Anatomy of a Conductor, Part 6: More on Branch Circuit Basics - Temperature Limitation
This issue continues our in-depth look at the branch circuit -- selection of the branch circuit conductor.
In the previous articles, we learned how to calculate the base current value for the branch circuit. Using this current value, the designer should determine / select the correct conductor size for the branch circuit. The selection of the conductors for the branch circuit should be based on the secondary factors and the auxiliary factors previously discussed or to be addressed in future articles in this series.
To refresh, the secondary factors are: (1) temperature limitation, (2) temperature correction factor, (3) adjustment factor, (4) equipment grounding conductor, (5) load type, (6) conductor insulation type, and (7) material the conductors are made of.
We will now examine one of the Secondary Factors - temperature limitation and its effect on selections related to the design of the branch circuit conductors.
The temperature limitation of branch circuit conductors is mentioned in two articles of the NEC.
(1) NEC Article 110 Requirement for Electrical Installations Section at 110.14 (C) Electrical Connections states: "The temperature rating associated with the ampacity of a branch circuit conductor shall be selected and coordinated so that as NOT to exceed the lowest temperature rating of any connected termination, conductor, or device." The same paragraph goes on to state: "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for adjustment, correction, or both."
(2) NEC Article 310 Conductors for General Wiring, Section 310.15 Ampacity for ConductorsRated 0 - 2000 Volts, Paragraph (3) Temperature Limitation of Conductors continues these requirements: "No conductor shall be used in such a manner that its operating temperatures exceeds that designed for the type of insulated conductor involved."
When you look at the ampacity tables, you will see that each table is broken into two parts that identify the material the conductors are made of (CU conductors and AL conductors). Each part is then broken into 3 columns that identify the different temperature rating of the branch circuit conductor. These temperatures are 60, 75, and 90 degrees Celsius. Each of these columns then lists the Type of Insulation (TW, THW, and THHW) used in the branch circuit conductor. Then, you can determine the branch circuit conductor size and its associated ampacity.
In summary, the temperature rating of the branch circuit conductor should match termination points where the branch circuit conductor lands. The higher rated ampacity for the branch circuit conductor(s) cannot be used unless the terminals where the branch circuit conductors land have comparable ratings. Most termination for systems rated at 600 V or less are rated for 60 degrees Celsius for branch circuits rated 100 Amps or less, and 75 degrees Celsius for branch circuits rated over 100 Amps. For systems rated over 600 V, the terminals are designed for branch circuit conductors rated for 90 degrees Celsius and higher. .
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