Resistor Derating Curves

Resistor Power ratings are normally specified at +25^oC and must be reduced as the resistor temperature increases. A derating chart is often used, always with Still-Air. Since parameters are application dependent, power de-rating curves or charts should be considered general rather than absolute, and only used as a guideline or recommendation. The most conservative or safest designs use the largest physical size operating at conservative temperatures and power ratings. This style of resistor will operate up to 125C before any derating is required. However the maximum power dissipation is reached at 155C, for types RNC, RNR, RNN [MIL-R-55182].

MIL-R-55182 Resistor Graph

When higher ambient temperatures exist or when resistors are mounted in enclosures which limit ventilation, the wattage dissipation of any resistor should be reduced so that the maximum hot-spot temperatures permissible for the resistor is never exceeded under the most severe combination of temperature conditions.

Derating Resistor Networks, Resistor Manufacturers, or Potentiometer Manufacturers

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The Derating Factor [percent Rated Load] shown in the table above is a general rule-of-thumb and can vary depending on the company or organization providing the guideline. In this particular case the Department of Defense [DOD] generated the permissible rated load over temperature. Stress Ratio is another term used to describe the derating factor. NASA uses 80% as a stress ratio for resistors, regardless of the ambient temperature.
Resistor Stress Ratio = Operating Power / Rated Power = 80%

Definitions
Near-by resistors; or resistor grouping, will also effect device derating. A resistors derating must be further reduced if it's effected by near by components or resistors that are radiating heat which would effect the surrounding ambient temperature. Derating curves only account for the resistor being tested. See Spacing.

Altitude; resistors must also be power derated based on altitude. Example numbers may indicate full power up to 5000 feet, than derate 10% for each addition 10,000 feet of altitude; in addition to temperature derating. Note that the figures for derating to altitude is just an example, check the manufacturers data sheet for a derating curve or recommendations.

Air Flow; or forced air cooling; Resistor derating curves or equations are routinely related to 25C; how ever what is not always stated is that the figures are for still air [Free Air]. Forced air will allow a resistor to operate above what is shown in the derating curves. Free Air rating is also called Full Rating, and Maximum Power Rating. Because resistor bodies may be smaller than other components on the printed wiring board any forced air added to the system may bypass the resistor as it's diverted around the device by other components.

Resistor Mounting; Resistor mounting may also be defined in the data sheet. Component pad sizes of a particular size or shape or thermal vias [High-Power Resistors] may be required for the device to comply with the derating curve provided by the data sheet. Check to insure that mounting instructions are given in the data sheet. Some derating curves may also specify the board type [as in FR4], but this is less common for resistors. Surface Mount resistor packages having a tab [TO-263] may require the tab attachment point [Pad] to be much larger than the actual size of the tab.
Some through-hole resistors may also have a tab to attach a heat sink; Example,
TO-126, TO-220, TO-247 package styles.

Resistor Shape; different series of resistors use different body types and sizes for the same resistance. Body size effects temperature rise because the size of the radiating surface is changing. Correct derating of a particular resistor series does not directly relate to another family that uses a different body shape, regardless of the resistance value. Also check the data sheet for the absolute derating recommendation.

Resistor Spacing; When resistors are mounted in rows or banks, they should be so spaced that, taking into consideration the restricted ventilation and heat dissipation by the nearby resistors, none of the resistors in the bank or row exceeds its maximum permissible hot-spot temperature. An appropriate combination of resistor spacing and resistor power rating must be chosen if this is to be assured.