Cable Gauge Selection for Battery Packs: A Practical Guide
A concise, practical reference for choosing wire sizes in lithium battery pack design — balancing safety, voltage drop and manufacturability.
Why cable gauge matters
Choosing the correct cable gauge (wire size) for battery packs affects three core areas:
- Safety — undersized wires overheat and risk fire.
- Electrical performance — excessive voltage drop reduces usable power and can affect device operation.
- Durability & manufacturability — correct flexibility, connector compatibility and routing ease.
Key concepts
- Current carrying capacity (ampacity): maximum continuous current the conductor can safely carry.
- Voltage drop: voltage lost across the wire due to resistance (important in low-voltage battery systems).
- Resistance: determined by conductor material (copper), cross-sectional area and length.
Quick reference: AWG ↔ mm² ↔ Typical ampacity
| AWG | Approx mm² | Typical continuous ampacity (air) |
|---|---|---|
| 18 AWG | 0.82 mm² | 10 A |
| 16 AWG | 1.31 mm² | 13 A |
| 14 AWG | 2.08 mm² | 17 A |
| 12 AWG | 3.31 mm² | 25 A |
| 10 AWG | 5.26 mm² | 35 A |
| 8 AWG | 8.36 mm² | 50 A |
| 6 AWG | 13.3 mm² | 65 A |
Voltage drop — formula and example
Voltage drop (V) ≈ I × R × L, where
I= current (A)R= resistance per unit length (Ω/m)L= one-way length (m)
Vdrop = 20 A × 0.00521 Ω/m × 0.5 m = 0.0521 V
Percentage drop = (0.0521 / 12) × 100% ≈ 0.43% — acceptable for most systems.
Design rules of thumb
- Target voltage drop < 3% for low-voltage packs (12V–48V). For sensitive equipment (sensors, comms), aim for <1%.
- Choose a wire gauge with ampacity ≥ 125% of expected continuous current (safety margin).
- Consider inrush/peak currents separately — fuses and transient-rated connectors may be required.
- Use multiple parallel conductors or busbars for very high current runs instead of a single large cable.
Temperature, insulation & derating
Ambient temperature, bundling of wires, and insulation type affect ampacity. If wires are routed in bundles or inside enclosures, apply derating factors (see your wire vendor tables). When in doubt, select one gauge larger.
Connector selection and contact resistance
Ensure connectors are rated for continuous current plus margin. Contact resistance adds to total loop resistance — inspect crimps and solder joints and use proper crimp tooling.
Practical checklist for engineers
- Estimate continuous current and worst-case peaks.
- Decide acceptable voltage drop percentage.
- Calculate required conductor resistance and choose gauge.
- Select connectors rated above peak current and compatible with chosen wire.
- Apply derating for temperature/bundling and re-check ampacity.
- Prototype and measure V-drop and temperature rise under load.
Advanced: quick calculation reference (Copper resistivity)
Copper resistivity ρ ≈ 1.724×10-8 Ω·m. Resistance R = ρ × length / area.
Common mistakes to avoid
- Using panel/table ampacity values without applying derating for enclosed runs.
- Neglecting voltage drop in low-voltage systems — can cause system resets or reduced efficiency.
- Ignoring inrush currents from motors or DC-DC converters.
Standards & references
- Refer to IEC and local wiring regulations for insulation and safety ratings.
- Wire vendors publish ampacity and resistance tables — use them for final selection.
