Many people refer to the enclosure and the interior panelboard as a “panelboard.” Panelboards do not include the enclosure or cabinet, so this is a misappropriation of a more complex application and piece of equipment. Misusing the term could be confusing and dangerous when the NEC requirements are not followed. Understanding the requirements for panelboards and enclosures will ensure that misapplications do not occur.
Panelboards, switchboards and switchgear are covered in Article 408, with Part I providing general requirements and Part III specifically covering panelboards. The busing, conductor terminations and bus support are covered in Part I, with Section 408.3(A) stating that conductors and busbars on a switchboard, switchgear or panelboard must be located so they are free from physical damage and held firmly in place, such as when placed in the appropriately sized enclosure. Section 408.3(B) states that the arrangement of busbars and conductors must be installed to avoid overheating due to inductive loads.
Where these panelboards are used as service equipment, 408.3(C) states the panelboard must be provided with a main bonding jumper sized in accordance with 250.28(D) or equivalent. In 408.3(E)(1), AC phase arrangement on three-phase buses must be A, B, C from front to back, top to bottom or left to right, as viewed from the front of the panelboard. In 408.3(E)(2), where the panelboard is used for DC, the ungrounded buses are permitted to be in any order but must be field marked to indicate polarity, grounding system and nominal voltage. In 408.3(F)(1), where a panelboard contains a four-wire delta-connected system with a high leg to ground, the high leg must be connected to the B-phase and must be legibly and permanently marked with “Caution ___ Phase Has ___ Volts to Ground.” This marking will warn anyone working on the panelboard that there is high leg to ground (usually 208V to ground) and to not accidentally connect any 120V equipment to this phase leg.
Misusing the term “panelboard” could be confusing and dangerous when NEC requirements are not followed. Understanding the requirements for panelboards and enclosures will ensure that misapplications do not occur.
In 408.3(F)(2), an ungrounded AC system connected to a panelboard, such as a three-phase ungrounded delta system that complies with 250.21, must be permanently and legibly field marked with “Caution Ungrounded System Operating—___ Volts Between Conductors.” According to the NEC Handbook, the intent of this requirement is to delineate grounded from ungrounded electrical systems.
When a ground fault occurs on a three-phase ungrounded system, the voltage to ground on the ungrounded system equals the line-to-line voltage. When one conductor goes to ground, the ground detection system, as required by 250.21, provides detection of the ground and a warning device will indicate this ground fault. This early warning will provide the advantage of using an ungrounded system for continuity of operation, which in some processes might create a safer condition than would be achieved by automatic or unplanned opening of the supply circuit. Ground detection will warn of the ground fault to permit an orderly shutdown of a process.
To tie down these requirements, 408.38 states that panelboards must be mounted in cabinets, cutout boxes or identified enclosures and must be of the dead-front type. Load centers are part of an assembled listed product with a fixed panelboard inside of an enclosure with a fixed size based on 312.6 and the accompanying Tables 312.6(A) and 312.6(B). Article 312 applies to the installation and construction of cabinets, cutout boxes and meter socket enclosures. It does not apply to equipment operating at over 1,000V, except as specifically referenced elsewhere in the NEC .
Based on text from the 2020 NEC Handbook, “Cabinets and cutout boxes are designed with a swinging door to enclose potential transformers, current transformers, switches, overcurrent devices (such as panelboards), meters, or control equipment. Cabinets and cutout boxes are required to be of sufficient size to accommodate all devices and conductors without overcrowding or jamming. Additional space is often provided through auxiliary gutters that are covered in Article 366.”
Section 312.6(A) and accompanying Table 312.6(A) provide width of the wiring gutters for the conductors installed in the enclosures with wider wiring gutter size for parallel conductors. The larger the conductors and the more in parallel, the wider the wiring gutter or sides of the enclosure must be from the panelboard busing, circuit breakers and incoming or outgoing conductors and terminations.
Table 312.6(A) also provides the appropriate distances where the conductors do not enter or leave the enclosure through the wall opposite its terminal or termination on circuit breakers. Section 312.6(B) and Table 312.6(B) provide distances where the conductors do enter or leave the enclosure through the wall opposite its terminal or termination on circuit breakers.
June’s column will further discuss the requirements for panelboard enclosures.