NEC 310.15(B)(3)(c): Approx 0.96 factor 33.5A × 0.96 = 32.16A
The wire’s ampacity table is a starting point , not an ending one. Ambient temperature, bundling, altitude, solar gain, and continuous operation all steal from the wire’s limited temperature budget. Your job as an engineer is to account for every thief.
16A continuous load. Required ampacity = 16A × 1.25 = 20A. After derating for ambient and bundling, the wire’s final adjusted ampacity must be ≥20A. Part 4: Advanced Derating Scenarios 4.1 High Altitude (Above 2,000 m / 6,500 ft) At higher altitudes, air density decreases, reducing convective cooling. The NEC (310.15(B)(3)(c)) mandates a correction factor of 0.95 to 0.80 depending on altitude. IEC 60364-2-2 has similar provisions. derating wire
Continuous load must not exceed 80% of the derated ampacity (or conversely, the derated ampacity must be ≥ 125% of the continuous load).
Table 310.15(C)(1): 7–9 conductors = 70% 47.85A × 0.70 = 33.5A NEC 310
Suddenly, the 30A wire becomes a 15A fire hazard.
| Number of Conductors | Percent of Ampacity | |----------------------|---------------------| | 1–3 | 100% | | 4–6 | 80% | | 7–9 | 70% | | 10–20 | 50% | | 21–30 | 45% | | 31–40 | 40% | 16A continuous load
is the process of reducing the current-carrying capacity (ampacity) of a conductor to account for operating conditions that increase its temperature. Since heat is the fundamental enemy of insulation, derating is not a suggestion—it is a thermodynamic necessity.