Wire & voltage
Wire Size by Voltage Drop Calculator
Wire size by voltage drop compares conductor resistance at each supported AWG size. The smallest size at or below the entered percentage target is returned.
Find the smallest supported AWG conductor that meets your editable voltage-drop target using a transparent resistance model.
Calculated result
Smallest size meeting the target
Calculating…
Calculated locally in your browser
Planning math only. Verify equipment specifications and installation requirements separately.
What this calculator returns
Use the adjacent-size comparison to see the tradeoff. A size that meets voltage drop can still be unsafe for the current or installation.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
For each supported size: Vdrop% = 100 × I × Rpath / Vsource. The first size at or below the target is returned.
- Vdrop
- Voltage lost across the conductor path, in volts.
- I
- Current through the conductor, in amperes.
- R
- Electrical resistance of the modeled conductor path, in ohms.
- ρ
- Material resistivity at the modeled temperature, in ohm-metres.
- L
- Conductor length used by the selected path model.
- A
- Metal cross-sectional area of the conductor.
Rpath uses AWG area, material resistivity, temperature, one-way length, and the selected circuit path factor.
Worked example
| Input | Value |
|---|---|
| System type | DC |
| Source voltage | 12 V |
| Load current | 20 A |
| Conductor material | Copper |
At 12 V, 20 A, and 20 ft one way, the calculator compares each copper AWG size and returns the smallest one that stays within the entered drop target.
Reference table
| AWG | Cu 20°C Ω/1000 ft | Cu 75°C Ω/1000 ft | Al 20°C Ω/1000 ft | Al 75°C Ω/1000 ft | Cu 20°C Ω/km | Al 20°C Ω/km |
|---|---|---|---|---|---|---|
| 14 | 2.5254 | 3.0712 | 4.1400 | 5.0576 | 8.2853 | 13.5825 |
| 12 | 1.5882 | 1.9315 | 2.6036 | 3.1807 | 5.2107 | 8.5421 |
| 10 | 0.9988 | 1.2147 | 1.6374 | 2.0004 | 3.2770 | 5.3722 |
| 8 | 0.6282 | 0.7640 | 1.0298 | 1.2581 | 2.0609 | 3.3786 |
| 6 | 0.3951 | 0.4805 | 0.6476 | 0.7912 | 1.2961 | 2.1248 |
| 4 | 0.2485 | 0.3022 | 0.4073 | 0.4976 | 0.8152 | 1.3363 |
Frequently asked questions
Does the selected AWG also verify ampacity?
No. The result is based only on resistive voltage drop; ampacity, protection, insulation, and installation rules require separate checks.
Why can a lower-voltage circuit need a larger wire?
The same absolute voltage loss is a larger percentage of a lower source voltage, so a lower-resistance conductor may be needed.
Why can the result differ from a wire-size chart?
Many charts combine ampacity or code rules. This page intentionally isolates resistive voltage-drop math.
Assumptions and limitations
- Voltage-drop-only sizing. This does not verify ampacity, overcurrent protection, conductor insulation, installation method, or electrical code.
- Only the displayed AWG set is searched.
- AC reactance is not included.
Method and sources
Review the AWG reference, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and verification. Last reviewed .