This page provides a number of guide-lines for Voltage and Temperature derating [Thermal Derating].
Derating guidelines are provided in tables for each component type, and should be regarded as design recommendations.
A good design always uses components that operate within their Safe Operating Region.
However there are absolute rules of design that should be followed when designing circuits.
The derating curves provided for Transistors, FETs, and Diodes should be considered rules to hold the junction temperature of these devices within specified limits.
These component derating recommendations do not account for mounting issues required by high vibration environments.
A short description is provided as required to understand How-to derate components;

Design Note; In all cases a component is considered to be operating alone, with no other components near-by.
In addition that the components are using ambient air [no conditioning], using natural cooling [no forced air].
Ambient air is considered to be normal room temperature, unless indicated otherwise.
Natural cooling is the dissipation of heat to surroundings by conduction, convection, radiation, or any combination thereof without the benefit of external cooling devices.

Connector Voltage Derating:

Derate connector voltage to 25% of rated operating voltage
.... Operating voltage = Connector rating / 0.25
Derate connector operational temperature by 50^oC
.... Connector rating = operating temperature + 50^oC
Current is normally specified as some number of pins carrying some specific amount of current, at some defined temperature. Increase the current carried over any particular pin and the number of pins carrying current most decrease. Decreasing the amount of current over the defined pins may allow more pins to carry current. Main section on Connector Derating. Refer to this page for a manufacturers listing for Connectors

Cable/Wire Derating:

Refer to this page for a manufacturers listing of Wire and Cable. Cable manufacturers will provide different numbers based on the insulation used for the wire. The table below lists copper wire with a Teflon [TFE] insulation. Teflon insulation has a higher operation temperature range then other insulators, for example PVC. The table below is based on data derived from MIL-STD-975, using 70^oC as the operating temperature. To derate based on number of wires in a bundle:
I_BW = I_SW x (29 - #_wire) / 28 @ [1 to 15 Bundled wires]
I_BW = I_SW x (0.5) @ [more then 15 Bundled wires]
I_SW = Single wire
I_BW = Bundled wires
To derate by temperature use; derate by 80% at 150⁰C, 70% at 135⁰C, or 50% at 105⁰C {Per MIL-STD-975}
To derate for Commercial wire use the table below;

Ampacity Tables for many conditions (Commercial Wiring):
IEEE Standard 835, IEEE Standard Power Cable Ampacity Tables
IEEE Standard 848, Procedure for the Determination of the Ampacity De-rating of Fire Protected Cables
ICEA P-54-440, NEMA Pub. No. WC 51 - Ampacities of Cables in Open-Top Trays
Use the National Electrical Code [NEC] system for premises wiring, and not the Military data on this page.

Voltage Drop per Foot:
V = I x L x (R/1000) x 1.004 x (T-20)
V = Voltage Drop, I = Current, R = Resistance per 1000 foot
L = Length of wire in feet, T = Estimated wire Temperature under load ^oC
Refer to the AWG Table for Resistance per length

{Component Derating Guidelines}

Potentiometer Derating:

Potentiometer Derating Curve

A list of Potentiometer manufacturers
This page shows the derating curve for an Adjustable WireWound resistor

{Component Derating Guidelines}

Resistor Derating Example:

Power ratings are normally specified at +25^oC and must be reduced as the resistors temperature increases. A derating chart is often used, with derating starting at 70^oC. Since parameters are application dependent, charts should be considered general rather than absolute, and only used as a guideline. However power de-rating curves should be considered more of a recommendation than a guideline The safest designs use the largest physical size operating at conservative temperatures and power ratings. Combining the resistor application and a power de-rating curve should start to form a design rule, once the location of the resistor on the PWB is known and the components around the device.

Military Resistor Derating Curves, Derating Resistor Networks, Derating Thermistors. A list of Resistor Manufacturers manufacturers

Design guidelines;
For resistors mounted in series, consider the heat being conducted through the leads to the next resistor. At the same time axial lead-mounted parts also act as a heat dissipator and allow heat to be conducted away from the device and into the printed circuit card, or pads on the PCB. Of course heat is also removed from the component via convection and radiation.
Large power resistors should be mounted to the metal chassis for heat dissipation. Do not mount resistors with power dissipation 1 Watt directly on terminal or printed wiring boards without use of heat sinks. A resistor that dissipates over one watt can damage a terminal board. A damaged board will have a lower insulation resistance. For the most efficient operation and even heat distribution, power resistors should be mounted in a horizontal position.

{Component Derate Guideline}

Fuse Derating:

Fuses are specified to interrupt within a maximum of 5 seconds when driven at 200% of their rated current for nominal ratings up to and including 10 amperes. A fuse with a nominal rating of 15 amperes is specified to interrupt within a maximum of 10 seconds when driven at 200% of its rated current. The power supply shall be capable of delivering appropriate levels of current to achieve short fusing times. There is an additional derating of 0.2%/°C for an increase in the temperature of fuse body. Main Fuse Derating Guidelines page. AC or DC Fuse operation. Fuse holders may also require derating.
A manufacturers listing for Fuses

{Component Derating Guidelines}

Switch Derating:

Note the derating factor changes with the type of load.
A manufacturers listing of Mechanical Switches

{Component Derating Guidelines}

Inductor Derating:

Inductors are also derated by reducing the maximum operating temperature based on the insulation class and reducing the operating voltage.
Transformer Stress Ratio = Operating VA Load / Rated VA Load = 80% [Rule-of-Thumb Operating Recommendation]
A manufacturers listing of Inductor Vendors, Transformer Vendors, Choke Vendors

{Component Derating Guidelines}

EMI Filter Derating:

The general recommendation is to derate an EMI Filter to 50% of rated voltage and 50% of rated current, using a maximum temperature of 850C. However that could be to strict for some applications, in which cause the next best option would be the dearting curve provided in the data sheet. This example shows 50% operation doesn't apply until 125C and maximum operation out to 150C [Case Temperature]. A manufacturers listing of EMI/RFI Components Note that many temperature-power derating curves are just a linear curve or set of curves, others are a bit more complicated. Some curves show maximum power dissipation, others show percent rated load [as current or voltage].

EMI Chip Derating Curve

{Component Derating Guidelines}

Diode Derating:

More data may reside on the Diode Derating page. Current-Power Diode Derating Curves for a number of diode types.

A manufacturers listing of Diodes, Zener Diodes and Varactor Diodes

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Power Supply Derating Guidelines:

Operating Temperature range is normally 0 to 50^oC, derate 20% for each additional 10^oC increase above 50^oC.
Operating Temperature range; 0^oC to +70^oC. Full power to +50^oC then derate 2.5%/^oC to +70^oC. Or, derate linearly from 100% load @ 50^oC to 75% @ 60, derated at 10% per ^oC to a Maximum of +65^oC
Forced air cooling [20CFM nominal]: a power supply may require a forced air flow to operate at its rated maximum temperature rating. Derate 20% with out cooling
Open Frame supplies: may require an additional 15% power de-rating when covered by a frame.
About: The power supply specification should detail its operation, use the above as a rule of thumb. Normal operational temperature range is defined as ambient _[Maximum]

Question; why derate electronic components. Answer; to reduce the stress they see and to keep them from over-heating.

Ambient temperature: The temperature of air or other media in a designated area, particularly the area surrounding equipment.
IEC 60134 Absolute maximum and design ratings of tube and semiconductor devices.
More data may reside on the Power Supply Derating page.
A list of Power Supply manufacturers.

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