DC Wire Size Calculator

Created by Luis Hoyos
Reviewed by Wojciech Sas, PhD candidate
Last updated: Jul 24, 2022

Calculating the DC wire size is vital for budgeting any electrical project, as a bigger wire size implies 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 additionally look at the FAQ section for other numeric examples, like what wire size for a 12V DC you should use.

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;
  • A — Cross-sectional area of the wire, in square meters; and

🙋 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 for small lengths, the wire doesn't result in excessively tiny sizes.

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:

  1. 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.
  2. That's it. The wire size for your 200 amp DC system should be 104.65 mm², or AWG = 0000 (4/0)
  3. You can check your results using the formula presented in the previous section.

Other useful tools like the DC wire size calculator

FAQ

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:

  1. Determine the electric current I (i.e., 20 A), cable length L (i.e., 5 m), conductor resistivity ϱ (typically 2.22 × 10⁻⁸ Ω⋅m for copper), and voltage drop V (typically 3% of the source voltage).
  2. Input the values in the formula:
    A = (2IϱL) / V = (2 ⋅ 20 ⋅ 2.22 × 10⁻⁸ ⋅ 50) / (0.03 ⋅ 12 V)
  3. The result should be 0.0001235 m² or 123.5 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:

  1. Determine the source voltage (i.e., 12 V), cable length L (i.e., 50 m), conductor resistivity ϱ (typically 2.22 × 10⁻⁸ Ω⋅m for copper), and voltage drop V (typically 3% = 0.03 of the source voltage).
  2. Input the values in the following formula:
    A = (2IϱL) / V = (2 ⋅ 30 ⋅ 2.22 × 10⁻⁸ ⋅ 50) / (0.03 ⋅ 12 V)
  3. The result should be 0.00018525 m² or 185.25 mm².

Disclaimer

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

Luis Hoyos
Source voltage
V
Allowable voltage drop (V)
%
Conductor material
Copper
Current (I)
A
One-way distance (D)
ft
Maximum wire temperature
°F
Recommended wire size per cable
Wire gauge
AWG
Wire cross-sectional area (A)
kcmil
Wire diameter (d)
in
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