Learning From the Past

Published On
May 15, 2020

Splices and taps of conductors have been in the National Electrical Code for a long time. The 2020 NEC contains major changes involving distribution blocks, splices and taps, and to understand the reasons for these changes, we need to review the past. In previous NEC editions, the requirements for taps and splices were in 312.8 for panelboard enclosures, in Article 314 for boxes and conduit bodies, in 366.56 for auxiliary gutters, in 376.56 in metal wireways and in 378.56 for nonmetallic wireways.

For many years, there weren’t many changes in these sections. A review of the past shows these sections all required splices and taps to be accessible; however, none dealt with power-distribution blocks. Conductor and splice fill couldn’t be more than 75% of the cross-section of the enclosures. Splices and taps could be accomplished using a variety of devices from butt splices, couple taps, c-crimps and terminal blocks. However, the types of splice devices were not specifically covered within the various boxes, enclosures, gutters and wireways. All of that started to change in the 2005 NEC .

In the 2005 NEC , power-distribution blocks were specifically added as new 376.56(B)(1) through (B)(4), where (B)(1) required power-distribution blocks installed in metal wireways to be listed.

Section 376.56(B)(2) required the enclosure size to be based on the 75% fill by 376.56(B)(1) and required the power-distribution block to have a wireway with dimensions that weren’t smaller than the spaces required by the manufacturer’s installation instructions. The spaces within the enclosures involved installations from one side of the enclosure to the other and the distance from termination points based on the installation voltage.

Wire-bending space to the power-distribution blocks in 376.56(B)(3) was required to comply with 312.6(B) and Table 312.6(B), which detailed the installation requirements from the entrance in the side of the enclosure to the conductor’s termination. Conductors entering directly opposite from the termination required more installation distance than a conductor being installed where it can be bent before insertion into the termination.

Finally, 376.56(B)(4) required power distribution blocks to not have “exposed live parts in the wireway after installation.” That wording was rather strange since the reason for the power-distribution blocks was to be energized. I believe the NEC panel really meant the power-terminal blocks were to have an insulated cover so no one could take the cover off and be exposed to energized parts. Evidently, someone noticed that mistake and changed the text in the 2008 edition to, “power distribution blocks shall not have uninsulated live parts exposed within a wireway, whether or not the wireway cover is installed.” That fixed the original problem from the 2005 NEC .

There were two changes in the 2014 NEC dealing with power-terminal blocks installed in a metal wireway. The first change stated, “power distribution blocks installed on the line side of the service equipment shall be listed for the purpose.” The other change that occurred involved a new requirement that any conductors installed in the wireway cannot obstruct the access to the power- distribution terminal blocks. Evidently, some installations of power-terminal blocks were having access blocked by subsequent conductor installation, which was dangerous to the electrician.

A subtle change occurred in the 2017 NEC that many electrical contractors, electricians and electrical designers may not have noticed but had serious consequences for electrical installations at and before electrical services. The new text in the 2017 edition stated power-distribution blocks were required to be “listed and marked as suitable for use on the line side of service equipment or equivalent” in accordance with 314.28(E)(1) where installed in boxes and 376.56(B)(1) for installations in metal wireways.

For the 2020 Code , the provisions for listing and marking apply to power-distribution blocks on the line side of the service equipment, and they apply the same requirements to splice and tap devices. Power blocks, splice devices and tap devices installed before the service overcurrent protective device must be listed and marked as suitable for use on the line side of the service equipment.

The change in the 2020 NEC will have far reaching consequences since the listing and marking of these devices will involve maximum fault current rating and other testing issues involving conductor termination pull out issues. The marking of the splice, tap and power-block devices may require marking based on the available fault current of the system. Be careful on these types of installations and be sure to check the devices to ensure compliance with all listing issues.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.