DC Wire Size Calculator
Calculate the minimum conductor area and recommended standard wire size using load current, one-way cable length, system voltage, allowable voltage drop, conductor material, and a selectable design-current margin.
DC Wire Size Results
The result is based on voltage-drop requirements. Final conductor selection must also satisfy ampacity, insulation temperature, installation, overcurrent protection, and applicable marine/electrical standards.
How does a DC wire size calculator work?
A DC wire size calculator determines how large a conductor needs to be so that the circuit does not lose more voltage than the selected limit. The calculation depends mainly on load current, conductor length, conductor resistivity, and allowable voltage drop.
In a typical two-wire DC circuit, current travels from the source to the load and then returns to the source. Therefore, the electrical calculation uses the complete round-trip conductor length.
What is the voltage drop formula?
For a conductor with resistance R, the voltage drop is calculated using Ohm's law:
$$V_D = I \times R$$
$$R = \rho \times \frac{L}{A}$$
$$V_D = I \times \rho \times \frac{L}{A}$$
- VD: Voltage drop in volts.
- I: Circuit current in amperes.
- ρ: Conductor resistivity.
- L: Electrical conductor length.
- A: Conductor cross-sectional area.
How much voltage drop is acceptable?
The acceptable voltage drop depends on the application and equipment. A 3% maximum is commonly used as a design target for many DC branch circuits, while some circuits may permit 5% or another value.
Sensitive electronics, lighting, navigation equipment, motors, pumps, and long cable runs can require special consideration because voltage drop affects their operating voltage and performance.
Why is DC wire sizing especially important on boats?
Marine DC systems commonly operate at relatively low voltages such as 12 V or 24 V. The same amount of voltage loss therefore represents a larger percentage of the supply voltage than it would in a higher voltage system.
For example, losing 1 volt in a 12 V system represents about 8.3% of the nominal voltage, while losing 1 volt in a 48 V system represents only about 2.1%.
Why does wire length matter?
Longer conductors have greater electrical resistance. In a two-wire DC circuit, both the positive and negative conductors contribute to the total resistance, which is why the round-trip length can be approximately twice the one-way distance.
Why does higher current require larger wire?
Voltage drop increases directly with current. Doubling the current through the same wire approximately doubles the voltage drop. Larger conductors have lower resistance and therefore reduce the resulting voltage loss.
What is the difference between AWG and mm²?
AWG, or American Wire Gauge, is a wire-size numbering system commonly used in North America. Smaller AWG numbers represent larger conductors. Metric wire sizing normally specifies the actual conductor cross-sectional area in square millimeters.
This calculator provides both representations where an appropriate standard size is available.
Does this calculator check wire ampacity?
The primary calculation on this page is based on voltage drop. Voltage-drop sizing and ampacity sizing are separate requirements. A wire can have sufficiently low voltage drop but still require a different size because of its permitted current-carrying capacity, insulation temperature, installation method, bundling, ambient temperature, or applicable standards.
Why can the recommended wire be larger than expected?
Low-voltage systems are particularly sensitive to voltage drop. A long 12 V circuit carrying a substantial current can require a surprisingly large conductor even when the current itself does not appear excessive.
Is this calculator suitable for marine wiring?
It is useful for preliminary voltage-drop sizing of DC conductors, including many boat and RV applications. Final marine wiring must also comply with the electrical code, vessel requirements, conductor insulation specifications, overcurrent protection requirements, temperature limits, installation conditions, and applicable marine standards.