Single-phase | 240 V | 40 A | 150 ft | Copper | 6 AWG | About 2.45% |
| Three-phase | 480 V | 80 A | 220 ft | Copper | 1/0 AWG | About 1.65% |
| DC | 48 V | 30 A | 60 ft | Copper | 4 AWG | About 1.12% |
Voltage Drop Planning
Voltage drop is the loss of voltage along a conductor. It appears when current flows through wire resistance and reactance. Long runs, high current, small conductors, and poor power factor increase the drop. A careful estimate helps equipment start well. It also helps lights stay steady.
Why It Matters
Every load needs enough terminal voltage. Motors may run hotter when voltage is low. Chargers may slow down. Sensitive controls may reset. Extra drop also wastes energy as heat in conductors. A small percent can become costly on long feeders. Many designers try to keep branch circuits near three percent. Feeders are often checked near two or three percent. Local rules and project standards should guide the final limit.
Key Inputs
The calculator uses phase, voltage, amperes, distance, material, wire size, power factor, temperature, conduit type, and parallel conductors. Copper has lower resistance than aluminum. Larger circular mil area lowers resistance. Higher temperature raises resistance. Three phase circuits use a different multiplier than single phase circuits. Direct current uses a two conductor path.
Design Notes
A result below the chosen limit is usually acceptable for planning. A result above the limit suggests a larger conductor, shorter route, higher voltage, or more parallel conductors. The recommended size is only an estimate. Real installations may need ampacity checks, derating, terminal ratings, conduit fill, voltage tolerance, and protection coordination.
Using Results
Review the voltage drop in volts and percent. Then compare the ending voltage with equipment needs. Check the power loss to understand heat and wasted energy. Use the CSV file for project records. Use the PDF file for quick sharing. Keep notes about assumptions, because conductor temperature and installation method can change the final answer.
Safe Application
This tool supports design review and estimating. It does not replace a licensed electrical professional. Always follow the applicable electrical code. Verify conductor ampacity before installation. Confirm equipment voltage ranges. Check all nameplates. When the project is critical, compare results with manufacturer data and engineered drawings.
Practical Comparison
Try several conductor sizes before choosing one. Compare cost, room in conduit, and future load growth. A slightly larger conductor can reduce losses for many years. It may also improve motor starting and service quality.
FAQs
What is voltage drop?
Voltage drop is the voltage lost as current moves through a conductor. It depends on current, distance, conductor size, material, temperature, and circuit type.
Is this an official Southwire tool?
No. This is an independent estimator inspired by common voltage drop methods. Always verify final designs with project standards and qualified electrical guidance.
Why does distance use one-way length?
Most voltage drop formulas use one-way conductor length. The circuit multiplier accounts for the return path or phase relationship.
Why does wire size change the result?
Larger conductors have more circular mil area. More area lowers resistance. Lower resistance reduces voltage drop and power loss.
Should I use copper or aluminum?
Choose the material used in the actual installation. Copper usually has lower resistance. Aluminum often needs a larger size for similar drop.
What is a good voltage drop limit?
Many designs target about three percent for branch circuits. Some feeders use two or three percent. Follow local code, project rules, and equipment needs.
Does this check conductor ampacity?
No. This calculator estimates voltage drop only. You must still check ampacity, derating, terminal temperature, conduit fill, and protection requirements.
Why include power factor?
Power factor affects AC voltage drop when impedance is considered. Motors and inductive loads often need this input for better estimates.