: Multiply by a percentage based on how many wires are in the conduit (e.g., 4–6 wires usually require an 80% adjustment). ExpertCE +2 Expert & Community Perspectives Professionals on forums often emphasize that overlooking derating is a common cause of electrical failure. “Remember that the code book considers the weakest link when derating wire.” Electrical PE Review “If you want to be conservative, some municipality require an extra 125% derating... when in doubt consult an electrical engineer.” Facebook · FANUC Robot Programmers
The Complete Guide to Derating Wire 1. What is Derating? Wire is sold with a standard ampacity rating (e.g., 12 AWG copper is rated for 20 amps). However, this rating assumes ideal conditions: a single wire, in open air, at a specific ambient temperature (usually 86°F / 30°C). Derating is the process of lowering the maximum current capacity of a wire to prevent overheating when installed in less-than-ideal conditions. Why is it necessary? When current flows through a wire, it generates heat (Joule heating). If the wire cannot dissipate that heat—because it is hot outside, surrounded by insulation, or packed tightly with other wires—the insulation can melt. This leads to short circuits and fire hazards.
2. The Two Main Factors for Derating There are two distinct scenarios where you must reduce the ampacity of a wire. You must calculate both and use the most restrictive value (the lowest number). Factor A: Ambient Temperature If the surrounding air is hotter than 86°F (30°C), the wire cannot cool itself effectively. You must reduce the amount of current allowed.
How to do it: Use NEC Table 310.15(B)(1)(1) (formerly Table 310.15(B)(2)(a)). The Math: Locate the temperature rating of your wire insulation (usually 60°C, 75°C, or 90°C). Find the correction factor for your ambient temperature. derating wire
Formula: Derated Ampacity = Base Ampacity × Correction Factor
Example: You have 6 AWG THHN wire rated at 75°C with a base ampacity of 65 amps. You are installing it in a boiler room where the ambient temperature reaches 104°F (40°C).
Looking at the table, the correction factor for 104°F is 0.88 . Calculation: 65 amps × 0.88 = 57.2 amps . : Multiply by a percentage based on how
Factor B: Number of Current-Carrying Conductors (Bundling) When wires are bundled together (in a conduit, a cable tray, or even a single jacket like Romex/NM cable), they radiate heat onto each other. The more wires you pack, the less heat can escape.
How to do it: Use NEC Table 310.15(C)(1) (formerly Table 310.15(B)(3)(a)). Thresholds:
1–3 conductors: No derating required (100%). 4–6 conductors: Reduce to 80% of ampacity. 7–9 conductors: Reduce to 70% of ampacity. 20–30 conductors: Reduce to 50% of ampacity. when in doubt consult an electrical engineer
Example: You are running eight 12 AWG THHN wires through a single conduit to power different circuits.
Base Ampacity (90°C column for THHN) is 30 amps. For 7–9 conductors, the table requires a 70% adjustment. Calculation: 30 amps × 0.70 = 21 amps . Note: Even though 12 AWG is normally breakered at 20A, the derating allows 21A, so it is still safe to use a 20A breaker.