Component Derating Guide
Derating means operating a component below its rated limit so that normal variation, ageing and transients never push it past what it can take. It is the cheapest reliability intervention available at design stage, and derating audits routinely find that a handful of over-stressed parts account for most predicted failures.
Formula
Worked example
A 100 V rated electrolytic capacitor operates on a 75 V rail with 12 V of ripple, peaking at 87 V.
- Applied peak = 87 V; rated = 100 V
- Stress ratio = 87 / 100 = 0.87
- Recommended limit for electrolytics is 0.80.
- Margin = (1 − 0.87) × 100% = 13%, against a 20% target.
The part fails the derating criterion once ripple is included. Auditing against the nominal 75 V rail alone would have given a passing ratio of 0.75 and missed it — derating audits must use the peak the component actually sees, including ripple, transients and start-up conditions.
How to interpret the result
| Electrolytic capacitor | Voltage ≤ 80% of rating; core temperature well below max — life roughly halves per 10 °C rise. |
| Ceramic capacitor | Voltage ≤ 50–60% of rating; note the large capacitance loss with DC bias in Class II dielectrics. |
| Resistor | Power ≤ 50–60% of rating at the actual ambient, after applying the manufacturer’s temperature derating curve. |
| Semiconductor junction | Junction temperature ≤ 80% of maximum rated, expressed in °C above ambient. |
| Connector / contact | Current ≤ 50% of rating; account for adjacent-pin heating in multi-pin housings. |
| Relay contact | Current ≤ 50–75% depending on load type; inductive and lamp loads need heavier derating. |
Common mistakes
- Auditing against nominal rather than worst-case applied stress.
- Ignoring the manufacturer’s temperature derating curve, which reduces the rating itself at elevated ambient.
- Derating voltage but not temperature for electrolytics, where temperature is the dominant life driver.
- Applying a generic ratio without checking the specific part family — ceramic and film capacitors have very different requirements.
Related tools
About Reliability Pro
Derating Guide is part of a suite of 126 reliability, maintenance and quality engineering tools covering life data analysis, accelerated testing, system reliability, FMEA and root cause, SPC, and design for reliability. It runs in the browser and as native iOS and Android apps, so the same calculation is available at a desk or in front of the asset.