Voltage Drop Calculator
Calculate voltage drop for a given current and wire resistance. Enter circuit values to calculate the selected electrical quantity.
Parameters
Electronics guide · Electrical power, energy, and wiring
About Voltage Drop Calculator
The Voltage Drop Calculator estimates how much voltage is lost across a wire resistance at a chosen load current. It also reports the remaining load voltage, percentage drop, and power dissipated in the wire, helping show why conductor resistance matters.
Power and energy calculations relate quantities such as voltage, current, resistance, time, and load. AC and DC systems can require different assumptions, and a calculated estimate is not a substitute for checking source capability, wire ratings, protection, efficiency, and equipment specifications.
How to Use This Tool
- 1.Enter source voltage and load current in the indicated units.
- 2.Enter the total resistance of the wire path, including both conductors if the circuit's outgoing and return paths are relevant.
- 3.Calculate and review voltage loss, voltage at the load, percentage, and wire power loss. Compare the result with the equipment and applicable wiring guidance.
Technical Details to Check
1.Check AC or DC assumptions
In AC calculations, voltage and current may refer to RMS values, and phase relationships can affect real power. Do not apply a DC relationship to an AC system unless the tool's stated assumptions fit.
2.Power is not the same as energy
Watts describe a rate of energy use; watt-hours and joules describe an amount of energy over time. A runtime or consumption estimate needs both a load and a duration or duty profile.
3.Real systems lose energy
Inverters, batteries, wiring, motors, and power supplies are not perfectly efficient. Confirm whether the selected calculation includes efficiency, power factor, voltage drop, or only ideal values.
4.Ratings and protection are essential
An estimated current or power value does not determine a safe wire size, fuse, battery connection, or supply. Follow applicable standards, product ratings, and qualified design guidance.
How the Calculation Works
Voltage drop is Vdrop=I×R. Load voltage is source voltage minus drop, percent drop is Vdrop/source voltage×100, and wire loss is I²R. The entered resistance should represent the complete path being modeled.
Worked Example
At 5 A through a total wire resistance of 0.1 Ω, drop is 0.5 V. A 12 V source would leave about 11.5 V at the load before other losses.
Where This Tool Can Help
Estimate low-voltage wiring losses, check a planned DC run, and understand how current or wire resistance affects load voltage.
Estimating a load
Use known voltage and current or power figures to understand a device's approximate demand before checking its label and startup behavior.
Planning energy use
Combine a power estimate with realistic operating hours to compare possible consumption, while accounting separately for standby use and efficiency.
Checking a wiring estimate
Use current or voltage-drop results as an early check, then verify conductor size and protection with the correct installation rules and environment.
Comparing supply options
Compare candidate sources on the same voltage, current, power, and duration basis, and include headroom where the equipment specification requires it.
Tips for More Reliable Results
- Confirm whether voltage and current are RMS, peak, nominal, or measured under load.
- Keep watts, watt-hours, joules, and time units distinct when entering values.
- Include efficiency or power factor only when the tool provides that input and you have a suitable value.
- Check conductor, connector, battery, and supply ratings before using an estimate in a real installation.
Limitations and Assumptions
Real wire resistance changes with conductor material, length, cross-section, temperature, connections, and AC frequency. The calculator does not certify wire size or safety. Follow local electrical codes and qualified guidance.
Electrical power tools estimate values from entered figures and stated assumptions. They do not measure a circuit, model every transient or loss, or certify wiring and equipment as safe. Confirm important designs with manufacturer data and applicable electrical requirements.
Frequently Asked Questions
Should resistance include both wires?
Use the total resistance of the outgoing and return path for a two-wire circuit.
Why does higher current increase the drop?
Ohm's law gives V=I×R, so a larger current through the same resistance creates a larger voltage loss.
Is a 5% threshold always acceptable?
No. Acceptable drop depends on the system, standard, equipment, and application.
Are watts and watt-hours interchangeable?
No. Watts are a rate; watt-hours measure energy used over time.
Does a calculated value make a circuit safe to build?
No. Safety depends on ratings, protection, installation conditions, and applicable standards as well as the arithmetic.