Wire Size Calculator
Pick the right AWG, by the numbers.
Size wire by current, run length and ambient temperature. References NEC Table 310.15 for building wire and SAE J1128 for automotive/chassis runs. Every row is engineer-verified.
- AWG range
- 30→4/0
- Max Vdrop
- 3%
- Base temp
- 30 °C
- Ref spec
- NEC
The selection rule
Three constraints, one gauge
Pick whichever gauge is larger to satisfy all three constraints. Never size for ampacity alone on long runs — voltage drop almost always wins.
Hot wire must stay under NEC table 310.15 / SAE J1128 limit for the insulation rating.
Long runs lose voltage — governs choice for wires over ~15 m, even at modest currents.
Below 22 AWG wires break under vibration or crimp pull-off — spec a minimum mechanical gauge.
Worked example
20 A · 15 m run · 12 V → what gauge?
Scenario: 12 V DC accessory run across a trailer. 20 A steady, 15 m one-way, 30 °C ambient, ≤ 3% voltage drop.
- Check 1 · Ampacity: 20 A → 12 AWG satisfies.
- Check 2 · Vdrop: 30 m loop × 20 A × 5.2 mΩ/m (12 AWG) = 3.12 V → 26% drop ✗
- Retry 10 AWG: 3.28 mΩ/m → 1.97 V = 16.4% ✗
- Retry 6 AWG: 1.30 mΩ/m → 0.78 V = 6.5% ✗
- Retry 4 AWG: 0.82 mΩ/m → 0.49 V = 4.1% ✗
- Retry 2 AWG: 0.52 mΩ/m → 0.31 V = 2.6% ✓
- Mechanical check: 2 AWG >> 22 AWG ✓
- Verdict: run 2 AWG for ≤ 3% drop
AWG Reference
NEC & SAE wire-size reference
| Signal gauges | |
| 30 AWG · 0.05 mm² | 0.86 A @ 30 °C · 337 mΩ/m |
|---|---|
| 24 AWG · 0.2 mm² | 3.5 A @ 30 °C · 84 mΩ/m |
| 22 AWG · 0.33 mm² | 7 A @ 30 °C · 52 mΩ/m |
| 20 AWG · 0.52 mm² | 11 A @ 30 °C · 33 mΩ/m |
| Building wire | |
| 18 AWG · 0.82 mm² | 14 A @ 75 °C (NEC) · 21 mΩ/m |
| 14 AWG · 2.08 mm² | 25 A @ 75 °C (NEC) · 8.3 mΩ/m |
| 10 AWG · 5.26 mm² | 40 A @ 75 °C (NEC) · 3.28 mΩ/m |
| Heavy gauge | |
| 6 AWG · 13.3 mm² | 75 A @ 75 °C · 1.30 mΩ/m |
| 4 AWG · 21.2 mm² | 95 A @ 75 °C · 0.82 mΩ/m |
| 2 AWG · 33.6 mm² | 130 A @ 75 °C · 0.52 mΩ/m |
| 2/0 AWG · 67.4 mm² | 195 A @ 75 °C · 0.26 mΩ/m |
| 4/0 AWG · 107 mm² | 260 A @ 75 °C · 0.16 mΩ/m |
| Temperature derating factor | |
| 30 °C ambient | × 1.00 |
| 40 °C ambient | × 0.88 |
| 50 °C ambient | × 0.75 |
| 60 °C ambient | × 0.58 |
Frequently Asked Questions
Wire sizing FAQ
01Should I size for ampacity or for voltage drop?
Both — but voltage drop usually wins on runs over ~10-15 m. Always run both calculations and pick the larger gauge.
02Do I need derate for bundled cables?
Yes. NEC 310.15(B)(3)(a) requires derate when 4 or more current-carrying conductors share a raceway: 4-6 × 0.80, 7-9 × 0.70, 10-20 × 0.50. We apply this automatically on engineered harness quotes.
03What’s the difference between NEC and SAE ampacity?
NEC is for building wire at steady state. SAE J1128 is for automotive / chassis wire in free air — typically allows 20-30% higher ampacity because of better cooling. Use the one that matches your application.
04Is this a substitute for a licensed electrician?
No. This is a design aid for harness and prototype work only. For building mains wiring, follow local code and engage a licensed professional. Published here for engineering reference.
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