DC Cable Size Calculator
Choosing the correct cable size is critical for any DC power application. Undersized cables can lead to excessive voltage drop, heat buildup, and potential fire hazards. Properly sizing your cables ensures efficient power delivery, minimizes energy loss, and extends the lifespan of your electrical components.
In this calculator, we help you determine the minimum AWG gauge needed based on your system voltage, current, distance, allowable voltage drop, and cable temperature rating. For details on how these calculations work, check out our How to Calculate Cable Size section below. For more info, see our Cable Size Reference Guide.
Why Cable Size Matters
Using the right cable size prevents excessive voltage drop and heat buildup, ensuring safe and efficient power delivery in DC systems. Undersized cables can damage equipment, waste energy, and increase the risk of fire. Oversized cables, while safer, can be unnecessarily expensive and difficult to work with. Striking the right balance is essential for both performance and safety.
When in doubt, always go one gauge thicker. If you plan to run wires through conduits or bundles, remember that heat dissipation is reduced, so upsizing the cable may be necessary. For a comprehensive guide, see our Cable Size Chart Reference Guide.
Comprehensive Guide to Calculating DC Wire Size
Correctly sizing your DC cable is essential for maintaining both efficiency and safety in your electrical system. This guide covers the necessary formulas, definitions, and a worked example to help you determine the required wire size. Use the links below to quickly jump to each section:
Formula to Calculate DC Wire Size
The formula to calculate the cross-sectional area (A) of a DC wire is:
A = (2 × D × I × ρ) / V
Where:
- A — Cross-sectional area of the wire (in square meters, m²);
- D — One-way cable length (in meters, m);
- I — Current through the wire (in amperes, A);
- ρ — Resistivity of the conductor material (in ohm meters, Ω·m);
- V — Voltage drop across the wire (in volts, V).
Once you determine A, the wire diameter (d) can be calculated with:
d = √(4A/π)
To determine the current-carrying capacity (ampacity) of the wire, you can rearrange the formula as:
I = (A × V) / (2 × ρ × D)
Voltage Drop Calculation
The voltage drop across a cable is given by:
Voltage Drop = I × R
For example, for a 12 V system with a 3% allowable drop:
Vdrop = 12 V × 0.03 = 0.36 V
Resistivity & Temperature Effects
Resistivity (ρ) indicates how strongly a material opposes the flow of electric current. It depends on both the wire material and its operating temperature. The temperature-adjusted resistivity is given by:
ρ = ρ₁ [1 + α (T - T₁)]
Where:
- ρ — Resistivity at the target temperature;
- ρ₁ — Resistivity at a reference temperature (T₁);
- α — Temperature coefficient for the material;
- T — Target temperature;
- T₁ — Reference temperature.
For example, for copper, if ρ₁ = 1.68×10⁻⁸ Ω·m at 20°C and α = 0.00404, then at 75°C:
ρ = 1.68×10⁻⁸ [1 + 0.00404(75 - 20)] ≈ 2.05×10⁻⁸ Ω·m
Worked Example
Consider a DC system with the following parameters:
- Source Voltage (V): 12 V
- Current (I): 15 A
- One-way Cable Length (D): 100 ft (≈ 30.48 m)
- Allowable Voltage Drop: 3% (i.e., 0.36 V for a 12 V system)
- Material: Copper (assume ρ = 1.68×10⁻⁸ Ω·m)
Step 1: Calculate the maximum allowable voltage drop:
Vmax = 12 V × 0.03 = 0.36 V
Step 2: Determine the maximum total circuit resistance:
Rmax = 0.36 V / 15 A = 0.024 Ω
Step 3: Calculate the required cross-sectional area using the formula:
Convert 100 ft to meters: 100 ft ≈ 30.48 m.
A = (2 × 30.48 m × 15 A × 1.68×10⁻⁸ Ω·m) / 0.36 V
Numerator: 2 × 30.48 × 15 ≈ 914.4; then 914.4 × 1.68×10⁻⁸ ≈ 1.537×10⁻⁵ m².
Thus, A ≈ 1.537×10⁻⁵ m² / 0.36 ≈ 4.27×10⁻⁵ m².
Converting to square millimeters (1 m² = 1,000,000 mm²):
A ≈ 42.7 mm²
Step 4: Calculate the wire diameter:
d = √(4A/π)
Step 5: Compare to standard AWG sizes for copper:
- AWG 2: ~33.6 mm²
- AWG 1: ~42.4 mm²
- AWG 1/0: ~53.5 mm²
Since our calculated area is approximately 42.7 mm², AWG 1 is very close to the requirement. However, to be on the safe side (or when accounting for temperature derating), choosing AWG 1/0 may be advisable.
This example demonstrates how the calculator uses the formulas to ensure the selected wire meets the required voltage drop limits.
For further details and additional examples, please refer to our Cable Size Reference Guide. Always consult a qualified electrician before proceeding with any installation.