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Every cable has resistance, so some voltage is lost along the way — a bigger issue on long runs, high currents and thin conductors. This calculator estimates that drop for a single-phase copper or aluminium run from the current, the one-way length and the conductor’s cross-section, and gives the voltage that actually reaches the load.
How is it calculated?
The calculation
Resistance of the run, then Ohm’s law:
R = ρ × (2 × length) ÷ area, then Vdrop = current × R
The 2× accounts for both conductors (out and back). ρ is the material’s resistivity:
| Conductor | Resistivity (Ω·mm²/m) |
|---|---|
| Copper | 0.0172 |
| Aluminium | 0.0282 |
Aluminium has about 60% higher resistance for the same size, so it drops more voltage.
Percentage drop and the 3% guideline
The percentage drop is Vdrop ÷ source voltage × 100. A common rule of thumb keeps voltage drop under about 3% for branch circuits (and ~5% overall), so equipment gets enough voltage. Exceed it and you fix it by using a larger conductor, a shorter run or a higher supply voltage.
Worked example
A 10 A load on a 50 m one-way run of 4 mm² copper: resistance is 0.0172 × (2 × 50) ÷ 4 = 0.43 Ω, so the drop is 10 × 0.43 = 4.3 V. On a 230 V supply that’s 1.87% — within the 3% guideline, leaving 225.7 V at the load. Switch to aluminium and the drop rises to about 7 V.
FAQ
How is voltage drop calculated?+
Find the run’s resistance — resistivity × (2 × one-way length) ÷ cross-section — then multiply by the current (Ohm’s law). The 2× covers the outgoing and return conductors. This tool does it for copper or aluminium.
What is an acceptable voltage drop?+
A widely used guideline keeps it under about 3% on a branch circuit and around 5% total from source to load, so equipment receives adequate voltage. Requirements vary by code and application, so treat this as a rule of thumb.
How do I reduce voltage drop?+
Use a larger conductor cross-section, shorten the cable run, reduce the current, or supply at a higher voltage. Increasing the conductor size is the most common fix, since drop is inversely proportional to cross-sectional area.
Why does aluminium drop more than copper?+
Because aluminium has a higher resistivity — about 0.0282 versus 0.0172 Ω·mm²/m for copper, roughly 60% more. For the same size and length it therefore has more resistance and a larger voltage drop, which is why aluminium runs are often sized up.
Does this cover three-phase systems?+
This estimate is for a single-phase two-wire run (the 2× length term). Three-phase drop uses a different factor (√3 instead of 2) and balanced-load assumptions, so for three-phase designs use a method suited to that.