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Voltage drop calculator

Work out how much voltage a cable run loses, as a value and as a percentage of the supply, for copper or aluminium conductors.

Voltage drop
0.672 V
As a percentage of supply5.6%
Voltage at the load11.328 V
Loop resistance67.2 mΩ
Power wasted in the cable6.72 W
VerdictToo much — go thicker

Conductors at 20 °C. Most wiring guidance treats 3% as the target for a branch circuit and 5% as the outer limit; low-voltage DC needs to be tighter still, because the same absolute drop is a far larger fraction of the supply.

Cable is not a perfect conductor. Every metre has resistance, and pushing current through it consumes voltage that never reaches the load. On a short run at low current this is negligible. On a long run at high current it is often the thing that decides what cable you need.

V_drop = 2 × I × ρL / A  (DC or single phase)

The factor of 2 is the detail people forget: current travels out along one conductor and back along the other, so a 5 metre run is 10 metres of copper. For balanced three-phase the factor is √3 instead, because the return currents partly cancel.

Why percentage matters more than volts

A 0.5 V drop is trivial on a 230 V circuit — about 0.2%. The same 0.5 V on a 12 V battery system is over 4%, enough to dim lights noticeably and to stop some devices working at all. This is why low-voltage DC installations need disproportionately fat cable: the current is higher for the same power, and the tolerance for loss is far smaller.

Common targets: 3% for a branch circuit, 5% total from the supply to the furthest point. For 12 V solar and automotive work, many designers aim for 2% or better.

Copper against aluminium

Aluminium’s resistivity is 2.82 × 10⁻⁸ Ω·m against copper’s 1.68 × 10⁻⁸, so it has roughly 68% more resistance for the same cross-section. To match a copper cable you need about 1.6 times the area, usually two AWG sizes up. Aluminium is cheaper and lighter, which is why it appears in service entrance and utility cable, but it creeps under termination pressure and oxidises into a resistive surface layer, so it needs connectors rated for it.

Reducing a drop that is too large

  • Thicker cable. Direct and usually the answer. Resistance is inversely proportional to area, so doubling the area halves the drop.
  • Shorter run. Move the supply closer, or feed from a nearer point.
  • Higher voltage. The most effective change of all. For a fixed power, doubling voltage halves current, which halves the drop and quarters the wasted power. It is the entire reason transmission lines run at hundreds of kilovolts.

Temperature

The figures here assume 20 °C. Copper gains about 0.39% resistance per degree, so a conductor at its rated 70 °C carries roughly 20% more resistance than calculated. On a run already near the limit, that margin matters — and it compounds, because the extra resistance produces extra heat.

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