Free Tool
Voltage drop calculator.
Length × gauge × current = voltage drop. Size your wire runs before sending a harness drawing. NEC 3% / 5% flagging, marine ABYC E-11 compatible.
- 3%NEC feeder · sensitive electronics
- 5%NEC combined feeder + branch
- 10%ABYC non-critical (bilge, horn)
- 3%Automotive 12 V bus · ADAS sensors
Under the Hood
Ohm’s law with round-trip length
| AWG | Ø mm² | Ω / kft | Max A (chassis) |
|---|---|---|---|
| 10 | 5.26 | 0.9989 | 55 |
| 12 | 3.31 | 1.588 | 41 |
| 14 | 2.08 | 2.525 | 32 |
| 16 | 1.31 | 4.016 | 22 |
| 18 | 0.823 | 6.385 | 16 |
| 20 | 0.518 | 10.15 | 11 |
| 22 | 0.326 | 16.14 | 7 |
| 24 | 0.205 | 25.67 | 3.5 |
Gotchas
Why the calculator might lie
- Temperature raises resistance (+0.4% per °C)
- Bundled cables derate per NEC 310.15
- Inrush / motor start can be 8 – 10× run current
- Long thin wires heat up → positive feedback
- Conduit fill limits cooling (see calculator)
- Terminations add ~5% equivalent length
Frequently Asked Questions
Voltage drop FAQ
01Why do I multiply length by 2?
Current flows to the load and back. The round-trip is what actually sees resistance, so voltage drop is 2 × L × R × I.
02What's safe for 12 V automotive?
Keep V-drop < 3% on critical rails (ABS, ECU, sensors). Non-critical (lighting, horn) can tolerate up to 10%. Anything driving a motor should stay under 5%.
03Does insulation type affect voltage drop?
Insulation doesn’t — resistance comes from the copper itself. Insulation affects ampacity (how much current you can flow before melting) and operating temperature — both of which indirectly affect resistance.
04Other calculators?
See all free tools: impedance, trace width, conduit fill, wire size.
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