Voltage Drop Calculator
Estimate cable or wire voltage drop, delivered load voltage, resistance, and I²R power loss. Use one-way or round-trip conductor paths for DC power distribution, automotive wiring, battery systems, and low-voltage hardware.
Engineering tool
Voltage Drop Calculator
Estimate cable resistance, voltage loss, delivered load voltage, and conductor power dissipation.
Voltage at the source before cable loss.
Use the worst-case steady or RMS load current.
One-way distance between the supply and load.
Conductor resistance at the expected operating temperature.
Round trip includes both supply and return conductor resistance.
Result console
- Wire resistance
- 80mΩ
- Voltage drop
- 400mV
- Load voltage
- 11.6V
- Power loss
- 2W
- Voltage drop percentage
- 3.333333%
- Conductor factor
- 2conductors
Voltage drop status
Warning
The estimated drop is between 3% and 5%. Consider a shorter cable, larger conductor, or lower-resistance connection.
Formula reference
Wire Voltage Drop Formulas
Wire length is the one-way source-to-load distance. The conductor factor accounts for the electrical return path.
Rwire = resistance per length × length × conductor factorVdrop = I × RwireVload = Vsupply - VdropPloss = I² × RwireDrop percentage = Vdrop / Vsupply × 100%Variable definitions
- Rwire
- total circuit wire resistance
- I
- load current
- Conductor factor
- 1 for one-way/chassis return or 2 for round trip
- Vsupply and Vload
- source and delivered load voltage
Worked Example
For a 12 V supply, 5 A load, 2 m one-way wire length, 0.02 Ω/m conductor resistance, and round-trip factor of 2:
Rwire = 0.02 × 2 × 2 = 0.08 Ω. Vdrop = 5 × 0.08 = 0.4 V. The load receives 11.6 V, wire loss is 2 W, and drop is approximately 3.33%.
Engineering Notes
- Wire voltage drop rises with current, conductor resistance, and total circuit length.
- Round-trip calculations normally include both supply and return conductors.
- Connector, fuse, switch, and terminal resistance should be added for worst-case analysis.
- Conductor resistance increases with temperature and manufacturing tolerance.
- Low-voltage loads are especially sensitive to the same absolute voltage drop.
- Use peak current and suitable design margin when wiring motors, radios, and pulsed loads.
Support reference
FAQ
How do you calculate wire voltage drop?
Multiply resistance per length by one-way length and the conductor factor, then multiply the resulting wire resistance by load current: Vdrop = I × Rwire.
Why does a round-trip calculation use two conductors?
Current travels from the source to the load and returns to the source. When both paths use wire, their resistances add, so the conductor factor is two.
What is an acceptable voltage drop?
The limit depends on the load, but 3% is a common design target and 5% is often a practical upper screening limit. Sensitive low-voltage loads may require less.
How can I reduce cable voltage drop?
Use a larger conductor, shorten the cable, reduce current, improve connectors, or raise distribution voltage where the system permits.
Does wire resistance change with temperature?
Yes. Metal conductor resistance generally rises with temperature. Use resistance per length at the expected operating temperature or include suitable margin.
Documentation
Related Engineering Guides
Design notes, guides, and engineering articles linked to this tool.
Engineering Guide
Power Dissipation and Component Power Ratings
Calculate actual component dissipation and choose safe power ratings using thermal resistance, derating, junction temperature, SOA, pulse conditions, PCB heat spreading, and real operating conditions.
20 min · Intermediate
Engineering Blog
10 Common Power Design Mistakes (and How to Avoid Them)
Avoid power design mistakes involving watts versus watt-hours, unit prefixes, RMS current, AC power factor, efficiency, derating, thermal design, startup current, voltage drop, batteries, PCB copper, and undersized components.
17 min · Intermediate
Engineering Guide
Understanding Electrical Power
Learn electrical power fundamentals, including voltage, current, resistance, watts, energy, Ohm's law, DC and AC power, efficiency, power dissipation, and practical calculations.
18 min · Beginner
Related Calculators
Power Calculator
Calculate voltage, current, or electrical power.
AvailablePower Dissipation Calculator
Calculate component power loss and derating margin.
AvailablePCB Voltage Drop Calculator
Calculate PCB trace voltage loss from copper geometry.
AvailablePCB Trace Width Calculator
Estimate required PCB trace width for current.
AvailableOhm's Law Calculator
Calculate voltage, current, resistance, and power.