DC Wire Size Calculator

Q: What wire size for a 12V DC should I use?

The wire size for a 12 V DC depends mainly on the current and the wire length. Follow these steps to calculate it: Determine the electric current I (i.e., 20 A), cable length L (i.e., 50 m), conductor resistivity ϱ (let's assume 2.05 × 10⁻⁸ Ω⋅m, the copper resistivity at 75 °C), and voltage drop V (typically 3% of the source voltage). Input the values in the formula: A = (2IϱL) / V = (2 ⋅ 20 ⋅ 2.05 × 10⁻⁸ ⋅ 50) / (0.03 ⋅ 12 V) The result should be 0.000114 m² or 114 mm².

Q: What size wire for 30 amp DC should I use?

The wire size for a 30 amp DC depends primarily on the source voltage and the wire length. Follow these steps to calculate it: Determine the source voltage (i.e., 12 V), cable length L (i.e., 50 m), conductor resistivity ϱ (typically 2.05 × 10⁻⁸ Ω⋅m for copper), and voltage drop V (typically 3% = 0.03 of the source voltage). Input the values in the following formula: A = (2IϱL) / V = (2 ⋅ 30 ⋅ 2.05 × 10⁻⁸ ⋅ 50) / (0.03 ⋅ 12 V) The result should be 0.0001711 m² or 171.1 mm².

Creators

Luis Hoyos

Luis is a mechanical engineer who works at Omni Calculator as a content creator and researcher specializing in math and physics calculators with a focus on thermodynamics and classical mechanics. With a strong foundation in mechanical engineering, Luis is passionate about creating tools that simplify complex concepts and improve everyday life. Outside of work, Luis likes weightlifting and applying his engineering knowledge to optimize the thermal conditions of his home, all while caring for four beloved cats. See full profile

Check our editorial policy

Reviewers

Wojciech Sas, PhD

Wojciech Sas

PhD, Institute of Physics in Zagreb

Wojciech, PhD, is a physicist at the Institute of Physics in Zagreb, investigating materials under extreme conditions such as low temperatures and high pressures. He specializes in experimental work, including participation in Large-scale User Facilities such as synchrotrons. Wojciech uses his experience and knowledge to create calculators in physics, math, and statistics categories. In his free time, he likes swimming, playing board games, and looking for meteors while stargazing. See full profile

Check our editorial policy

New

Calculating the DC wire size is vital for budgeting any electrical project, as bigger wires imply more expensive cables. For that reason, we've created this tool.

As an example of how to size a DC wire, we'll calculate the wire size for a 200 amp DC system using this calculator.

You can also look at the FAQ section for other numeric examples, such as what wire size you should use for a 12V DC.

How to calculate the size of a DC wire

The formula to calculate the wire size of a DC system is:

A = (2DIϱ) / V

where:

V — Voltage drop between the source and the farthest end of the wire, measured in volts;
I — Current running through the wire, in amps;
ϱ — Resistivity of the conductor material, in ohm × meters;
D — One-way distance (how far is the cable run from its source to the farther point), in meters; and
A — Cross-sectional area of the wire, in square meters.

🙋 Important remarks:

The 2 coefficient before D accounts for the return path after the current passes the load.
V is the product of the allowable voltage drop percentage and the source voltage.
ϱ is not constant; it varies with the operating temperature.
For source voltages above 50 V, if L<16 m, this calculator assumes a distance of 16 m so that the wire doesn't result in excessively tiny sizes for small lengths.

Example: How to size a DC wire of 200 amp

Suppose an electrical system with a 120 V source and allowable drop of 3%, a copper cable with a maximum temperature of 50°C, operating at 200 amps at DC. The wire size is unknown, and the one-way distance (from source to load) is 50 m.

Follow these steps to know the wire size:

In the DC wire size calculator, input:
- Source voltage: 120 V;
- Allowable drop: 3%;
- Material: Copper;
- Current: 200 A;
- Distance (D): 50 m; and
- Maximum temperature: 50°C.
That's it. The wire size for your 200 amp DC system should be 104.65 mm², or AWG = 0000 (4/0)
You can check your results using the formula presented in the previous section.

Other useful tools like the DC wire size calculator

As you already know how to size a DC wire, you can take a look at other calculators:

Disclaimer

These results are only a guide for informational purposes. Always consult a qualified electrician before proceeding with any electrical installation.

FAQs

What wire size for a 12V DC should I use?

The wire size for a 12 V DC depends mainly on the current and the wire length. Follow these steps to calculate it:

Determine the electric current I (i.e., 20 A), cable length L (i.e., 50 m), conductor resistivity ϱ (let's assume 2.05 × 10⁻⁸ Ω⋅m, the copper resistivity at 75 °C), and voltage drop V (typically 3% of the source voltage).
Input the values in the formula:

A = (2IϱL) / V = (2 ⋅ 20 ⋅ 2.05 × 10⁻⁸ ⋅ 50) / (0.03 ⋅ 12 V)
The result should be 0.000114 m² or 114 mm².

What size wire for 30 amp DC should I use?

The wire size for a 30 amp DC depends primarily on the source voltage and the wire length. Follow these steps to calculate it:

Determine the source voltage (i.e., 12 V), cable length L (i.e., 50 m), conductor resistivity ϱ (typically 2.05 × 10⁻⁸ Ω⋅m for copper), and voltage drop V (typically 3% = 0.03 of the source voltage).
Input the values in the following formula:

A = (2IϱL) / V = (2 ⋅ 30 ⋅ 2.05 × 10⁻⁸ ⋅ 50) / (0.03 ⋅ 12 V)
The result should be 0.0001711 m² or 171.1 mm².