Voltage Drop Calculator for 12V, 24V and 60V Systems
Use this free voltage drop calculator to estimate cable voltage loss in automotive, campervan, marine and off-grid copper cable circuits. Enter voltage, load, cable length and cable size to calculate voltage drop, percentage loss, power dissipation and a practical cable suitability check.
Enter your cable and load details
Results
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How to use the voltage drop calculator
Select system voltage, then choose whether to enter the load as current in amps or power in watts. Enter one-way cable length and the cable size you plan to use. The calculator then estimates voltage drop across the full circuit length and shows both voltage loss in volts and percentage loss relative to system voltage.
- Choose current in amps if you already know the load current.
- Choose power in watts if you only know appliance wattage.
- Enter one-way cable length only.
- Select the actual cable size you plan to use.
- Use Auto if you want a sensible recommended target for the selected circuit type.
Why voltage drop matters
In low-voltage systems such as 12V campervan, automotive, marine and off-grid installations, voltage drop can have a major effect on equipment performance. Excessive voltage drop may cause dim lights, poor motor performance, reduced charging efficiency and overheating. Longer cable runs and smaller cable sizes increase voltage drop.
Typical voltage drop guidelines
| Circuit type | Typical target | Comment |
|---|---|---|
| LED lights, electronics, sensitive loads | 3% | Preferred where stable voltage is important. |
| Pumps, fans, sockets, general accessories | 5% | Often acceptable for common 12V accessory circuits. |
| Less critical circuits | Up to 10% | Can be acceptable depending on the application. |
| Solar and battery charging circuits | 3% | Lower voltage drop usually improves charging efficiency. |
How voltage drop is calculated
This calculator uses a simplified copper cable resistance model. It estimates total cable resistance using the selected cable cross-section and the full circuit length, then applies Ohm’s law to calculate voltage drop.
In simple terms:
- Resistance increases with cable length.
- Resistance decreases as cable size increases.
- Voltage drop increases with current.
Example voltage drop checks
Example 1: A 12V fridge drawing 5A on a long cable run may suffer noticeable voltage loss if the cable is undersized.
Example 2: A 12V water pump drawing 10A through 2.5 mm² cable over several metres may show a moderate voltage drop, especially on a long return path.
Example 3: The same power load in a 24V system usually means lower current than in a 12V system, which can reduce percentage voltage drop.
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FAQ
For many 12V lighting, electronics and charging circuits, 3% is a good target. For general accessories, 5% is often acceptable. Some less critical circuits may tolerate more.
Because the system voltage is low, even a small voltage loss can represent a large percentage of the available voltage and affect equipment performance.
This calculator asks for one-way length only and automatically uses the full circuit length for the voltage drop calculation.
Yes. In power mode the calculator estimates load current from power and system voltage before calculating cable voltage loss.
You can reduce voltage drop by using a larger cable size, shortening the cable run, lowering current where possible or increasing system voltage where appropriate.
These calculator results are provided as a practical technical guide only. Final cable size, fuse protection, installation method and equipment selection should always be verified against the real application, installation environment and manufacturer specifications. Electrical systems should be installed using appropriate protection devices and suitable cable routing for the intended load conditions.