Voltage Drop Calculator
Estimate voltage drop, drop percentage, load-end voltage, and cable resistance for DC, single-phase AC, and three-phase AC wiring checks.
Enter values to estimate voltage drop.
What is a voltage drop calculator?
A voltage drop calculator estimates how much voltage is lost between the source and the load. In a perfect world, a 120 V supply would deliver exactly 120 V at the device. In real wiring, the wire has resistance, so some voltage is lost along the way. The longer the wire, the smaller the conductor, and the higher the current, the more voltage drop you can expect.
This tool is useful for checking long cable runs, low-voltage DC wiring, outdoor circuits, temporary power layouts, lighting runs, pumps, small solar wiring checks, and general electrical learning. It converts the circuit inputs into four practical outputs: voltage drop in volts, percentage drop, estimated voltage at the load, and total wire resistance used in the formula.
CalcMora keeps this tool in the Converters section because it converts wire size, length, current, voltage, and conductor resistance into electrical loss values. If you are also working with length measurements, try the cm to inches calculator or the mm to inches converter. For non-electrical conversions, the Roman numerals converter can help with labels, clocks, outlines, and design notes.
Voltage drop formula
The calculator is based on the relationship between current, resistance, and voltage. The simple idea is that voltage drop equals current multiplied by total conductor resistance. The total conductor resistance depends on cable length, wire material, wire size, and circuit type.
DC and single-phase AC
Voltage drop = I × 2 × L × R ÷ 1000
The factor 2 accounts for the outgoing and returning path. Enter the one-way length only.
Three-phase AC
Voltage drop = 1.732 × I × L × R ÷ 1000
The factor 1.732 is the square root of 3, used for a three-phase line-to-line estimate.
In these formulas, I means current in amps, L means one-way length in feet, and R means wire resistance in ohms per 1000 feet. If you enter meters, the calculator changes the length to feet before calculating. If you enter a custom resistance value, the calculator uses that value instead of the built-in wire table.
How to use this voltage drop calculator
Select DC, AC single phase, or AC three phase. This changes the circuit factor used in the formula.
Enter the source voltage and the expected load current in amps. Higher current usually creates more voltage drop.
Enter only the distance from the source to the load. Do not double it for DC or single-phase circuits.
Choose copper or aluminum and select the wire size. Larger wires usually have lower resistance.
Check voltage drop, drop percentage, load voltage, and the status message before comparing other wire sizes.
Worked examples
Example 1: 120 V single-phase circuit
Suppose a 120 V single-phase load draws 15 amps through 100 feet of 12 AWG copper wire. The calculator uses the 12 AWG copper resistance, applies the factor of 2, and estimates how many volts are lost before the power reaches the load. If the percentage is high, you can compare a larger wire size or shorter cable run.
Example 2: 480 V three-phase load
Suppose a 480 V three-phase load draws 30 amps through a long feeder. The calculator uses the square-root-of-three factor instead of the round-trip factor. A higher system voltage often keeps the percentage drop lower, but long distance and high current can still create a meaningful loss.
Important notes before using the result
This calculator is for educational purposes and early planning. It does not check every safety rule, and it does not replace a licensed electrician. Real electrical design can depend on wire temperature, insulation type, conduit, cable bundling, breaker size, voltage rating, terminal limits, equipment instructions, and local electrical code.
Voltage drop is only one part of wire selection. A wire can have a low voltage drop but still be unsafe if it does not meet ampacity rules. For real installations, confirm both voltage drop and ampacity with official code tables and qualified guidance.
Voltage drop calculator FAQ
What does a voltage drop calculator do?
A voltage drop calculator estimates how much voltage is lost as current moves through a wire or cable. It uses source voltage, current, one-way cable length, conductor material, wire size, and circuit type to estimate voltage drop in volts, voltage drop percentage, and the voltage that may remain at the load. This is useful for early electrical planning, comparing wire sizes, and checking whether a long cable run may create too much loss.
Why does voltage drop happen?
Voltage drop happens because every wire has resistance. Copper and aluminum are good conductors, but they still resist current slightly. When current passes through that resistance, part of the source voltage is lost along the cable. The drop becomes larger when the wire is longer, the current is higher, the wire is smaller, or the conductor has higher resistance. That is why a short cable run and a larger wire size usually reduce voltage drop.
What is a good voltage drop percentage?
Many simple planning guides aim for about 3% or less for branch circuits and about 5% or less for some longer combined runs. The right target depends on the device, installation, local rules, and equipment instructions. Sensitive electronics, motors, pumps, and long-distance loads may need a lower drop. This calculator gives an estimate only, so important electrical work should still be checked by a qualified professional.
Does this calculator work for DC and AC circuits?
Yes. This calculator supports DC, single-phase AC, and three-phase AC. DC and single-phase AC use a round-trip factor because current travels through an outgoing and return path. Three-phase AC uses the square root of 3 factor for a line-to-line estimate. The calculator changes the formula automatically when you select the circuit type.
Should I enter one-way length or total cable length?
Enter the one-way distance from the power source to the load. The calculator applies the circuit factor internally. For DC and single-phase AC, it accounts for the return path by using a factor of 2. For three-phase AC, it uses a factor of 1.732. If you enter the round-trip length by mistake, the result for DC or single-phase circuits will be too high.
Can this calculator choose the final wire size for my project?
No. This calculator is for education and rough planning only. It estimates voltage drop, but it does not check every safety rule. Real wire sizing also depends on ampacity, insulation rating, breaker size, cable temperature, conduit fill, installation method, terminal limits, and local electrical code. For real wiring work, use official rules and ask a licensed electrician or qualified electrical professional.
This tool is for educational purposes only. Always verify important results with a qualified professional.
