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Sizing Conductors, Part II

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Published In July 2011

Many factors are involved when sizing conductors. Specifications for sizing conductors are not just in one section or even one article of the National Electrical Code (NEC). Although the maximum allowable ampacities for conductors are listed in Table 310.15(B)(16) (formerly Table 310.16), other provisions must be considered.

Section 110.14(C) must be acknowledged when sizing conductors. In accordance with 110.14(C), the temperature rating associated with a conductor’s ampacity shall be selected and coordinated, so the lowest temperature rating of any connected termination, conductor or device is not exceeded. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or (C)(1)(b). Section 110.14(C)(1)(a) covers circuits rated 100 amperes (A) or less or marked for 14 AWG through 1 AWG conductors. Section 110.14(C)(1)(b) covers circuits rated greater than 100A or marked for conductors larger than 1 AWG.

Last month’s column covered the first three of four provisions for circuits rated 100A or less or marked for 14 AWG through 1 AWG conductors. This month, the discussion continues.

Circuits rated 100A or less
The fourth provision in 110.14(C)(1)(a) pertains to motors marked with design letters B, C or D. Conductors having an insulation rating of 75°C (167°F) or higher can be installed, provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity. In Table 310.15(B)(16), the temperature ratings of conductors are 60°C (140°F), 75°C (167°F) and 90°C (194°F). This means the ampacity of a 75°C conductor can be based on the 75°C column if it is supplying power to a motor marked with a design letter B, C or D. A conductor with a temperature rating of 90°C can also be installed, but the ampacity must not exceed the 75°C ampacity. For example, THHN conductors will have 75°C terminations on one end and a motor marked with a design letter D on the other end. After complying with the applicable requirements in Article 430, the conductors supplying power to this motor must have an ampacity of at least 55 amperes (A). What is the minimum size of THHN conductors required to supply power to this motor? The allowable ampacity of an 8 AWG THHN conductor (in the 90°C column) is 55A. Although conductors with a temperature rating of 90°C can be installed, the ampacity must not exceed the 75°C ampacity. The ampacity of an 8 AWG conductor in the 75°C column is 50A. Because this motor requires a minimum ampacity of 55A, installing 8 AWG conductors is not permitted. The allowable ampacity of a 6 AWG conductor in the 75°C column is 65A. Conductors supplying power to this motor must be at least 6 AWG (see Figure 1).

Circuits rated greater than 100A
There are two provisions for circuits rated greater than 100A or marked for conductors larger than 1 AWG. The conductor selection must be based on one of these two provisions. The first provision requires the use of a conductor rated 75°C. Although there are a limited number of conductors with a temperature rating of 75°C, there are definitely more conductors rated 75°C than conductors rated 60°C. See Table 310.15(B)(16) and Table 310.104(A) for maximum operating temperatures of conductors rated 600 volts (V). For example, THWN conductors (larger than 1 AWG) will supply power to a circuit rated greater than 100A. The conductors will have 75°C terminations on both ends. The ampacity can be based on the 75°C column because the conductors are larger than 1 AWG, and the circuit it is supplying is rated greater than 100A (see Figure 2).

In accordance with the second provision in 110.14(C)(1)(b), it is permissible to use a conductor with a higher temperature rating, provided the ampacity of such conductors is determined based on the 75°C ampacity of the conductor size used. For example, what is the maximum ampacity for a 1/0 AWG THHN copper conductor fed from a 150A breaker? Assume an ambient temperature of 30°C and no more than three current-carrying conductors in the raceway. The conductors will have 75°C terminations on one end, but the temperature rating of the terminations on the other end is unknown. To comply with 110.14(C)(1)(b)(2), do not exceed the 75°C ampacity for this conductor. Although the temperature rating on one end is unknown, the ampacity of this 90°C conductor can be based on the 75°C column because the conductors are larger than 1 AWG and the circuit it is supplying is rated greater than 100A. This 1/0 AWG THHN copper conductor has a maximum ampacity of 150A (see Figure 3).

The provision in 110.14(C)(1)(b) also states that, if conductors with higher temperature ratings are installed and the equipment is listed and identified for use with such conductors, the ampacity can be based on the higher temperature ratings. This means the ampacity of a 90°C conductor can be based on the 90°C column if all the terminations are rated at least 90°C. When looking to see if the equipment (panelboard, switchboard, disconnect, etc.) is listed and identified for use with 90°C conductors, look at the listing and labeling for the equipment, not just the marking on the lugs. Quite often, the lugs that are installed in equipment will have a 90°C temperature rating. A marking of AL9CU or CU9AL on the lug indicates the lug is listed for copper and aluminum conductors. The number 9 indicates the lug has a 90°C conductor temperature rating. A marking of AL7CU or CU7AL on the lug indicates the lug is listed for copper and aluminum conductors. The number 7 indicates the lug has a 75°C conductor temperature rating. If the equipment is not listed and identified for use with 90°C conductors, the ampacity of the conductor shall not exceed the 75°C ampacity for that size conductor. For example, the lugs in the top of a panelboard have a marking of AL9CU, but a label inside the panelboard states that the terminals are approved for 60°C and 75°C wire. Because the listing states the panelboard is for use with 60°C and 75°C conductors, the ampacity of the conductors supplying power to this panelboard shall not exceed the 75°C ampacity (see Figure 4).

When the conditions of use require a correction and/or adjustment of the conductor’s maximum ampacity, the last sentence of 110.14(C) is very useful. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction or both. Conditions of use include adjustment factors for more than three current-carrying conductors in a raceway or cable and ambient temperature correction factors. Correction and adjustment factors will be discussed later in this series on sizing conductors.

Next month’s column will continue the discussion of sizing conductors.

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, charles@charlesRmiller.com and www.charlesRmiller.com.

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Summary | 10 Annotations
110.14(C)(1)(a)
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motors marked
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design letters B, C or D
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an insulation rating of 75°C
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a 75°C
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conductor
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based on the 75°C column
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supplying power
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motor marked
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design letter B, C or D.
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