CQD Spotlight features questions and answers from NECA and ELECTRICAL CONTRACTOR’s Code Question of the Day program. Whether you’re a seasoned contractor, an apprentice or simply a National Electrical Code enthusiast, you’ll gain knowledge with a nod to the legacy of Charles M. “Charlie” Trout, a true NEC legend.
From the archives— March 1, 2005
On a quiz for my code class, one question was, “What is the ampacity of an 8-AWG THHN conductor supplying a hoist motor with a 30-minute rating?” I used Table 610.14(A), and answered 69A. The instructor referred to 110.14(C)(1)(4), which states that “For motors marked with design letters B, C, D, or E, conductors having an insulation rating of 75°C or higher shall be permitted to be used provided the ampacity of such conductors does not exceed the 75°C ampacity.“ The instructor said that you would have to use the 75°C column, and answer 60A. My question is, why would the table even have the 90°C column if you couldn’t use it?
Good question. However, I believe that the instructor is correct. NEC 110.14(C)(1)(4) should be followed. Even though 90.3 permits Chapter 6 to amend the general rules of Chapter 1, it is my opinion that Table 610.14(A) permits the use of conductors with higher temperature ratings only where the conductor termination point is not exceeded.
To my knowledge, at this time there are no conductor termination points rated higher than 75°C for use with crane or hoist motors. I believe the table is confusing and leads the user to believe that using conductors with 90°C or 125°C temperature ratings is permitted without considering the temperature rating of the termination points.
From the archives— Feb. 19, 2024
Section 110.14(C)(1)(a) tells us the wiring to use for terminations of circuits of 100A or less, or marked for use with No. 14 through No. 1 AWG wire. The next section, 110.14(C)(1)(b), tells us the wiring to use for terminations of circuits more than 100A, or marked for use with wire larger than No. 1 AWG.
In the case where we have a circuit that is 100A or less, and it can accept wire larger than No. 1 AWG, can we pick which section to use for wire ampacity?
In general, the temperature rating is determined for the application of conductor ampacity. The lowest temperature rating of any connected termination, conductor or device shall be selected in accordance with Section 110.14(C).
In the example submitted with the question, a circuit rated 100A or less would be permitted to use a temperature rating higher than the 60ºC rating if the circuit complied with Section 110.14(C)(1)(a)(3) requiring all of the equipment (terminations, connectors, devices, conductors, etc.) in the circuit to be listed and identified for use with the higher temperature rating.
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Understanding conductor ampacity and termination limitations can be complex. The following process provides a foundational framework applicable to a variety of situations. While there are some exceptions, these principles serve as essential guidelines for most applications.
Step 1: Establish the load current. This is a critical step. Understand the difference between a continuous load and a noncontinuous load. Continuous load is a defined term in Article 100.
Step 2: Determine minimum conductor ampacity and size (210.19 and 215.2). Select the resulting larger conductor of the following as the minimum conductor size for the application.
- Determine the minimum ampacity required based only on conditions of use. Use the column of Table 310.16 that aligns with the insulation rating of the conductor and select a conductor with an ampacity equal to or greater than the adjusted load current [load current divided by the ambient temperature correction factors of 310.15(B) and adjustment factors of 310.15(C)].
- Determine the minimum conductor size base only on the equipment termination limiations found in 110.14(C). Use the column of Table 310.16 that aligns with the lowest temperature rating of the terminations on each end of the conductor.
Step 3: Select an OCPD rating that satisfies the following three criteria. If an OCPD cannot satisfy all three, increase the size of the conductor and repeat.
- Less than or equal to the ampacity of the conductor. The OCPD rating can exceed the ampacity of the conductor when the ampacity does not equate to a standard ampere rated OCPD and is less than 800A [see 240.4(B)].
- Less than or equal to the maximum current permitted for the termination. The OCPD rating selected cannot exceed the value in Table 310.16 that aligns with the termination temperature rating [see 110.14(C)].
- The OCPD rating must be greater than or equal to the noncontinuous plus 125% of continuous current.
