Free electrical planning tool
Voltage Drop Calculator
Estimate cable voltage loss in a 12V, 24V or 48V two-conductor DC circuit. Compare copper cable sizes against your voltage-drop target — not against their current-carrying capacity.
Your system
Voltage loss can reduce lighting output, interrupt electronics or prevent equipment receiving its required supply voltage.
Compare standard cable sizes
| Area (mm²) | Drop (V) | Drop (%) | Target |
|---|---|---|---|
| 1 | 1.75 | 14.58 | Exceeds |
| 1.5 | 1.17 | 9.72 | Exceeds |
| 2.5 | 0.7 | 5.83 | Exceeds |
| 4 | 0.44 | 3.65 | Exceeds |
| 6 | 0.29 | 2.43 | Meets |
| 10 | 0.18 | 1.46 | Meets |
| 16 | 0.11 | 0.91 | Meets |
| 25 | 0.07 | 0.58 | Meets |
| 35 | 0.05 | 0.42 | Meets |
| 50 | 0.04 | 0.29 | Meets |
| 70 | 0.03 | 0.21 | Meets |
| 95 | 0.02 | 0.15 | Meets |
| 120 | 0.01 | 0.12 | Meets |
Your results
- Voltage drop
- 0.29 V
- Voltage drop percentage
- 2.43 %
- Estimated voltage at the load
- 11.71 V
Meets your 3% voltage-drop target
Minimum size for voltage-drop target only
6 mm²
Planning estimates, not installation approval.
How to use this calculator
Enter the nominal supply voltage, current at the load, one-way cable length and copper cross-section. Select the maximum drop you wish to allow. The return conductor is included automatically.
Calculation method
Voltage drop = 2 × one-way length × current × 0.0175 ÷ cable area. Divide the drop by supply voltage and multiply by 100 for the percentage. The 0.0175 Ω·mm²/m copper resistivity assumes 20°C.
What the results mean
A passing target only means the calculated voltage drop is within your limit. Minimum size for voltage-drop target only is the smallest listed cross-section meeting that limit; it is not a safe cable recommendation.
Worked example
At 12V, a 10A load with 5m of one-way 6mm² copper cable loses 0.292V (2.43%). The load sees approximately 11.708V. For a 3% target the smallest listed size is 6mm².
Common mistakes
Do not enter the combined outgoing and return length: this doubles the distance twice. Use mm² cross-section, not cable diameter. A chassis return or unequal conductors needs a different resistance model.
Frequently asked questions
Why does the calculation use twice the cable length?
Current travels through both the positive and negative conductors. A 5m one-way run has a 10m electrical circuit in this model.
Is a 3% target always required?
No. Equipment specifications and the circuit purpose determine a suitable target. Sensitive equipment and charging circuits may need a tighter limit.
Does thicker cable guarantee safety?
No. Voltage drop is only one check. Cable ampacity, fuses, terminations and installation methods also matter.
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