Voltage drop calculator
Single- or three-phase, copper or aluminium, metric or US units (AWG). Enter the load current, run length and conductor size — the drop in volts and percent updates as you type.
Common design limits: ≤3% for lighting, ≤5% for power circuits. Always check the limit in your local standard.
NEC recommendation (informational notes to 210.19 and 215.2): up to 3% on a branch circuit or feeder, and 5% for feeder and branch circuit combined. It is a recommendation, not a requirement — check your local amendments.
Voltage drop formula
ΔU = k · I · L · (ρ/S · cos φ + x · sin φ), where k = 2 for single-phase and √3 for three-phase, ρ is the conductor resistivity (copper 0.0225, aluminium 0.036 Ω·mm²/m at operating temperature), x ≈ 0.00008 Ω/m the reactance per metre, L the one-way length and S the cross-section. The percentage is ΔU divided by the nominal voltage. Figures are indicative — confirm against your project conditions and local standard.
ΔU = k · I · L · (R · cos φ + X · sin φ) / 1000, where k = 2 for single-phase (120 V or 240 V) and √3 for three-phase, L is the one-way length in feet, R the conductor resistance at 75 °C from NEC Chapter 9, Table 8 (Ω per 1,000 ft) and X ≈ 0.05 Ω per 1,000 ft a typical reactance for conductors in conduit. With a power factor of 1 this is the familiar 2 · K · I · L / CM method. Figures are indicative — confirm against your project conditions and the code edition adopted where you work.
Worked example
A 16 A single-phase load over 25 m of 2.5 mm² copper, at 230 V and cos φ 0.95: ΔU ≈ 6.9 V — about 3.0%, within the 5% limit for power circuits. Halve the length or double the cross-section and the drop roughly halves.
A 20 A load at 120 V on 12 AWG copper, 100 ft one way, power factor 1: ΔU = 2 × 20 × 100 × 1.93 / 1,000 ≈ 7.7 V — 6.4%, well over the 3% recommendation. 10 AWG brings it to about 4.8 V (4.0%), 8 AWG to about 3.1 V (2.5%).
Frequently asked
What is an acceptable voltage drop?
As a common design rule, keep it within about 3% for lighting and 5% for power circuits, from the origin of the installation to the load. In the US, the NEC informational notes recommend 3% for a branch circuit or feeder and 5% for both combined. Always confirm the exact limit in your local standard.
How is voltage drop calculated for three-phase?
Use ΔU = √3 · I · L · (R·cos φ + X·sin φ), where R = ρ/S is the resistance per metre; for single-phase the factor √3 becomes 2. The percentage is ΔU divided by the nominal voltage.
Copper or aluminium — how much does it matter?
Aluminium has roughly 60% higher resistivity than copper (about 0.036 vs 0.0225 Ω·mm²/m), so for the same cross-section its voltage drop is higher. Compensate with a larger aluminium cross-section.
How can I reduce the voltage drop?
Increase the conductor cross-section, shorten the run, split the load across circuits, or supply at a higher voltage. Cross-section and length have the largest effect.
Need the whole set drawn?
Send us the architect's floor plan — we return the complete drawing set and the bill of quantities as editable, unstamped DWG. You review, stamp and sign under your own licence.