dc wire size calculator

DC Wire Size Calculator

DC Wire Size Calculator

Estimate the minimum conductor size (AWG) for DC circuits to control voltage drop. This calculator uses conductor resistance and a chosen voltage drop percentage. For final installations, consult local code and an electrician.

Results show estimated minimum AWG and check on voltage drop.
Reference conductor resistances (Ω per 1000 ft) — copper
AWGΩ / 1000 ft (Cu)
Note: This tool estimates wire size based on voltage drop only. It does not replace ampacity checks, temperature derating, or code compliance. For safety and code compliance, always verify with NEC tables or a licensed electrician.

DC Wire Size Calculator — Quick Guide

Choosing the right wire size for DC circuits is essential for safety, efficiency, and reliable performance. The primary factors are the circuit current (amps), the length of the run, allowable voltage drop, and the conductor material (usually copper). This calculator helps you estimate the minimum conductor needed and shows common practical choices.

How it works (simple steps)

  1. Determine current (I) — the maximum continuous DC current the wire must carry (in amps).
  2. Measure one-way length (L) — the distance from source to load in feet (round-trip = 2×L for DC).
  3. Choose allowable voltage drop (%) — common choices are 3% for sensitive electronics or 5% for general power.
  4. Select conductor material — copper is the most common; aluminum requires a larger size.
  5. Calculate required conductor resistance using the formula below, then pick the nearest larger standard wire (AWG or mm²).

Key formulas

  • Round-trip length (ft) = 2 × L
  • Maximum allowable voltage drop (V) = System voltage × (allowed % / 100)
  • Required maximum resistance (Ω) = Vdrop_max / I
  • Look up copper resistance per 1000 ft for candidate wire sizes, compute R = (resistance_per_1000ft × roundtrip_ft) / 1000, and choose the smallest wire where R ≤ required maximum resistance.

Example (worked)

System: 12 V DC, Load = 30 A, One-way length = 10 ft, Allowed drop = 5%

  1. Round-trip = 2 × 10 = 20 ft.
  2. Vdrop_max = 12 × 0.05 = 0.6 V.
  3. Required max resistance = 0.6 V ÷ 30 A = 0.02 Ω.
  4. AWG 10 copper ≈ 1.0 Ω per 1000 ft → R = 1.0 × (20 ÷ 1000) = 0.02 Ω → meets requirement.
    So AWG 10 would be a practical choice for this example.

Practical tips & considerations

  • Always round up to the next larger standard conductor for safety and heat margin.
  • Temperature, bundling, and insulation affect ampacity — derate when necessary.
  • For long runs or low-voltage systems (12/24 V) keep voltage drop low because percent drop has a larger effect.
  • Consult local electrical codes (NEC or local standard) for ampacity tables and safety requirements. For critical installs (solar, EV, battery banks), get a qualified electrician or engineer to verify final sizing.

Final Thought

Proper conductor sizing is essential for safety, efficiency, and long-term reliability. Taking a conservative approach—accounting for heat, voltage drop, and local code requirements—helps prevent failures and costly rework. When in doubt, choosing the next larger conductor and consulting a qualified professional ensures your electrical system performs safely under real-world conditions.

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