Voltage Drop Calculator for DC and AC Circuits
Estimate conductor voltage change, load-end voltage and resistive loss. You can also find a theoretical conductor area, maximum one-way run, voltage-drop-limited current or millivolt drop from measured resistance.
Last Updated: August 2, 2026- DC, 1-phase and 3-phase
- AWG, kcmil and mm²
- Power factor and reactance
- Private browser calculation
Online Voltage Drop and Wire Size Calculator
Choose one calculation, enter one-way route length and review every assumption. Display rounding never feeds another result.
The estimated drop is 2.8785% of 230 V and is within your selected 3% voltage-change target.
Result Conversions
Every row uses the same unrounded calculated value.
| Quantity | Unit | Value |
|---|---|---|
| Voltage change | mV | 6620.54 mV |
| Voltage change | V | 6.62054 V |
| Voltage change | kV | 0.00662054 kV |
Calculation Steps
- Convert the one-way length, current, voltage and conductor area to the internal SI basis.
- Adjust copper resistivity from 20°C to the entered conductor temperature.
- Find resistance per metre and apply the equal two-wire single-phase path factor once.
- Calculate the unrounded voltage drop, percentage, load-end voltage and resistive loss.
How to Use This Voltage Drop Calculator
The seven-step check usually takes about two minutes.
- Choose the result. Select voltage drop, theoretical conductor area, maximum length, voltage-drop-limited current or measured-resistance drop.
- Choose the circuit. Identify DC one-conductor, equal two-wire DC, equal-impedance two-wire single-phase AC or balanced three-phase AC. Three-phase voltage must be line-to-line.
- Choose conductor data. Use material and metallic area, or enter cable resistance per length from suitable manufacturer data.
- Enter one-way length and RMS current. Do not double the route. The calculator applies the selected circuit factor once.
- Set the AC model. Use ohmic drop for an I×R estimate. Use the projected model only when power factor and cable reactance are known.
- Enter source voltage and your design target. The percentage result uses the entered source or nominal voltage.
- Review every limit. Check load voltage, conductor loss, assumptions and actual cable data before using the result.
What Voltage Drop Means
Current flowing through conductor impedance creates a voltage difference between the source and the load. A larger current, longer route or higher conductor resistance increases the change. A larger metallic cross-sectional area lowers resistance and usually lowers ohmic drop.
The percentage shown here uses nominal or source voltage:
For a positive drop, the simple load-end estimate is Vload = Vsource − ΔV. A leading AC case can produce a negative projected change. The calculator labels this as voltage rise instead of hiding the sign.
Voltage Drop Formulas Used
In every conductor mode, L is the one-way source-to-load length. Current I is DC current or RMS AC line current. Resistance r and reactance x are per conductor per unit length.
| Circuit or mode | Calculated voltage change | Length rule |
|---|---|---|
| DC, one included conductor | ΔV = I L r | Only the entered conductor is included |
| DC, equal two-wire path | ΔV = 2 I L r | One-way route; return factor is applied |
| Single-phase AC, equal conductor impedances | ΔV = 2 I L r | One-way route; equal outgoing and return impedance |
| Balanced three-phase AC, ohmic | ΔV = √3 I L r | One-way; voltage is line-to-line |
| Single-phase projected R/X | ΔV ≈ 2 I L (r cos φ ± x sin φ) | Plus for lagging; minus for leading |
| Balanced three-phase projected R/X | ΔV ≈ √3 I L (r cos φ ± x sin φ) | Line-to-line first-order estimate |
| Measured total resistance | ΔV = I Rmeasured | No length factor is added |
The AC projected formula assumes sinusoidal steady state, fixed RMS current, stated receiving-end power factor and a balanced three-phase load where selected. It does not solve how a constant-power device, motor or regulated load changes current as terminal voltage changes.
Resistive conductor loss
Reactance changes voltage phase but does not create real I²R heating. The calculator therefore uses resistance only for conductor loss. It applies I²Lr to one included DC conductor, 2I²Lr to a two-wire DC or single-phase circuit and 3I²Lr to a balanced three-phase circuit.
Conductor Resistance, Material and Temperature
For a uniform conductor, resistance per length follows r = ρ/A. The tool stores preset resistivity in Ω·mm²/m and adjusts it from the 20°C reference with:
| Preset | ρ20, Ω·mm²/m | α20, per °C |
|---|---|---|
| Annealed copper, 100% IACS | 0.017241 | 0.00393 |
| Aluminum EC-0, 61.8% IACS | 0.027898 | 0.00408 |
| Aluminum EC-H19, 61% IACS | 0.028264 | 0.00403 |
| Aluminum 5005-H19 | 0.032226 | 0.00353 |
| Aluminum 6201-T81 | 0.032840 | 0.00347 |
These NIST reference values describe identified material conditions. Real cable resistance also depends on alloy, stranding lay, compaction, manufacturing tolerance, skin effect, proximity effect, joints and temperature. Use published AC resistance and reactance for the actual cable when the decision matters.
The temperature equation is a linear reference model. A 20°C default is traceable but often optimistic for a loaded warm conductor. The calculator warns outside its 0 to 100°C reference interval and does not present extrapolation as measured accuracy.
Numeric fields accept at most 15 significant digits. Longer decimal inputs are rejected instead of being silently rounded across a validation boundary.
Wire Size, Maximum Length and Current
The inverse modes rearrange the selected voltage-change formula. For a resistance-only two-wire DC circuit:
The result is a continuous theoretical metallic area. The tool then chooses the first displayed metric or AWG/kcmil size whose nominal area is at least that value. It recalculates the expected drop for the suggested size before displaying it.
Maximum length and current are voltage-drop limits, not safety limits. A cable with acceptable voltage drop can still fail ampacity, terminal-temperature, fault-current, short-circuit, insulation or overcurrent-protection requirements.
AWG and kcmil conversion
AWG nominal diameter follows d = 0.005 × 92(36−n)/39 inch. The tool converts that diameter to circular area. For example, 12 AWG is about 3.30877 mm², while 4/0 AWG is about 107.219 mm². One kcmil is about 0.506707479 mm². A converted area does not make an AWG and metric cable interchangeable.
Worked Voltage Drop Examples
12 V DC two-wire circuit
A 10 A load sits 10 m from the source on 2.5 mm² annealed copper at 20°C. Resistance per metre is 0.017241/2.5 = 0.0068964 Ω/m. The loop resistance is 2 × 10 × 0.0068964 = 0.137928 Ω. Drop is 10 × 0.137928 = 1.37928 V, or about 11.494% of 12 V. Estimated load voltage is 10.62072 V.
230 V single-phase conductor at 75°C
For 20 A, 30 m one-way and 4 mm² copper, temperature-adjusted resistivity is about 0.0209676 Ω·mm²/m. The two-wire resistance is about 0.314515 Ω. The ohmic drop is about 6.29029 V, or 2.73491% of 230 V. Resistive cable loss is about 125.806 W.
400 V balanced three-phase projected drop
Use 50 A, 100 m, R = 0.4 Ω/km, X = 0.08 Ω/km and 0.8 lagging power factor. Since sin φ = 0.6, the effective coefficient is 0.0004×0.8 + 0.00008×0.6 = 0.000368 Ω/m. The projected line-to-line change is √3 × 50 × 100 × 0.000368 ≈ 3.18697 V, or about 0.796743%.
Millivolt drop test
If 40 A flows through a measured 2.5 mΩ path, ΔV = IR gives 0.1 V, or 100 mV. Across a 12 V source this is 0.833333%. Acceptance still depends on the drawing, specification, equipment instructions and measurement procedure.
Accuracy, Assumptions and Common Errors
- Do not double length twice. Enter one-way route length. The factor of two is already built into the equal-conductor DC and single-phase formulas.
- Use line-to-line voltage for balanced three-phase. A phase-to-neutral branch is a two-wire calculation, not the balanced three-phase mode.
- Use RMS AC current. Peak current does not belong in these steady-state formulas.
- Do not invent cable reactance from gauge. Frequency, spacing, geometry and installation affect x.
- Use conductor temperature. Ambient temperature and loaded conductor temperature are not the same.
- Include joints when relevant. Terminations, connectors, splices, busbars and chassis return paths add resistance outside a uniform-wire estimate.
- Model distributed loads by segment. A tapped run does not carry the same current along its full length.
- Check voltage rise. Leading power factor can reverse the projected reactance term. Upper-voltage limits then matter.
The calculator excludes motor starting and inrush, harmonics, nonlinear loads, unbalanced three-phase systems, shared neutrals, parallel-conductor rules, transformer and source impedance, long-line capacitance, earth-return assumptions and changing current in regulated constant-power loads.
Electrical Safety and Design Limits
This page is an educational planning tool. Voltage-drop sizing does not verify ampacity, conductor heating, insulation voltage, terminal temperature, bundling, ambient derating, overcurrent protection, fault-loop impedance, grounding, bonding, short-circuit withstand, conduit fill or local minimum size.
Do not use the calculator as permission to work on energized equipment. De-energize and verify where required. Qualified electrical personnel should handle mains systems, live measurements and consequential designs under the current rules for the installation location.
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Voltage Drop Calculator FAQs
What is the basic voltage drop formula?
For a known total resistive path, voltage drop is ΔV = IR. For equal two-wire DC conductors entered with one-way length, it becomes ΔV = 2ILr, where r is resistance per conductor length.
Should I enter one-way or round-trip wire length?
Enter one-way source-to-load length. The calculator applies factor 2 for equal outgoing and return conductor impedance in two-wire DC and single-phase circuits, factor √3 for balanced three-phase, and factor 1 for the one-conductor DC mode.
How is three-phase voltage drop calculated?
For a balanced three-phase circuit, the ohmic estimate is √3ILr. The projected impedance estimate is √3IL(r cos φ ± x sin φ), using line-to-line voltage and one-way length.
Does power factor affect voltage drop?
Power factor enters the projected AC R/X model. It does not change the separate ohmic I×R conductor-drop result when RMS current is already known. Reactance and leading or lagging direction also matter.
Why does conductor temperature matter?
Copper and common aluminum conductors have higher resistance as temperature rises. The material mode applies a linear reference correction from 20°C, while entered cable-resistance mode assumes your value already represents the relevant condition.
Is 3% voltage drop always required?
No. Three percent is a common editable design target, not a universal legal limit. Use the equipment limits, project specification and electrical rules that apply to your circuit and location.
Does the required area result give a safe wire size?
No. It gives a theoretical metallic area for the entered voltage-change target. You must separately verify ampacity, insulation, terminals, derating, protection, fault performance, installation method and local rules.
What is the difference between voltage drop and power loss?
Voltage drop describes the potential difference along the path. Real conductor loss is I²R and becomes heat. Reactance affects the AC voltage phasor but is not included in real resistive heating.
Why might a measured drop differ from the result?
Actual results can differ because of conductor temperature, cable construction, AC resistance, joints, contact resistance, source impedance, changing load current, harmonics, instrument uncertainty and installation geometry.
Can I use this calculator for a millivolt drop test?
Yes. Choose measured total resistance, enter test current and resistance, then read volts and millivolts. Use a safe approved measurement method, and compare the result with the applicable drawing or specification.
Method and Review Basis
The formulas, reference values, units and limits were reviewed against primary or official engineering sources:
- Seattle Public Utilities electrical design calculations, one-phase and three-phase approximate voltage-drop formulas.
- U.S. Department of Energy Electrical Science, Volume 1, resistance, resistivity and circuit relationships.
- NIST Copper Wire Tables, IACS copper reference values and AWG data.
- NIST Aluminum Wire Tables, alloy-specific resistivity and temperature coefficients.
- NIST SP 811 conversion factors, length, area and circular-mil conversions.
- NIOSH Electrical Safety in the Workplace, qualified-person and energized-work guidance.
Electrical disclaimer: Use this calculator for voltage-drop planning and education. Voltage-drop sizing does not verify ampacity, protection, installation method or code compliance. Check the actual cable data and applicable local rules, or consult a qualified electrical professional.