Where the drop comes from
Every conductor has resistance, and current through it drops voltage — plain Ohm's law applied to the wire. Over a run of length L the drop is:
Vdrop = ρ × (2 × L) × I / A
ρ is the resistivity (copper ≈ 0.0172 Ω·mm²/m, aluminum ≈ 0.0265), L is the one-way length — the current has to get there and back, hence the 2 — I is the current, and A the conductor's cross-sectional area. Thicker wire (bigger A) drops less; longer runs and higher currents drop more.
Worked example
10 A down a 5 m run of 12 AWG copper (≈ 3.31 mm²): Vdrop = 0.0172 × 10 × 10 / 3.31 ≈ 0.52 V — about 4.3% of a 12 V supply, and 5.2 W wasted as heat in the cable. Step up to 10 AWG (≈ 5.26 mm²) and the drop falls to ~0.33 V (2.7%).
How much drop is acceptable?
- 3% — the usual target for branch circuits feeding a load.
- 1–2% — sensitive electronics, long sensor runs, solar charge lines where end voltage matters.
- 5%+ — sometimes fine for lighting or short intermittent loads.
Low-voltage systems feel drop the most: 0.5 V is nothing on 120 V but is over 4% of a 12 V battery, which is why automotive and solar wiring is so thick.
Frequently asked questions
- Is this the voltage at my device?
- Close — the "load voltage" line is the source voltage minus the drop, the voltage your device actually sees. It ignores the load's own behavior and connector resistance, which add a little more.
- Why one-way length, not total?
- You enter the one-way run; the formula doubles it because current flows out on one conductor and back on the other, so it crosses twice the length of copper.
- Does this account for wire heating?
- No — it's the drop at 20 °C. Hot conductors have slightly higher resistance (and thus drop). It's also not an ampacity/code check: a wire can be within its current rating yet still drop too much over a long run. Use the wire size calculator to pick a gauge by drop, and your electrical code for ampacity.
- Copper or aluminum?
- Aluminum has about 1.5× the resistivity, so for the same drop it needs roughly 1.5× the cross-section — and connectors rated for aluminum.