Good Placement = Prevention

In the past, one of the thorny issues in M/E coordination has been the requirements of the National Electrical Code (NEC) governing the exclusion of mechanical systems from the area around indoor electrical equipment, particularly the overhead space above certain types of equipment. The latest (1999) edition of the NEC contains revisions to these requirements that may make our lives easier. Let’s review the rules that are now in force.

Water and Electricity Don’t Mix

In general, Article 110 of the NEC excludes piping, ductwork, and equipment “foreign to the electrical installation” from “dedicated electrical space.” We all know water and electricity don’t mix safely, and excluding piping and ductwork (in which condensation may occur) is meant to protect the electrical equipment from leaks and drips. In addition, this requirement is intended to preserve the space directly above panelboards and motor control centers for the installation of the large number of conduits that typically exit this equipment. In the past, this space was required to be clear for a distance of 25 ft or to the bottom of the structural ceiling, whichever was less. That requirement was felt to be impractical and unnecessary in many installations, and has been relaxed in the 1999 NEC.

“Foreign to the electrical installation” means equipment that does not serve the electrical equipment, or a dedicated room or space provided for the electrical equipment. Fire protection sprinkler system branch piping serving the electrical room or the area in which the electrical equipment is installed is not considered “foreign.” Neither is ductwork installed in an electrical room for the purpose of ventilating or cooling that space. A sprinkler main or riser for another part of the building or a duct that passes through an electrical room to serve another space would not be permitted.

The “dedicated electrical space” from which these systems are excluded varies with the type of electrical equipment involved. We will consider three separate categories: the low-voltage (600 V) equipment typical in buildings, medium-voltage (600 V) installations, and transformer vaults.

600 Volts and Under

Dedicated space for low-voltage equipment consists of two types: workspace around the equipment and conduit entry space above the equipment.

Clear workspace is required around electrical equipment to permit safe access for operation and maintenance of the equipment. Where energized access is required to operate, test, or adjust the equipment, a clear work space a minimum of 6.5 ft high, 2.5 ft wide, and of a depth that varies with the operating voltage must be provided. Where access is required only with the equipment de-energized, a minimum depth of 2 in. to 6 in. must be provided. This work space is required for all electrical equipment, and no construction of any type is permitted to obstruct it.

In addition, dedicated overhead space is established for switchboards, panelboards, and motor control centers to permit the installation of conduits. This space is the width and depth of the equipment and extends to a height of 6 ft above the equipment or to the structural ceiling, whichever is lower. Foreign ductwork and piping must be kept completely out of this space, and if installed above the 6 ft limit, it must be provided with drip pans or equivalent means of protecting against leaks and condensation.

Over 600 Volts

The workspace requirement for low-voltage equipment applies to medium-voltage installations as well, although the higher voltage increases the depth required in the direction of access to energized parts. When it comes to overhead space, however, the NEC leaves room for interpretation. There is a general requirement that foreign systems be excluded “from the vicinity” of the equipment, but without the detailed guidance that is provided for low-voltage equipment.

Higher voltage increases both the susceptibility of electrical equipment to contamination-induced failure and the potential hazards of such a failure. For this reason, “in the vicinity” should be interpreted to mean anywhere from which leakage or dripping could reach the equipment; in the case of pressurized piping systems, avoiding the space directly over the equipment may not be sufficient. Whenever possible, keep all mechanical systems well clear of medium-voltage installations.

Transformer Vaults

Transformer vaults are rooms with special construction features required to house transformers with flammable insulating fluids. They are of 3-hr, fire-rated construction, often located below-grade outside the building footprint. Their ventilation openings must discharge out of doors with separation from doors, windows, and building air intakes. Foreign systems are excluded from the entire vault. Fire protection piping and transformer cooling equipment is permitted.

Review for compliance with these requirements should always be part of the final M/E coordination check of design drawings. In addition, noting the excluded areas on all plan drawings will help ensure that field-routed piping and field-located equipment does not require later relocation. ES