Cabinets and Panelboards in Article 312: Be specific when it comes to ‘panelboards,’ part 2

National Electrical Code
Published On
Jun 15, 2022

In last month’s column, I explained the proper use of the term “panelboards,” which are the interior busing and breaker terminations covered by parts I and III of Article 408. That article was more involved in covering the concept behind Article 408 and preventing the misuse of terminology and misconceptions surrounding panelboards. The requirements in Article 312 are also important in panelboard and cabinet applications.

In the past, I have written about placing the panelboard busing in the cabinet for proper bending and appropriate space for termination of conductors, but users need to know much more about Article 312 than space requirements.

Let’s start out by looking at the spacing requirements in 312.6(A) and (B). Before the 2017 National Electrical Code , conductor space requirements for wire sizes in Table 312.6(A) only applied to standard stranded building wire as provided in Table 5 in Chapter 9 and not compact stranded AA-8000 aluminum alloy conductors as in Table 5B. Table 312.6(B) had compact stranded AA-8000 aluminum alloy conductors, but the other table did not until it was added to Table 312.6(A) in the 2017 NEC .

Tables 312.6(A) and (B)

Compact stranded conductors are much stiffer and more difficult to pull into raceways and bend into place within enclosures. Thus, they take up more space than regular flexible standard stranded building wire.

Where cables are installed in parallel (those are limited by 310.10(G) to sizes 1/0 and larger), more space is required based on the number of conductors within the enclosures. Tables 312.6(A) and 312.6(B) clearly provide that extra space for larger cables and the number installed in parallel.

Table 312.6(A) provides two applications. The first is the size of the pull section area between the enclosure sides and back and the busbar/bus termination. Table 312.6(A) provides the necessary space for termination to the circuit breakers and supply lugs and includes space so large conductors can easily be bent into 90-degree or U-bends, since the conductors are not entering into the enclosure directly opposite of the termination. Where conductors are entering into the enclosure directly opposite the termination point, extra space is necessary due to the large conductor sizes and lack of flexibility of standard stranded and compact stranded large conductors. That is provided by Table 312.6(B).

Read the notes

Many NEC users do not read the notes at the bottom of the tables and miss some very important points that may permit spaces to be less than the spaces given in Table 312.6(B). For example, Note 2 at the bottom of this table states that, for removable and lay-in wire terminals intended for only one wire (plus some limited sizes of parallel wires), bending space for the wire may be decreased by superscripts a through e, located within the table and based on the sizes of conductors.

The reductions in distances in Table 312.6(B) are as follows: superscript a is ½-inch, superscript b is 1 inch, superscript c is 1½ inches, superscript d is 2 inches and superscript e is 3 inches and based on the capability of removing the lug or having a lay-in lug. Always check the notes at the bottom of any table in the NEC , since most are mandatory and not for information only.

What 312.8 permits

Many times, the pull section area in the cabinet may be used for purposes other than just installing conductors. Where enclosures contain switches and overcurrent devices, 312.8 permits the wiring spaces within enclosures to contain other wiring and electrical equipment, as provided in 312.8(A) and (B).

The wiring space can have conductors feeding through the enclosure to others, and can be spliced and tapped, as long as the total of all conductors does not exceed 40% of the cross-sectional area of that space plus the total area of all conductors, splices and taps installed at any cross-section of the wiring space and doesn’t exceed 75% of the cross-sectional area of that space. A label must also be installed at the enclosure that identifies the closest disconnecting means for any feed-through conductors.

Section 312.8(B) allows power management and energy management equipment to be installed in that area with proper identification, not exceeding the 75% rule, among other requirements.

Articles 408 and 312 are closely interlinked. Review both for proper application of NEC rules for panelboards and cabinets.

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.

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