Calculator inputs
Designed for typical 12V DC wiring. Use the round-trip option for most two-wire runs.
Tip: Set allowable drop to see maximum length.
Example data table
| Use case | Wire | Current | Length (one-way) | Path | Typical target |
|---|---|---|---|---|---|
| LED light bar | AWG 14 copper | 8 A | 12 ft | Round-trip | ≤ 3% drop |
| Water pump | AWG 12 copper | 12 A | 18 ft | Round-trip | ≤ 5% drop |
| 12V fridge | 6 mm² copper | 6 A | 5 m | Round-trip | ≤ 3% drop |
| Inverter feed | AWG 2 copper | 120 A | 6 ft | Round-trip | Keep as low as possible |
These examples are illustrative; always follow local code and equipment requirements.
Formula used
- R_total = (R_per_1000ft / 1000) × Length_ft × PathFactor ÷ ParallelRuns
- V_drop = I × R_total
- Drop% = (V_drop / V_system) × 100
- V_load = V_system − V_drop
- P_loss = I × V_drop
- Temperature adjustment (typical): R_T = R_20 × (1 + α × (T − 20°C))
- Metric option uses R = ρ × L / A to derive resistance.
How to use this calculator
- Enter system voltage, expected current, and one-way cable length.
- Select round-trip for most DC two-wire circuits.
- Choose copper or aluminum, then pick AWG, mm², or custom resistance.
- Set temperature and parallel runs if your installation uses them.
- Click Calculate to see voltage drop, delivered voltage, and heat loss.
- Use allowable drop to estimate the maximum safe run length.
- Download CSV or PDF to share with your build notes.
FAQs
1) Why does low-voltage wiring need thicker cables?
At 12V, small resistance causes a larger percent drop. Thicker conductors reduce resistance, keeping devices within their operating voltage and improving efficiency.
2) Should I use one-way or round-trip length?
Use round-trip for most DC circuits with a positive and return wire. One-way applies when the return path is not in the cable, such as chassis return assumptions.
3) What drop percentage is acceptable for 12V loads?
Many installers target about 3% for sensitive electronics and about 5% for general loads. Motors and inverters often benefit from even lower drop to reduce heat and nuisance trips.
4) How does temperature affect voltage drop?
Conductor resistance increases as temperature rises. Hot engine bays or bundled cables can show higher drop than a 20°C table value, so adding a temperature estimate improves realism.
5) Do parallel cables always halve the voltage drop?
If the cables are identical in length and termination quality, two runs share current and roughly halve resistance. Unequal lengths or poor terminations can unbalance current sharing.
6) Why might real-world drop be higher than calculated?
Lugs, crimps, switches, breakers, fuse holders, and corrosion add contact resistance. Measure under load if performance is critical, and consider upgrading connectors as well as cable size.
7) Can I use this for 24V or 48V systems?
Yes. Enter your system voltage, and the percent drop recalculates automatically. Higher voltages tolerate the same absolute drop better, but current, length, and cable heating still matter.