Complete Guide to Cable Sizing (NEC 310.15)
The Cable Sizing Tool selects conductor size using load current, voltage drop, ampacity, and derating per NEC 310.15(B)(16), 310.15(C), and Annex B.
Key Sizing Criteria
- Ampacity: ≥ 125% of continuous load (NEC 215.2, 230.42)
- Voltage Drop: ≤3% feeder, ≤5% branch + feeder
- Derating: >3 conductors, ambient >30°C
- Conduit Fill: ≤40% for >2 conductors
NEC 310.15(B)(16) 75°C Ampacity (Cu)
| AWG | 14 | 12 | 10 | 8 | 6 | 4 | 2 | 1/0 |
|---|---|---|---|---|---|---|---|---|
| Amps | 20 | 25 | 35 | 50 | 65 | 85 | 115 | 150 |
Derating Factors
- 4–6 conductors: 80%
- 7–9: 70%
- Ambient 41–50°C: 0.82
Voltage Drop Formula
VD = (2 × L × I × R) / 1000 (1φ) or (√3 × L × I × R) / 1000 (3φ)
R = DC resistance from NEC Chapter 9 Table 8
Example: 45A, 150ft, 240V 3-Phase, Cu
- 125% = 56.25A → #6 Cu (65A)
- VD = 2.7% → OK
- 4 conductors → 80% → 65×0.8 = 52A → Need #4 Cu
Aluminum vs Copper
Al: Larger size, lower cost, requires antioxidant. Use 75°C column.
Conduit Fill & Parallel Runs
>3 conductors → derate. >250A → parallel per phase.
Common Mistakes
- Using 60°C column for THHN.
- Ignoring neutral as current-carrying.
- Forgetting voltage drop in long runs.
- Not applying 125% for continuous loads.
Advanced Tips
- Use Voltage Drop for fine-tuning.
- Pair with Conduit Fill.
- Check short-circuit withstand.
Standards & Codes
- NEC 310.15: Ampacity
- NEC 215.2: Feeder sizing
- NEC 110.14(C): Temperature limits
Conclusion
Proper cable sizing prevents overheating and voltage issues. Our Cable Sizing Tool automates NEC compliance. Use with Breaker Size and Load Balance. Build smarter with 1000 Calculators.