Members of a workshop I instructed on the National Electrical Code (NEC) asked me how to determine the ampacities of conductors based on how they are used in the electrical system. Covering all the Code sections that listed such requirements helped, so I decided to outline these rules in this month’s Code Comments column.
Article 100 defines ampacity as the current in amperes that a conductor can carry continuously under conditions of use without exceeding its temperature rating. The items that govern these conditions are outside temperature (correction factors) surrounding the enclosed conductors, calculated load, termination ratings and number of current carrying conductors (adjustment factors).
The ungrounded phase conductors are usually considered to be current-carrying, and they must be derated, as outlined in Table 310.15(B)(2)(a), based on the number. However, the grounded neutral conductor may not be classified as current-carrying when the provisions of 310.15(B)(4)(c) are applied. Table 310.15(B)(2)(a) requires that each current-carrying conductor be derated, but Table B.310.11 recognizes loading only half of the conductors at one time as described in Example 1 below the table. If more than half but less than the total number is loaded, then Example 2 describes the procedure used to calculate the ampacity of each conductor.
Article 100 defines a continuous load as one that operates for at least three hours. Naturally, a noncontinuous load operates for a period of less than three hours. When continuous and noncontinuous loads are calculated, they are added to other loads that are subjected to demand factors as permitted by specific sections of the Code.
Note: a demand factor is considered less than one, and a diversity factor is considered more than one.
Applying 310.10, (1) through (4)
CORRECTION FACTORS: Ambient temperature may vary along the conductor length and from time to time. When this condition exists, select the correction factor and percentage below Table 310.16, which is based on the temperature surrounding the enclosed conductors. Next step, use this percent to derate the ampacity of the conductors and derive the reduced ampacity.
To aid in determining if the length of the enclosed conductors is exposed to a greater distance than the 10 foot or 10 percent rule permits, review the information in the example to 310.15(A)(2). Also, the FPN 2 to 310.10 contains useful information that serves in determining if a heating problem might exist. For example, a temperature of 102°F per Table 310.16 requires that a correction factor of 91 percent be applied to the ampacity of the conductors subjected to such temperature.
CALCULATED LOAD: To calculate the load, apply the requirements of 210.19(A)(1) for branch circuits, 215.2(A)(1) for feeder circuits and 230.42(A)(1) for services. After the load calculations are performed, the size of the conductors should handle the heat generated internally as the result of load current flow. Harmonic currents can be a problem; evaluate carefully and determine if they are present and correct as necessary. Section 220.61 covers the requirements necessary to calculate the load for sizing the neutral conductor. Understand that when calculating the load for sizing the conductors, 210.19(A)(1) and 215.2(A)(1) do not include the correction factors or the adjustment factors. Additional calculations are required for adjusting the ampacity of the conductors to accommodate the correction factor and adjustment factor conditions.
TERMINAL RATINGS: When selecting terminal ratings, use the concept that the insulation that covers or surrounds conductors affects the rate of heat dissipation. Therefore, the final ampacity of the conductors must comply with the provisions of 110.14(C)(1)(a) or (b). Basically, a 100-amp circuit breaker, if existing, usually has terminals rated at 60°C, while modern- day circuit breakers are rated at 60/75°C. Note that the ampacities of conductors are selected from the 60° or 75°C column in Table 310.16 based on one of these ratings outlined in 110.14(C)(1)(a) or (b).
ADJUSTMENT FACTORS: If there are four or more adjacent load-carrying conductors, then one of the derating factors outlined in Table 310.15(B)(2)(a) must be selected based on the number of conductors pulled through the raceway or cable. For example, a raceway enclosing 9–12 AWG copper current-carrying conductors requires that an adjustment factor of 70 percent be used, when the usable ampacity of each conductor is calculated. Adjacent conductors can have the dual effect of raising the ambient temperature and impeding heat dissipation that has the capability of overheating the conductor’s insulation as well as other circuit components. Don’t forget, if not properly calculated, such conditions might cause operational problems. EC
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.