Calculate derated capacity by multiplying a base rating by temperature, conductor-grouping and site derating factors, or find the base rating you need.

Multiply applicable correction factors; 100% means no adjustment.

All capacities use this unit. Changing it reinterprets the numbers; it does not convert them.

NEC Table 310.15(C)(1) adjustment for conductors in one raceway or cable; choose Custom for other standards.

Optional load check
Blank omits the load check; an entered zero is a zero load.

Select 1.25 only when the governing rule requires it.

Related Calculators

Derating Factor Formula (Linear Temperature Derating)

The widget multiplies compatible entered temperature, grouping and site factors: adjusted capacity = base capacity × (temperature % / 100) × (grouping % / 100) × (site % / 100). It also reverse-solves base capacity or the site factor. A blank optional load omits the load check; an entered load is multiplied by the selected allowance and (1 + margin % / 100). Select 1.25 only where the governing rule requires it; it is not universal. This arithmetic does not establish code compliance. The separate formula below illustrates a linear temperature derating factor, only when the equipment manufacturer specifies such a curve. Calculate that factor from the applicable temperatures before entering its percentage in the widget.

DF = (Tₘₐₓ - Tambient) / (Tₘₐₓ - Trated)

Variables:

  • DF is the derating factor (unitless, typically between 0 and 1)
  • T_max is the maximum allowable temperature (°C)
  • T_ambient is the ambient temperature (°C)
  • T_rated is the rated/reference temperature where the device is at 100% of its nominal capacity (°C)

To calculate the derating factor for linear derating, subtract the ambient temperature from the maximum allowable temperature and then divide by the temperature span between the maximum allowable temperature and the rated/reference temperature. In many datasheets, the rating is constant (DF = 1) at or below T_rated and derates down to DF = 0 at T_max.

What is a Derating Factor?

A derating factor adjusts a nominal rating for specified operating conditions. Select factors for the same equipment, capacity basis and reference installation; do not double-count a condition already included in the base rating. Applicable correction factors can exceed 1 in favorable conditions, but an increased rating needs explicit source support. The calculator does not determine safe operating limits, protective-device sizing or installation compliance.

How to Calculate Derating Factor?

The following steps outline how to calculate the Derating Factor.

  1. First, determine the maximum allowable temperature (T_max) of the device (often specified by the datasheet as a maximum ambient/case/junction temperature, depending on the part).
  2. Next, determine the rated/reference temperature (T_rated) where the device can operate at 100% of its nominal capacity (for example, many resistors are rated at 70°C ambient; many photovoltaic ratings reference 25°C cell temperature).
  3. Then, determine the ambient temperature (T_ambient) where the device will operate.
  4. Use the formula DF = (T_max – T_ambient) / (T_max – T_rated) to calculate the derating factor for linear derating between T_rated and T_max.
  5. Finally, apply the factor to the matching nominal rating (adjusted capacity ≈ nominal capacity × DF), only within the device-specific curve and all other applicable limits. Do not extrapolate beyond the curve.
  6. For the temperature-only example, enter DF × 100 in Temperature factor, use 100% for grouping and site only when no other adjustment applies, and enter the nominal capacity. The widget does not infer a temperature curve from a device type.

Example Problem:

Use the following variables as an example problem to test your knowledge.

maximum allowable temperature (T_max) = 150°C

ambient temperature (T_ambient) = 35°C, rated/reference temperature (T_rated) = 25°C. For this hypothetical linear curve, DF = (150 − 35) / (150 − 25) = 115 / 125 = 0.92. Enter 92% as the temperature factor. These example temperatures are not generic equipment ratings.