How to use this wire size calculator
Enter the load current, one-way run length, system voltage, and acceptable voltage drop. The calculator returns the smallest AWG conductor that satisfies both the current-carrying capacity (ampacity) and your voltage-drop limit — plus the nearest European standard metric size (mm²) shown alongside for cross-reference. A wire thick enough to carry the current isn’t always thick enough to keep voltage drop acceptable over a long run.
AWG and mm² — same physics, different labels
North America specifies wire by American Wire Gauge (AWG) — a numbering system where larger numbers mean thinner wire (14 AWG is small, 4/0 AWG is huge). Continental Europe and most of the world specify cables by cross-sectional area in square millimeters: 1.5, 2.5, 4, 6, 10, 16, 25 mm² and so on. The physics is identical; only the naming differs. This calculator gives you the AWG answer with the nearest metric equivalent so European electricians can translate to a standard cable size on the shelf.
Why voltage drop matters
Every conductor has resistance, so some voltage is “lost” along the run. A common rule of thumb is to keep voltage drop under 3% on a branch circuit (5% total for feeder plus branch). Excessive drop causes dim lighting, motors that run hot or fail to start, and controllers that behave erratically. On long runs, voltage drop — not ampacity — is usually what forces you up to a larger wire.
The formula
For a single-phase or DC circuit, voltage drop is calculated as:
Vdrop = 2 × I × L × (ρ / A)
where I is current in amperes, L is one-way length in meters, ρ (rho) is the resistivity of the conductor material (about 0.0225 Ω·mm²/m for copper at operating temperature, 0.036 for aluminium), and A is the conductor’s cross-sectional area in mm². The factor of 2 accounts for the current travelling out and back. For three-phase, the multiplier is √3 instead of 2.
Copper vs aluminium
Aluminium has roughly 1.6× the resistance of copper for the same cross-section, so an aluminium conductor needs to be about two AWG sizes larger to match a copper one. Aluminium is lighter and cheaper for large feeders but requires terminations specifically rated for aluminium contact.
Important limitations
This calculator is an educational estimate. It uses generic 75°C ampacity figures and does not apply temperature correction, conduit-fill or grouping derating, terminal temperature ratings, or continuous-load factors (the 125% rule). Real installations must follow the applicable wiring code — NEC in the US, IEC 60364 / BS 7671 / LST EN in Europe — and be verified by a qualified electrician.