is the process of lowering the maximum current capacity of a wire to prevent overheating when installed in less-than-ideal conditions.
The National Electrical Code provides Table 310.16 (ampacities) and mandates derating via:
Failing to derate properly is one of the leading causes of overheating, insulation failure, and electrical fires. Why Derating is Necessary
Required ampacity = 45A continuous × 1.25 = 56.25A derating wire
Disclaimer: This guide is for educational purposes. Always consult a licensed electrician and your local electrical inspector, as local codes may amend or supersede the NEC.
In electrical engineering, derating wire refers to the practice of reducing the maximum current-carrying capacity of a wire or cable to ensure safe and reliable operation. This is done to account for various environmental and operational factors that can affect the wire's performance and longevity. Derating wire is essential to prevent overheating, insulation damage, and even fires, which can occur when a wire is subjected to excessive current, high temperatures, or other stressors.
In electrical engineering, is the practice of intentionally reducing the maximum current-carrying capacity (ampacity) of a conductor to account for environmental factors that hinder heat dissipation. Simply put, if a wire cannot stay cool because of its surroundings, you cannot let it carry as much electricity as its "standard" rating suggests. is the process of lowering the maximum current
When you derate correctly, you don’t just follow code—you guarantee that the insulation remains intact, the breaker stays closed, and the building stays standing.
| Number of Conductors | Percent of Ampacity | |----------------------|---------------------| | 1–3 | 100% | | 4–6 | 80% | | 7–9 | 70% | | 10–20 | 50% | | 21–30 | 45% | | 31–40 | 40% |
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. Always consult a licensed electrician and your local
You are running eight 12 AWG THHN wires through a single conduit to power different circuits.
: 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
Skin effect and proximity effect cause additional heating in AC conductors, but ampacity tables already account for 60 Hz. At higher frequencies (e.g., 400 Hz aircraft, or harmonic-rich VFD outputs), additional derating is required (not covered by NEC—use engineering analysis).