There are two requirements in the NEC for derating ampacities: (1) ambient temperature and (2) number of conductors in a raceway or cable. There are other code sections that affect the size of conductor to be used (e.g., motor circuits or continuous loads) but only the two requirements above are used to regulate ampacity to keep the conductor within its maximum temperature rating.
NEC Section 310-10 contains the basic rule that, in effect, requires derating for the two reasons above. Section 310-10 states that no conductor shall be used in such a manner that its operating temperature will exceed that for which it is designed. Then, four items are mentioned that are the principal determinants of operating temperature: (1) ambient temperature, (2) heat generated internally in the conductor as a result of current flow, (3) the rate at which generated heat dissipates from the conductor, and (4) adjacent current-carrying conductors. It is items (1) and (4) for which derating is done.
DERATING FOR AMBIENT TEMPERATURE
All ampacity tables necessarily must use some ambient temperature as a basis and NEC Table 310-16 uses 30°C (86°F). At the bottom of the table, correction factors are given for situations where the ambient is expected to be higher or lower than 30°C (86°F). Use of these factors is fairly straightforward; for example, what is the ampacity of a 1/0 AWG, aluminum, type THHN conductor when the ambient temperature is 100°F? The answer is found by taking the ampacity from the table and multiplying it by the appropriate correction factor. In this example, we would have 135 x 0.91 = 122.85 amperes.
DERATING FOR NUMBER OF CONDUCTORS
Note 8 to the ampacity tables in the NEC contains the requirement columns for derating ampacity because of adjacent current-carrying conductors. This note states that when the number of conductors in a raceway or cable exceeds 3, the ampacities are to be reduced by the appropriate percentage. In the 1999 NEC, a column of factors is given. For example, what is the ampacity of twelve No. 12 copper THHN conductors installed in one conduit? From Table 310-16 the ampacity (in the table) is 30 amperes. From Note 8, the derating factor for 12 conductors is 50 percent. 30 x 0.5 = 15 amperes per conductor.
DERATING FOR BOTH AMBIENT TEMPERATURE AND NUMBER OF CONDUCTORS
When derating for both conditions is necessary, then both calculations must be made. For example, what is the ampacity of four 1/0 THW copper conductors when the ambient temperature is expected to reach 110°F? From Table 310-16 the ampacity is 150 amperes, derating factor for 110°F is 0.82, and derating factor from Note 8 for 4 conductors is 80 percent. 150 x 0.82 x 0.8 = 98.4 amperes.