When is an electrical vault required?


Article 110.31 defines enclosures for electrical installations. 1) When is an electrical vault required [section A of code]? and 2) If I am providing a 4160-volt motor control center for an industrial plant [qualified personnel], am I required to meet the fire rating as shown for a vault? Or can I supply this motor control center in a generic modular building? — D. G.



It is difficult to give a concrete response to “When is an electrical vault required,” as the use of a vault could be a design consideration or a mandatory Code requirement. There are conditions and other criteria that must be considered.

For example, where equipment is operating at 50 volts or more, Section 110.27(A) allows the use of a vault for guarding of live parts against accidental contact. This is a design consideration. The vault could be used for guarding live parts in lieu of an electrical enclosure, such as a typical switchboard or panelboard enclosure. Section 230.6(3) also allows the use of a vault for service conductors to be considered outside the building.

Again, this is design consideration and not a mandatory Code requirement. The Code allows the service conductors to be considered outside the building where installed in a vault. Keep in mind that the service conductors inside the vault could be exposed and insulated or uninsulated.

An example of mandatory requirements is Article 110, Part III, which applies to over 600-volt installations, and it indicates that over 600-volt installations must comply with Part I and the requirements of this section, which supplement or modify the requirements of Part I. As such, Section 110.31 allows the enclosure for over 600 -volt installations to be a vault.

Such vault must comply with the mandatory requirement of 110.31(A), Fire Resistivity of Electrical Vaults. Keep in mind that the over 600-volt conductors inside the vault could be exposed and insulted or uninsulated, too. Again, the vault could be used in place of a typical electrical enclosure, such as metal-enclosed switchgear enclosure.

The 4160-volt motor control center in question is likely constructed as metal-enclosed switchgear. Generally, metal-enclosed switchgear is not required to be installed in a vault. For additional information on construction requirements for metal-enclosed switchgear, see Article 490, Part III, Metal-Enclosed Power Switchgear and Industrial Control Assemblies. The 4160-volt motor control center should meet the requirements of Part III.

Further, if the 4160-volt equipment is metal-enclosed, installed indoors, and accessible to unqualified persons, Sections 110.31(B)(1) and 110.31(D) come into play. Section 110.31(B)(1) provides information on openings in the equipment and signage requirements; Section 110.31(D) provides information on ventilating openings, protection against physical damage, and specific requirements where the general public may have access to the equipment.

To summarize, a vault may or may not be required for over 600-volt installations. The use of a vault is dependent on the system design and the type of equipment installed. Where the electrical installation contains exposed energized parts accessible to qualified persons only, a fire resistive electrical vault or other type of enclosure is likely required.
Where metal-enclosed equipment is installed, and the equipment is accessible to unqualified persons, generally a vault is not required.

I hope this provides some guidance to your questions. If of any reassurance, based on the information provided, and if all other applicable code requirements are considered, I am comfortable in saying that the motor control center could be installed in a “generic modular building!” — Lanny McMahill, CMP-1