Things You'll Need
NEC table 310-15(B)(2)(a)
NEC table 310-16
The derating tables are based on the amount of heat the current will produce, plus the ambient temperature and the temperature of other wires close by. Derating applies to current carrying conductors only. Ground wires are not considered current carrying conductors. Therefore, if a conduit has four wires in it and one of those wires is a ground wire, the conductors do not require derating since only three carry current. The NEC tables refer to a circuit breaker as an OCPD which is an acronym for Over Current Protection Device.
NEC table 310-16 lists a maximum ampacity for 14, 12 and 10 gauge wires, which is higher than the allowed circuit breaker size. The maximum size circuit breakers are 15, 20 and 30 amperes for 14, 12 and 10 gauge wire, respectively. After derating wires, you must still adhere to the maximum circuit breaker size. For example: Derating a type THHN 14 gauge wire at 80 percent might lead you to believe you can use a 20 ampere circuit breaker. But the maximum size breaker allowed by the NEC is 15 amperes for a 14 gauge wire.
Conductor derating reduces the amount of current that wires are allowed to carry. The idea behind it is to protect the wire's insulation from degradation by excessive heat. As current passes through a wire, heat is produced. As current levels rise, the amount of heat produced also rises. National Electrical Code (NEC) section 310-10 requires conductor derating in two situations: when the ambient temperature rises above 30 degrees Celsius, and when you are bundling more than three wires in a conduit or cable. You will need access to NEC tables for the following instructions.
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Ambient Temperature Derating
Determine if the highest ambient temperature a conductor is subjected to exceeds 30 degrees Celsius. For example, the temperature in an attic reaches 125 degrees F (52 C) in the summertime. The conductor must be derated according to NEC table 310-16 because the ambient temperature exceeds 30 C.
Determine the conductor's ampacity and temperature rating by gauge and wire type in table 310-16. Find the conductor size and follow it across to the wire type column for the temperature rating and ampacity (See "Resources for a link to table 310-16). For example: A 12 Gauge type TW wire has a maximum ampacity 25 amperes and a temperature rating of 90 degrees Celsius.
Find the ambient temperature range in NEC table 310-16 in the ambient temperature column. Follow the row across to the temperature rating for the wire and read the derating percentage. Multiply the conductor's ampacity by the derating percentage. For example: The 12 gauge wire TW wire in the 52 C attic is derated to 76 percent of its maximum ampacity; 25 amperes x .82 = 19 amperes. Protect the wire with a circuit breaker rated at 19 amps or less. A 15 amp circuit breaker meets this requirement.
Bundled Conductor Derating
Determine if the number of current carrying conductors bundled in a cable or conduit requires derating. For example: Eight 14-gauge type TW current carrying wires travel through a conduit to serve two branch circuits in a house. Since more than three current carrying wires are bundled in a conduit, the wire ampacity must be derated.
Determine the maximum ampacity of the wire according to table 310-15. Find the wire gauge in the wire gauge column. Follow the row across to the wire type and read the listed ampacity (See "Resources" for a link to table 310-15). For example: The 14-gauge type TW wire is listed as having a maximum ampacity of 20 amperes.
Determine the rating percentage from the bottom of table 310-15. For example: The number of current carrying wires is eight, which must be derated to 70 percent ampacity; 20 amperes x .70 = 14 amperes. Protect the wires with circuit breakers rated at or below 14 amperes. The closest available circuit breaker is 10 amperes.
Bundled Conductor and Temperature
Determine if conductors require derating for both ambient temperature and the number of bundled conductors. For example: A conduit carrying six TW type 12-gauge wires travels through an attic that reaches 110 degrees F (43 C) in the summertime. Derate the wires for ambient temperature because the attic temperature rises above 30 C. Derate bundled wires because there are more than three conductors in the conduit.
Determine the ambient temperature derating as shown previously. For example: The 12 gauge type TW wires have a maximum ampacity of 25 amperes. According to table 310-16, derate the wires to 87 percent of maximum ampacity.
Determine the bundled wire derating as shown previously. For example: Derate six wires bundled in a conduit to 80 percent of maximum ampacity.
Multiply the maximum ampacity by both derating factors. For example: 25 amperes x .87 x .80 = 17.4 amperes. Protect the wires with a circuit breaker rated at 15 amperes -- the closest circuit breaker size that does not exceed 17.4 amperes.