NEC - De-rating Factor Calculation

Total de-rating can be calculated once each of following de-ratings are selected:

a. De-rating factor of Ambient Temperature ((Cat)

b. De-rating factor for Cable Grouping (Ccg)

Total De-rating factor = (a * b )

There is a third contributing factor which is:

c. Ambient Temperature Adjustment for Circular Raceways Exposed to Sunlight on or Above Rooftops

This factor is used by altering the value of De-rating factor of Ambient Temperature (Cat).

If De-rating is not used the factor is set to 1.

The user can select a de-rating factor which will be used instead of the calculated value.

To select a de-rating factor, click the De-rating Factors button. The Derating Factors window is then displayed:

Select the required factor from the list. The list includes the de-rating factors that are part of the standard and user-defined project specific de-rating factor set up using the Cable Sizing Configuration window.

If a value is 1 then the rating will have no effect on the calculation.

Click Save. The selected factor is then displayed in the Chosen De-rating Factor field.

The Chosen De-rating Factor field value can only be set by the user to be equal to or less than the Calculated De-rating Factor field value.

If this field is blank when the Size Cable button is clicked, it will be automatically populated with the calculated value.

Reference NEC standard tables used for de-rating factors

Table 310.15(B) (2) (a) and Table 310.15(B) (2) (b) covers De-rating factor of Ambient Temperature (Cat)

Table 310.15(B) (3) (a) covers De-rating factor for Cable Grouping (Ccg)

Table 310.15(B) (3) (c) covers Ambient Temperature Adjustment for Circular Raceways Exposed to Sunlight on or Above Rooftops

Note:
Depending on installation arrangement, methods and external situation, the factors may differ.