Calculator
Example Data Table
| System (V) | Current (A) | Length | Unit | Material | AWG | Drop (V) | Drop (%) |
|---|---|---|---|---|---|---|---|
| 24.0 | 5 | 2 | m | Copper | 18 | 0.68 | 2.83% |
| 12.0 | 10 | 3 | m | Copper | 14 | 0.46 | 3.83% |
| 12.0 | 20 | 5 | m | Copper | 10 | 0.66 | 5.50% |
| 48.0 | 15 | 20 | m | Aluminum | 6 | 1.28 | 2.67% |
| 12.0 | 30 | 2 | ft | Copper | 8 | 0.22 | 1.83% |
Formula Used
The calculator models a DC round-trip cable run. Voltage drop is computed from current and total series resistance:
- R20 = rho * (2L) / A
- Rcable = R20 * (1 + alpha * (T - 20)) / N
- Rtotal = Rcable + Rextra + Rinternal
- Vdrop = I * Rtotal
- %Drop = 100 * Vdrop / Vsystem
- Ploss = I^2 * R
Symbols: rho is resistivity, L is one-way length, A is conductor area, alpha is temperature coefficient, T is cable temperature, N is number of parallel conductors, and I is load current.
How To Use This Calculator
- Enter your system voltage and expected load current.
- Enter one-way cable length and select meters or feet.
- Select copper or aluminum, then choose AWG or mm2.
- Optionally add parallel runs and temperature.
- Add extra and internal resistance if you want total sag.
- Press Calculate to view results above the form.
- Use the CSV/PDF buttons to download the reported values.
Engineering note: For long runs, small increases in conductor area can reduce losses and heating significantly.
FAQs
1) Why does the calculator use twice the cable length?
The circuit includes a supply and return path. Current travels out to the load and back, so resistance is based on the round-trip distance.
2) What is a good voltage drop target?
Many low-voltage systems aim for 2-3% drop on feeders. Sensitive electronics may need less, while noncritical loads can tolerate more.
3) Why does temperature matter?
Conductor resistance rises with temperature. Warmer cables create more drop at the same current, especially in high-current or poorly ventilated installations.
4) When should I use parallel conductors?
Parallel runs can lower resistance and heating when a single cable becomes too large or hard to route. Ensure each run is equal length and properly terminated.
5) What counts as extra series resistance?
Connectors, terminals, relays, fuses, and switches add small resistances that can create noticeable sag at high currents. Enter their combined value in milliohms.
6) Should I include battery internal resistance?
If you want total voltage sag at the load, include it. If you only want cable drop, keep it at zero and focus on conductor sizing and length.
7) Does this work for AC circuits?
This tool is intended for DC battery systems. AC design often needs power factor, reactance, and different standards for allowable drop.
8) Why might my measured drop differ from the result?
Real systems vary by strand count, terminations, temperature, and actual current waveform. Measure under steady load, and check connectors for heating or corrosion.