Non-metallic sheathed cable, commonly known as Romex or NM-B, is a staple in residential and light commercial wiring. However, its current-carrying capacity, or ampacity, must be reduced—or "derated"—under certain conditions to prevent overheating and fire hazards.

The primary derating factors include ambient temperature, conductor bundling, and conduit fill. According to the National Electrical Code (NEC), when more than three current-carrying conductors are bundled together without adequate ventilation, heat dissipation decreases. NEC Table 310.15(B)(3)(a) requires a 70% derating factor for 4-6 conductors, 60% for 7-9, and 50% for 10-20. For instance, a 12 AWG copper NM cable with a baseline ampacity of 25A drops to 17.5A when four conductors are bundled.

Ambient temperature also matters. The NEC 310.15(B)(2) correction factors reduce ampacity when temperatures exceed 30°C (86°F). For a 60°C rated NM cable at 40°C, you apply a 0.82 multiplier, reducing 25A to 20.5A. When both bundling and high temperature apply, multiply all derating factors together.

Another critical scenario is when NM cable is run in conduit for physical protection. Per NEC 334.80, the cable must be considered as a single conductor, and fill calculations follow Chapter 9 tables. The ampacity is then derated per Table 310.15(B)(3)(a).

Ignoring derating can lead to insulation breakdown, voltage drop, and electrical fires. Always consult local codes and use a licensed electrician for complex installations. Proper derating ensures long-term safety and code compliance.