Derating Wire [top] -

| Number of Conductors | Percent of Ampacity | |----------------------|---------------------| | 1–3 | 100% | | 4–6 | 80% | | 7–9 | 70% | | 10–20 | 50% | | 21–30 | 45% | | 31–40 | 40% |

12 current-carrying THHN #12 wires in a conduit. Base 90°C ampacity = 30A. 12 wires = 50% derate. Result = 15A. Suddenly, that 20A circuit is illegal. Pillar 3: Continuous Loads (>3 Hours) Even if a wire is sized perfectly for non-continuous load, running at 100% for hours allows heat to saturate the entire assembly (conduit, wall, junction boxes).

Required ampacity = 45A continuous × 1.25 = 56.25A derating wire

Neutrals that carry only unbalanced current (e.g., in a 3-phase wye system) are not counted. Neutrals that carry full load (e.g., single-phase, or non-linear loads with triplen harmonics) are counted.

NEC Table 310.15(B)(2)(a) for 45°C ambient, 90°C insulation = 0.87 55A × 0.87 = 47.85A | Number of Conductors | Percent of Ampacity

NEC 310.15(B)(3)(c): Approx 0.96 factor 33.5A × 0.96 = 32.16A

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. Result = 15A

Table 310.15(C)(1): 7–9 conductors = 70% 47.85A × 0.70 = 33.5A