Updated: Jun 21, 2021
With the rising cost and difficulty in getting electrical materials on site, it is important to minimize the material and construction costs. For electrical construction, the wiring (conductors) is a significant overall cost for any project and a good starting point for reducing initial construction costs. For most, this means potentially switching from copper wiring to aluminum wiring. However, another option to consider is utilizing the 75-degree ampacity rating for conductors rated under 100 amperes rather than the 60-degree ampacity rating. Although the 60-degree ampacity rating is typically taught to engineers, the National Electrical Code (NEC) will permit using the 75-degree rating based upon a couple of factors.
Insulation Temperature Rating
The temperature rating of the equipment and the respective conductors is first discussed in Article 110.14(C) of the NEC. Within this article, the NEC restricts the ampacity of the conductor based upon the temperature rating of the lowest rated connected termination, conductor, or device within the equipment served.
The article further elaborates on the requirements in Article 110.14(C)(1)(a) which includes requirements for termination of equipment rated 100 amperes or less or marked for 14 AWG through 1 AWG conductors. Lines (1) and (2) within this article reference utilizing conductors rated for 60-degree (Celsius) C for this application and permits the use of conductors with higher temperature ratings as long as the ampacity is determined based upon the 60-degree C rating. These lines are most commonly referenced when specifying conductors under 100 amperes at the 60-degree ampacity ratings included in Table 310.16 of the NEC.
However, line (3) states, “Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.” This line permits the use of the 75-degree C rating in Table 310.16 as long as the connectors and equipment you are terminating to also include a 75-degree C rating. As this is most commonly the case with modern electrical distribution equipment it permits the use of the 75-degree C ampacity rating which helps to reduce the overall cost of feeders and branch circuits. However, caution should still be taken in case you are unfamiliar with the equipment being served especially when you are using this approach for the branch circuits.
It should also be noted that Article 110.14(C)(1)(b) of the NEC includes requirements for termination of equipment rated over 100 amperes or marked for conductors larger than 1 AWG using the 75 degree C ampacity rating.
Understanding the insulation temperature rating of the conductors you intend to use is important, however, the selection of the correct conductor type and size includes several factors including environment (dry, wet, corrosive, etc.), ampacity, installation (free air, conduit, direct buried, cable tray, etc.), and ambient temperature to name a few. Each of these factors will guide the engineer and contractor in determining the correct conductor size, type, and insulation that best meet the requirements of the design and installation.
Article 310 of the NEC covers the requirements for conductors including types, insulation characteristics, ampacity ratings, installations, and uses. Within this article Table 310.4(A) is included which includes a thorough list of conductor types, the temperature ratings, applications, insulations, and thicknesses. Within this table, you will find the most common wire types used within a building which include THHN (dry and damp locations) and THWN (wet locations). Both of these cables (THHN/THWN) are dual rated for 60 / 75-degree C or higher but as noted above, can be utilized at the 60-degreeampacity ratings outlined in Table 310.16 of the NEC.
When using the 75-degree ampacity ratings for conductors 100 amperes and less, it permits the ability to use at least one (1) wire size smaller in most cases. Although this doesn’t seem significant, the cost savings will start to add up quickly if you have equipment branch circuiting rated at 25 amperes and above.
Utilizing the higher rated insulation temperature (75-degree C) for the feeder and branch circuit conductors serving the electrical distribution equipment and devices for a building can provide significant labor and material cost savings for an owner. However, always ensure the equipment that you are using is properly rated for the insulation type.
Cameron Bellao, PE, LEED AP
Electrical Engineering Manager