| Mistake | Why It’s Wrong | |---------|----------------| | Derating from the wrong temperature column | Always use the wire’s (e.g., 90°C for THHN), not the terminal rating. | | Forgetting the neutral | In a 3-phase wye with non-linear loads (LEDs, VFDs, computers), the neutral is a current-carrying conductor. | | Derating ground wires | Equipment grounding conductors do not count for bundling derate. | | Ignoring both factors | You must apply both temp AND bundling derates (multiply them). | | Using 100% for 3 wires | Correct. 4-6 wires = 80% already. Many assume 3 wires need derating—they don't. |
If you don't follow a derating chart, several things can happen: derating chart
Suppose we have a voltage regulator with a maximum operating voltage of 30V and a maximum operating temperature of 125°C. The derating chart for this component might look like this: | Mistake | Why It’s Wrong | |---------|----------------|
To use the chart, you find your operating temperature on the X-axis, move up to the curve, and then look across to the Y-axis to see the maximum percentage of the load you are allowed to apply. Why is Derating Necessary? | | Ignoring both factors | You must
While temperature is the biggest factor, other variables can shift the curve on a derating chart: