Wire Voltage Drop Calculator
Calculate the voltage drop and power loss in a wire run based on current, wire length, and conductor size in AWG or mm².
How to Use This Calculator
Enter current in amps, the one-way wire length in metres, and either select a standard AWG gauge from the dropdown or enter the cross-section area in mm². Optionally add supply voltage to see the drop as a percentage. The calculator uses round-trip length (×2) automatically.
Formula
ρ = resistivity of copper (1.72 × 10⁻⁸ Ω·m), L = total wire length in metres (round trip = 2× one-way), A = cross-section area in m², I = current in amps.
Example
R_wire = (1.72×10⁻⁸ × 20) / (2.081×10⁻⁶) ≈ 0.165 Ω
V_drop = 5 × 0.165 ≈ 0.83 V
P_loss = 5² × 0.165 ≈ 4.1 W
Frequently Asked Questions
Why does wire length matter for voltage drop?
Resistance is proportional to length (R = ρL/A). Longer wires have more resistance, causing a greater voltage drop at the same current. This is why high-current loads should always be close to their power source, or served by thicker cable.
Why does the calculator double the wire length?
Current flows out through one conductor and returns through another. The round-trip path is twice the one-way distance, so both conductors contribute resistance. Most voltage drop calculators expect one-way length for this reason.
What is an acceptable voltage drop?
For 12V automotive and solar systems, a drop of 3% or less (0.36V) is typically acceptable. For sensitive electronics, keep it below 1–2%. Mains wiring standards usually require ≤3–5% drop across the full circuit.
What if I'm using aluminium wire?
Aluminium has a higher resistivity (2.65 × 10⁻⁸ Ω·m) than copper — about 60% more resistance for the same cross-section. Aluminium is common in mains distribution but rarely used in low-voltage DC circuits.