Table 310-16 Conductor Ampacities

Choosing the right size and type of conductor for electrical installations is imperative. Although specific conductor sizing and overcurrent protection requirements exist throughout the National Electrical Code (NEC), Table 310-16, Table 310-15(b)(2)(a), and Section 240-3 are the most commonly used.

Table 310-16 provides maximum allowable ampacities of insulated conductors rated from 0 through 2,000 volts. This table includes copper and aluminum (or copper-clad aluminum) conductors. Each conductor is divided into three categories: 60 degrees Celsius (140 F), 75 degrees Celsius (167 F), and 90 degrees Celsius (194 F).

Ampacities vary depending upon both the type of conductor, and the insulation properties. The ampacities shown here are based on a 30 degrees Celsius (86 F) ambient temperature. Correction factors appear below the conductor's maximum ampacities. These factors should be used when the ambient temperature is anything other than 30 degrees Clesius (86 F). This table presumes three (or fewer) current-carrying conductors installed in raceway, cable, or earth (directly buried). Equivalent data for single conductors is found in Table 310-17.

Table 310-15(b)(2)(a) Adjustment Factors

Table 310-15(b)(2)(a) contains adjustment factors applicable wherever more than three current-carrying conductors are used. In previous NEC editions, this information appeared as "Note 8" following the ampacity tables.

The occurrence of more than three current-carrying conductors requires that the ampacity be reduced, or derated. For example, a No. 3 THW copper conductor (per Table 310-16), has an ampacity of 100 amperes. Where the number of current-carrying conductors is between four and six, the ampacity must be derated to 80 amperes (100 x 80 percent = 80). Along with the four exceptions in Section 310-15(b)(2)(a), Sections 310-15(b)(4) and (5) are also associated with Table 310-15(b)(2)(a).

240-3 Conductor Overcurrent Protection

Section 240-3 provides requirements for selecting the proper size overload protection (fuses or breakers). Conductors, other than flexible cords and fixture wires, must be protected against overcurrent in accordance with their ampacities as specified in Section 310-15, unless otherwise noted in 240-3(a) through (g). [240-3] Of particular importance within this section is: (b) devices rated 800 amperes or less, (c) devices rated over 800 amperes, and (d) small conductors.

240-3(b) Devices Rated 800 Amperes or Less

If after all adjustment (or derating) factors have been applied in determining a conductor's ampacity rating, the rating does not match a standard size fuse or circuit breaker, the next higher standard size is permitted, provided the three stipulations in Section 240-3(b)(1), (2), and (3) are met. Refer to Section 240-6(a) for standard ampere ratings for fuses and inverse time circuit breakers.

One set of 500 kcmil THW copper conductors can be fed from a 400 ampere overcurrent protective device. Each 500 kcmil THW copper conductor has a rating of 380 amperes, which is not a standard-size overcurrent device rating. Since the next higher size is 400 amperes, this installation is permitted.

In similar fashion, two paralleled sets of 500 kcmil THW copper conductors can be fed from an 800-ampere overcurrent device. Because the paralleled conductor's combined rating of 760 amperes is not a standard-size fuse or breaker, and the next higher size is not more than 800 amperes, an 800 ampere overcurrent protective device is permitted.

240-3(c) Devices Rated over 800 Amperes

Where the overcurrent device is rated more than 800 amperes, the protected conductors' ampacity must equal or exceed the overcurrent device's ampacity rating. Having three paralleled sets of 500 kcmil THW copper conductors fed from a 1,200-ampere overcurrent protective device violates this requirement. These conductors have a combined rating of only 1,140 amperes (380 x 3) per phase. Therefore, since the combined rating is not equal to (or greater than) the overcurrent device's rating, a 1,200-ampere overcurrent protective device is not permitted to protect these conductors.

240-3(d) Small Conductors

Small conductors are not ordinarily affected by temperature limitation requirements because of Section 240-3(d). As a general rule, Nos. 14 through 10 conductors have a maximum overcurrent protective device rating that is no higher than the ampacity rating listed in the 60 degrees Celsius column. Maximum overcurrent protection for copper conductors are: 15 amperes for No. 14, 20 amperes for No. 12, and 30 amperes for No. 10.

Two aluminum (or copper-clad aluminum) conductors are also listed: 15 amperes for No. 12, and 25 amperes for No. 10. Where specific requirements are met in Sections 240-3(e) through (g), overcurrent devices for these conductors are not limited to these ampere ratings.

110-14(c) Temperature Limitations

One ordinance that is frequently overlooked when choosing a conductor is Section 110-14(c). The temperature rating associated with a conductor's ampacity must be selected and coordinated so that the lowest temperature rating of any connected termination, conductor, or device is not exceeded. [110-14(c)] Just because a conductor has an insulation with a temperature rating of 90 degrees Celsius, does not mean the ampacity is automatically selected from the 90 degrees Celsius column. Instead, the appropriate column for conductor ampacity selection depends upon the temperature rating of the termination (or connection) points.

Conductor temperature limitations can be compared to the strength of a chain. A chain is only as strong as its weakest link. Every termination point is a potential weak link for conductors. Therefore, the lowest temperature rating of the termination points, along the conductor's path, determines the maximum ampacity. This temperature limitations section is divided into two categories: (1) circuits rated 100 amperes or less and (2) circuits rated over 100 amperes. In the next issue, these two categories will be explained in the conclusion of temperature limitations for electrical connections.

MILLER, owner of Lighthouse Educational Services, teaches custom-tailored classes and conducts seminars covering various aspects of the electrical industry. He is the author of Illustrated Guide to the National Electrical Code. For more information, visit his Web site at www.charlesRmiller.com. He can be reached at (615) 333-3336 or charles@charlesRmiller.com.