General requirements for conductor connections, terminations, splices and terminal temperatures are covered in 110.14 of the National Electrical Code (NEC). Since 1990, there have been some very substantial changes in 110.14 and other parts of the NEC involving terminals, terminations and splicing of conductors.
In the 1993 NEC, temperature limitation requirements were added as 110.14(C). The temperature rating associated with the ampacity of a conductor must be selected and coordinated to not exceed the lowest temperature rating of any connected termination, conductor or device, such as a circuit breaker at the power source and a receptacle or other similar device at the load end. For electrical equipment, the determination of the termination provisions are based on 110.14(C)(1)(a) or (b).
Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions are based on Table 310.15(B)(16) in either the 60°C column or the 75°C column of the table.
Termination provisions for electrical equipment rated 100 amperes (A) or less, or marked for No. 14 AWG through No. 1 AWG, must be used only for one of the following four applications: (1) conductors rated at 60°C, (2) conductors rated higher than 60°C if the conductors’ ampacity are based on the 60°C column from the Table, (3) conductors with higher temperature ratings can be used at higher ampacities if the equipment on both ends of the circuit are listed and identified at the higher temperature rating, and (4) for motors marked with design letters (usually NEMA motors) B, C or D, conductors having an insulation rating of 75°C or higher are permitted to be used if the conductors’ ampacity, based on the table, do not exceed the 75°C ampacity column value.
Termination provisions for equipment rated over 100A or marked for conductors larger than No. 1 AWG can be used for one of the following applications: (1) the 75°C column can be used with conductor insulation rated 75°C or higher as long as the ampacity does not exceed the 75°C column ampacity. The addition of 110.14(C) to the 1993 NEC was not really new to the electrical industry because 110.3(B) in the NEC and the UL listing standards required these provisions; however, this addition provided the text to electricians, electrical contractors and electrical inspectors.
New 110.14(D) for installation of electrical equipment was added in the 2017 NEC requiring application of tightening torque as indicated on equipment or in the manufacturer’s installation instructions. A calibrated torque tool must be used to achieve the indicated torque value unless the equipment manufacturer has provided installation instructions for alternative methods of achieving the required torque.
For many years, power distribution blocks were installed as an integral part of many electrical installations but were not specifically covered as power distribution blocks. Power distribution blocks were added to metal wireways for the 2005 NEC in 376.56, newly titled “Splices, Taps, and Power Distribution Blocks,” and a new (B) covering power distribution blocks. Based on 376.56(B)(1), the Code required power distribution blocks to be installed in metal wireways. Power distribution blocks cannot have exposed energized parts in the wireway after the installation.
For example, a plastic cover could be placed over the terminal blocks. The conductors, including splices, taps and terminal blocks, cannot fill the wireway to more than 75 percent of the area at that point. In addition to this sizing space requirement, the power distribution block shall be installed in the wireway with dimensions not smaller than specified in the power distribution block’s installation instructions. Wire-bending space at the terminals must comply with 312.6(B).
Additional changes to 376.56(B) in subsequent NEC editions ensure conductors are arranged so the power blocks are unobstructed following the installation and power distribution blocks because uninsulated energized parts cannot be exposed within a wireway, regardless of whether the wireway cover is installed.
The 2017 NEC added a requirement in 376.56(B)(1) for the power distribution blocks installed in metal wireways on the line side of the service equipment to be marked as “suitable for use on the line side of service equipment” or equivalent. These wireway requirements have been added to 314.28(E). Compliance is easier than fixing an installation.