Why are Temperature Ratings Important?

Conductors carry a specific temperature rating based on the type of insulation used on the conductor. Common insulation types can be found in Table 310.4(A) of the NEC, and corresponding ampacities can be found in Table 310.16(B)(16). Ampacity of a 1 AWG Copper Conductor with different insulation Types shows the ampacity of a 1 AWG copper conductor based on different conductor insulation types.

The ampacity of the 1 AWG Cu conductor depends on the temperature rating of the insulation. At the same ampacity, a smaller conductor with higher-rated insulation can be used instead of a larger conductor with lower-rated insulation. As a result, the amount of copper and even the number of conduit runs needed for the job may be reduced.

One of the most common misapplications of conductor temperature ratings occurs when the established temperature rating of the equipment termination is ignored. This is particularly true for equipment rated for 600 V and less since the equipment is tested as a complete system using conductor sizes by the NEC rules. Reduced conductor sizes result in the system having less ability to dissipate heat and therefore increase the operating temperature of the equipment terminations. Conductors must be sized by considering where they will terminate and how that termination is rated. If a termination is rated for 75°C, the maximum temperature at that termination is 75°C when the equipment is loaded to it’s ampacity. If 60°C insulated conductors are used in this example, the additional heat at the connection above 60°C could result in conductor insulation failure.

When a conductor is selected to carry a specific load, the user/installer or designer must know the termination ratings for the equipment in the circuit. For example, consider a circuit breaker with 75°C terminations and a 125 A load. If a THHN
(90°C) conductor is chosen for the job, review Table 310.16(B)(16) in the NEC and look for a conductor that will carry the 125 A. Although a 90°C conductor is being used, ampacity must be chosen from the 75°C column because the circuit breaker termination is rated at 75°C. Based on the table, a 1 AWG copper conductor is acceptable. The installation would be as shown in Properly Sized Conductor, with proper heat dissipation at the termination as well as along the conductor length. Had the temperature rating of the termination not been a consideration,
a 2 AWG conductor might have been chosen, based on the 90°C ampacity. This may have led to overheating at the termination or premature opening of the overcurrent device due to the smaller conductor size (see Improperly Sized Conductor).

In this same example, a conductor with a 75°C insulation type (THW, RHW, USE, etc.) also would be acceptable since the termination is rated at 75°C. A 60°C insulation type (TW) is not acceptable since the temperature at the termination could rise to a value greater than the insulation rating.