Calculator inputs
This engineering estimate evaluates both voltage drop and a derated ampacity screen. Always verify terminal ratings, fuse sizing, and local code requirements.
Wire size comparison table
| AWG | Area (mm²) | Voltage drop (V) | Drop (%) | Adjusted ampacity (A) | Power loss (W) | Status |
|---|---|---|---|---|---|---|
| Enter values and press calculate to populate the comparison table. | ||||||
Plotly graph
The chart compares estimated voltage drop and derated ampacity across available wire sizes.
Example data table
| Use case | Current | One-way length | Allowed drop | Suggested starting point |
|---|---|---|---|---|
| LED lighting branch | 5 A | 3 m | 3% | AWG 14 |
| Portable fridge feed | 8 A | 5 m | 3% | AWG 10 |
| Water pump circuit | 15 A | 4 m | 3% | AWG 8 |
| Inverter supply lead | 50 A | 2 m | 2% | AWG 4 |
| Starter-adjacent heavy feed | 120 A | 1.5 m | 2% | AWG 1/0 |
The example table is illustrative. Actual selections depend on actual current, total path length, ambient conditions, connector losses, and protection strategy.
Formula used
1) Design current
Design Current = Load Current × Continuous Factor × (1 + Safety Margin)
Continuous Factor = 1.25 when continuous loading is enabled, otherwise 1.00.
2) Round-trip length
Round-Trip Length = 2 × One-Way Length
3) Required conductor area by voltage drop
Required Area (mm²) = Resistivity × Round-Trip Length × Design Current ÷ Allowed Drop Voltage
4) Resistance, drop, and loss for each wire size
Resistance = Resistivity × Round-Trip Length ÷ Area
Voltage Drop = Design Current × Resistance
Power Loss = Design Current² × Resistance
5) Derated ampacity screen
Adjusted Ampacity = Base Ampacity × Temperature Factor × Bundle Factor
How to use this calculator
- Enter the expected load current for the 12V circuit.
- Enter the one-way cable length. The calculator doubles it automatically for the full loop.
- Choose meters or feet, then set the maximum acceptable voltage-drop percentage.
- Select copper or aluminum, then choose the insulation temperature rating.
- Enter ambient temperature, bundled conductor count, and any extra safety margin.
- Enable continuous-load treatment when the circuit may operate for long periods.
- Press the calculate button to view the result card, comparison table, and graph.
- Download the summary as CSV or PDF for records, quoting, or design review.
FAQs
1. Why is wire size important on 12V systems?
Low-voltage systems lose a higher percentage of supply voltage across cable resistance. A wire that seems adequate on higher voltages can perform poorly on 12V circuits, causing dim lights, weak motors, hot cables, and reduced equipment life.
2. Why does the calculator use round-trip length?
Current travels to the load and back to the source. Both legs add resistance. Using only the one-way length would understate voltage drop and heat, which usually leads to selecting a wire that is too small.
3. What voltage-drop percentage should I choose?
Many designers target about 3% for general branch circuits and about 2% or less for sensitive electronics, pumps, and inverter feeds. Tighter limits improve performance but often require larger, more expensive conductors.
4. Why does ambient temperature affect the answer?
Hot surroundings reduce how much current a conductor can carry safely. The calculator derates ampacity when ambient temperature rises. This makes the recommendation more conservative for engine bays, enclosures, rooftop areas, and tightly packed wiring.
5. Why include bundled conductor count?
Grouped conductors retain heat. As the bundle gets larger, each wire can safely carry less current. That thermal crowding is why bundle derating matters when several current-carrying wires share the same loom, tray, or conduit.
6. Can I use aluminum on a 12V circuit?
You can, but aluminum has higher resistance and lower practical ampacity than copper for the same size. It usually needs a larger cross-sectional area and careful termination methods to control heat, oxidation, and connection looseness.
7. Does this replace code or manufacturer guidance?
No. This tool is an engineering estimate for screening wire sizes. Final selection should also consider insulation type, installation method, fusing, short-circuit protection, terminal ratings, code rules, and the equipment manufacturer’s instructions.
8. What if no listed wire size passes both checks?
That means the current, run length, or voltage-drop target is too demanding for the listed sizes. Reduce length, allow more drop, split the load, use parallel conductors, or move to a larger cable family than the table currently shows.