Voltage Drop Calculator
Enter wire size, length and current to see the voltage lost along a cable.
What this tool does
A voltage drop calculator estimates voltage loss based on current, conductor properties, distance and circuit type. On a single-phase circuit the drop is 2 × resistivity × one-way length × current ÷ cross-section, and on three-phase it is √3 × resistivity × length × current ÷ cross-section. A 50 ft run of 12 AWG copper carrying 20 A at 120 V loses about 3.2 V, or 2.65% of the supply.
Estimates voltage loss based on current, conductor properties, distance and circuit type.
Supports single-phase and three-phase circuits with the correct formula for each.
Shows the result as both absolute voltage drop and percentage of supply voltage.
Reports the voltage left at the load and the power lost as heat in the conductor.
How to use it
- Choose copper or aluminium, and the wire size in AWG or a custom cross-section in mm².
- Enter the one-way length of the cable in feet or metres.
- Enter the current in amps, or the watts and volts if you know the load instead.
- Enter the supply voltage and choose single-phase or three-phase.
- Read the drop in volts, its percentage, the voltage at the load and the power lost.
Why use this calculator
Estimates voltage loss based on current, conductor properties, distance and circuit type, because a cable that is fine on ampacity can still be wrong for the job. Every conductor has resistance, so current flowing along a run turns some of the supply into heat and leaves less at the far end. On a long run to a shed, garage, pump, pool light or EV charger that loss is large enough to make equipment run hot, run slowly or not start at all.
Why we built it
We built the voltage drop calculator to cover copper and aluminium, AWG or mm², and single and three phase, and to show the heat lost in the cable as well as the volts. The wire size calculator runs the same physics forwards to pick a conductor, and the Ohms Law calculator covers the underlying relationship between voltage, current and resistance.
If your task goes further, try the watts to amps converter to turn a load in watts into the current this calculation needs, and the Power Factor Calculator when the load is AC and inductive. For apparent versus real power on three-phase kit there is the kVA to kW converter.
Who this tool is for
- Homeowners running cable to a shed, garage or garden
- Electricians and installers checking a long run before it is hidden
- Solar, DC and EV installation planning
- Electronics students learning why cable length matters
When to use it
- Checking a cable to a garage, shed or outbuilding.
- Choosing a wire size for a pump, heater or EV charger run.
- Estimating the loss in a low-voltage lighting circuit.
- Verifying an existing installation that behaves oddly at the end of a run.
How it works
The resistance of a conductor is resistivity × length ÷ area. Copper is about 1.724 × 10⁻⁸ Ω·m and aluminium about 2.82 × 10⁻⁸ Ω·m at 20 °C. On a single-phase circuit the current flows out and back, so the drop is 2 × ρ × L × I ÷ A. On three-phase it is √3 × ρ × L × I ÷ A, because the return path is shared between the other two phase conductors rather than doubling one wire. AWG cross-sections come from the standard diameter formula, 0.127 mm × 92^((36 − n) ÷ 39).
Worked example: 12 AWG copper (3.31 mm²), 50 ft (15.24 m) one way, 20 A, 120 V single-phase. The drop is 2 × 1.724 × 10⁻⁸ × 15.24 × 20 ÷ 3.31 × 10⁻⁶ = 3.18 V, which is 2.65% of the supply, leaving 116.82 V at the load. The resistance of both conductors together is 0.159 Ω, so the cable dissipates about 63.5 W as heat - energy you paid for and no one used.
A drop of about 3% on a final circuit and 5% overall are the usual design targets, with local wiring rules taking precedence. This calculation ignores temperature rise, reactance and power factor; have fixed wiring checked by a qualified electrician.
Single-phase versus three-phase
The difference is the return path. In a single-phase circuit the full current leaves through one conductor and returns through the neutral or earth-side conductor, so both conductors contribute and the length effectively doubles - hence the factor of 2. In a three-phase circuit there is no dedicated return for balanced loads: current splits across the other phases, and the combined effect across the run works out at √3 times a single conductor's drop.
| Circuit | Formula | Example (20 A, 15.24 m, 3.31 mm² Cu) |
|---|---|---|
| Single-phase DC or AC | 2 × ρ × L × I ÷ A | 3.18 V |
| Single-phase, two-way run | 2 × ρ × L × I ÷ A | 3.18 V (length is one way) |
| Three-phase | √3 × ρ × L × I ÷ A | 2.74 V |
| Three-phase, same cable and current | √3 × ρ × L × I ÷ A | About 14% less drop than single-phase |
What you give it, and what you get back
| Item | Details |
|---|---|
| Material and size | Copper or aluminium, with an AWG size or custom cross-section in mm². |
| Length | One-way cable length, in feet or metres. |
| Current and supply | Current in amps and the supply voltage. |
| Circuit type | Single phase or three phase. |
| Output | Voltage drop in volts and percent, voltage at the load, resistance and power lost. |
A worked example
A worked example you can repeat in the calculator above. Change the size, the length or the current and every figure updates from the same formula.
| Input | Value |
|---|---|
| Conductor material | Copper |
| Wire size | 12 AWG (3.31 mm²) |
| One-way length | 50 ft (15.24 m) |
| Current | 20 A |
| Supply voltage | 120 V |
| Supply | Single phase |
With those inputs, the calculator returns:
- Voltage drop: 3.18 V (2.65%)
- Voltage at the load: 116.82 V
- Resistance of both wires: 0.159 Ω
- Power lost in the cable: 63.5 W
How cable length and gauge change the answer
Both effects are linear, which makes them easy to reason about. Double the length and the drop doubles - 100 ft of the same cable at 20 A loses 6.4 V instead of 3.2 V. Double the cross-section and the drop halves, because resistance falls as area rises. That is why upgrading one gauge step is often enough on a marginal run, and why a slightly thicker cable over a very long run costs little more for a large improvement.
| Wire size | Cross-section | Voltage drop | Percentage of 120 V |
|---|---|---|---|
| 14 AWG | 2.08 mm² | 5.05 V | 4.21% |
| 12 AWG | 3.31 mm² | 3.18 V | 2.65% |
| 10 AWG | 5.26 mm² | 2.00 V | 1.67% |
| 8 AWG | 8.37 mm² | 1.26 V | 1.05% |
Aluminium needs about 1.64 times the cross-section of copper for the same drop, so 12 AWG aluminium behaves more like 16 copper in this table. Terminations and joint practice matter as much as the conductor itself, which is why aluminium is restricted to larger sizes in many wiring rules.
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| The drop is too high | The cable is thin or long for the current. | Use a thicker conductor, a shorter run, or a higher supply voltage. |
| I measure a different drop | Joints, warm cable and load changes affect the real value. | Treat the result as an estimate and measure under load. |
| Aluminium needs a bigger size | Aluminium has higher resistivity than copper. | That is expected - about 1.6 times the area for the same drop. |
| Three-phase result is lower than expected | The √3 formula applies instead of doubling. | Choose the right circuit type before comparing. |
Assumptions and limitations
- DC and unity power factor AC only - reactance, harmonics and power factor are not modelled.
- Resistivity is quoted at 20 °C; a hot cable has more resistance and a slightly larger drop.
- One-way length is what the calculation expects; entering the round-trip distance doubles the result.
- An estimate for planning, not a substitute for the wiring rules that apply where you live.
Frequently asked questions
How do I calculate voltage drop?
For a single-phase circuit: 2 × resistivity × one-way length × current ÷ conductor cross-section. For three-phase, replace the 2 with √3. Copper's resistivity is about 1.724 × 10⁻⁸ Ω·m and aluminium's about 2.82 × 10⁻⁸ Ω·m, and the result is expressed in volts or as a percentage of the supply.
What is acceptable voltage drop?
Around 3% on a branch circuit and about 5% from the supply to the furthest point are the common design targets, including what the National Electrical Code recommends as informational guidance. Lighting on long runs is often held to the tighter figure, and low-voltage DC systems usually need an even stricter allowance because the same loss is proportionally more damaging.
What is the difference between single-phase and three-phase?
In single-phase the current returns through one conductor, so the calculation uses a factor of 2 and the one-way length. In three-phase the return is shared across the other phases, giving a factor of √3 instead - about 14% less drop for the same cable, current and length. Choose the circuit type before comparing results.
How does cable length affect voltage drop?
Directly in proportion: resistance grows with length, so doubling the run doubles the drop. It is the one-way length that counts on single phase, because the current makes the journey both ways. This is why drop, not ampacity, usually decides the size on long feeders to sheds, pumps and chargers.
How does wire gauge affect voltage drop?
Inverse to the cross-sectional area: double the area and the drop halves. A step from 14 AWG to 12 AWG cuts the drop on a 50 ft, 20 A run from about 5.1 V to 3.2 V, and 10 AWG brings it to 2.0 V. Aluminium needs roughly 1.6 times the area of copper to achieve the same reduction.
What happens if voltage drop is too high?
Lights dim at the end of the run, motors run hotter with less torque and can fail to start, chargers slow down, and electronic equipment may reboot or misbehave. The missing voltage appears as heat in the cable itself, so sustained excess drop wastes energy and can push the conductor and its terminations beyond their temperature rating.