Voltage Drop Calculator — Wire & Cable Loss
Calculate how much voltage is lost in a cable run by entering the conductor material, cross-sectional area, one-way length and load current. See the remaining voltage at the load and whether the design meets the 3–5 % engineering guideline.
Conductor material
mm²
m
A
V
Potential lost in the cable conductors due to resistance
- 1
Cross-section in m²
2.5 × 10⁻⁶ = 0.0000025Convert mm² to m² for SI resistivity calculation. - 2
Round-trip resistance R
2 × 0.00000001724 × 30 ÷ 0.0000025 = 0.41376 - 3
Voltage drop V = I × R
16 × 0.41376 = 6.620
How does this calculator work?
Voltage drop in a cable = 2 × ρ × L × I / A, where ρ is resistivity (copper: 1.724×10⁻⁸ Ω·m), L is one-way length (m), I is current (A) and A is cross-section (m²). NEC/IEC guidelines say keep drop below 3 % for branch circuits and 5 % total. Use a larger conductor or shorter run to reduce drop.
Formula
How this is calculated
When current flows through a conductor it meets resistance, and some of the supply voltage is spent overcoming that resistance rather than doing useful work at the load. The resistance of a round-trip cable run (outward and return conductors) is R = 2 × ρ × L / A, where ρ is the material's resistivity in Ω·m, L is the one-way length in metres and A is the cross-sectional area in m². The voltage drop is then V = I × R.
Copper has a resistivity of 1.724 × 10⁻⁸ Ω·m at 20 °C; aluminium is about 54 % more resistive at 2.65 × 10⁻⁸ Ω·m. Larger cross-sections and shorter runs reduce resistance proportionally, so doubling the wire area halves the drop. These resistivity values assume 20 °C; resistance rises roughly 0.4 % per °C for copper, so a hot cable (60 °C) has about 16 % higher resistance than the room-temperature value shown.
NEC (National Electrical Code, US) and most IEC-based standards recommend total voltage drop — including from the service panel to the final outlet — should not exceed 5 %, with branch circuit drops ideally below 3 %. Excessive drop wastes energy as heat in the cable, dims lights, causes motor heating, and can reduce equipment performance or lifespan.
Frequently asked questions
Because electricity flows from the source to the load and must return along a separate conductor. Both the outward ("line" or "phase") and return ("neutral") conductors carry the same current and each contributes resistance. A 30 m cable run therefore produces the same drop as 60 m of a single conductor.
Most electrical codes (NEC Article 210, IEC 60364-5-52) recommend a maximum total voltage drop of 5 % from the supply point to the point of use, with a suggested limit of 3 % for branch circuits alone. Drops above 5 % cause efficiency losses, motor heating and premature equipment failure. Data and sensitive electronic circuits often require <1 % to prevent errors.
Use a larger conductor cross-section (drop is inversely proportional to area), shorten the cable run, increase the supply voltage (if permitted by the equipment), or use copper instead of aluminium for the same gauge. In long runs — for example, feeding a sub-panel in an outbuilding — running at higher voltage (e.g. 400 V three-phase) and transforming down at the load end is far more efficient.
Also known as
TG we-Calculate Editorial Team. (2026). Voltage Drop Calculator — Wire & Cable Loss [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/voltage-drop-calculator
TG we-Calculate Editorial Team. "Voltage Drop Calculator — Wire & Cable Loss." TG we-Calculate. 2026. https://we-calculate.com/calculator/voltage-drop-calculator.
TG we-Calculate Editorial Team, "Voltage Drop Calculator — Wire & Cable Loss," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/voltage-drop-calculator
@misc{wecalculate_voltage_drop_calculator, title = {Voltage Drop Calculator — Wire & Cable Loss}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/voltage-drop-calculator}}, year = {2026}, note = {TG we-Calculate} }
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