On a 12 V circuit, a drop of one volt is more than 8 per cent of the supply. A lamp dims, a motor slows or a controller resets, and none of it shows up on a continuity test. Sizing harness wire for current alone is therefore not enough. The voltage drop has to be calculated for the actual length.
The method
For a two-wire DC circuit, the current flows out through the supply wire and back through the return, so the resistance counted is that of twice the one-way length.
Voltage drop = current × 2 × length × (resistivity ÷ conductor area)
The calculation here uses copper at 20°C, resistivity 0.01724 ohm mm² per metre. Resistance rises by roughly 0.4 per cent for every degree of heating, so a conductor running warm in an engine bay or a loaded bundle drops more than the table shows. Where the return is through the chassis, only the supply wire length counts, but chassis joints add their own resistance.
Voltage drop at 5 A on a 12 V circuit
| Conductor area | Resistance, mΩ per metre | 2 m one way | 4 m one way | 6 m one way |
|---|---|---|---|---|
| 0.5 mm² | 34.5 | 0.69 V (5.7%) | 1.38 V (11.5%) | 2.07 V (17.2%) |
| 0.75 mm² | 23.0 | 0.46 V (3.8%) | 0.92 V (7.7%) | 1.38 V (11.5%) |
| 1.0 mm² | 17.2 | 0.34 V (2.9%) | 0.69 V (5.7%) | 1.03 V (8.6%) |
| 1.5 mm² | 11.5 | 0.23 V (1.9%) | 0.46 V (3.8%) | 0.69 V (5.7%) |
| 2.5 mm² | 6.9 | 0.14 V (1.1%) | 0.28 V (2.3%) | 0.41 V (3.4%) |
| 4.0 mm² | 4.3 | 0.09 V (0.7%) | 0.17 V (1.4%) | 0.26 V (2.2%) |
The percentage is of a 12 V supply. For the same current on a 24 V supply, the volt figures are unchanged and the percentages halve. Since the drop is proportional to current, the figures scale: at 10 A they double, at 2.5 A they halve.
Using the chart
- Find the load current and the longest one-way run.
- Read the drop for the candidate wire size, and scale for the actual current.
- Compare it with the drop limit set by the vehicle designer or equipment maker. This limit is not a universal figure and varies by circuit type.
- Move up a wire size if the limit is exceeded, then check the wire still fits the connector terminal.
Where the chart stops
This chart covers resistance only. Fuse selection, bundle heating and wire temperature class are separate checks, and the right wire for a given circuit also depends on insulation type. APAR’s automotive wires and cables range and the automotive and EV harness service are on the product pages. Drawings, wire lists and quantities for a quote are listed in the harness RFQ checklist, and the harness testing guide explains the checks run on the finished harness.