1. PURPOSE
1.1 This engineering standard defines the methods of selecting, sizing, installing, and supporting terminal boxes.
- SCOPE
2.1 This standard applies to all terminal box installations. While directed toward Air Products-owned and -operated facilities, it shall be considered the minimum requirements for any facility design. For sale of equipment facilities, it is the responsibility of the electrical designer to verify these requirements with the customer representatives before proceeding with facility design.
- RELATED DOCUMENTS
3.1 Air Products Engineering Documents
STD-P338A Low Point Drain Installation Details (Sht. 1 and 2)
4AEL-620300 Electrical Work
- National Electrical Manufacturers Association (NEMA)
250 Enclosures for Electrical Equipment (1000 Volts Maximum)
- TERMINAL BOX CONSTRUCTION
4.1 All terminal boxes located outdoors shall be NEMA 4X. Aluminum is preferred due to its mechanical strength, resistance to most adverse environments, and moderate cost.
4.1.1 The use of nonmetallic terminal boxes shall only be considered when low initial cost is requested and long-term reliability is not an issue. Air Products’ operating experience has been that, while most nonmetallic boxes are made of material that is resistant to continued exposure to sunlight and other outdoor elements, the material becomes brittle and more easily susceptible to damage after years of exposure to these elements.
4.1.1.1 The use of nonmetallic terminal boxes must also be carefully reviewed when multiple cutouts are provided in the terminal box door for items such as controls or pilot lights. The removal of the door material for the installation of these devices can significantly reduce the structural integrity of the door unless the door is properly designed to accept these devices.
4.1.2 The use of stainless steel terminal boxes shall be considered only when required to withstand specific environmental conditions that other available materials cannot withstand or by project‑specific requirements.
4.2 All terminal boxes located indoors in process areas (for example, compressor buildings, storage buildings) shall be NEMA 12, except where the area is subject to occasional hose-down for cleaning purposes. In those installations, NEMA 4 boxes shall be used.
4.3 Terminal boxes located indoors in clean, dry, nonprocess areas (for example, control rooms, electrical rooms, office areas), where the presence of dust, dirt, or dripping noncorrosive liquid is mostly nonexistent, may be NEMA 1.
- TERMINAL BOX INSTALLATION
5.1 Separate terminal boxes shall be installed for control (120 volt), instrumentation (4–20 mA) and thermocouple wiring. In applications when the number of field wiring pairs for instruments and thermocouples combined does not exceed 12 pairs for either wiring type, the wiring may be combined in one common terminal box provided instrumentation wiring is terminated on a separate terminal strip from thermocouple wiring.
5.2 All boxes shall be readily accessible and shall be supported securely and independent from the conduit system.
5.3 All metallic boxes containing ac wiring shall be grounded with a grounding conductor installed in at least one of the conduits entering the terminal box which, at the other end, shall be solidly connected to a grounding source connected to the plant grounding electrode system.
5.4 All metallic and nonmetallic terminal boxes containing 120-volt wiring shall be equipped with a separate 12-point ground bar, Square D Co., catalog #PK12GTA or equal. This ground bar shall be used to terminate all ground wires within the terminal box not terminated on terminal blocks (ground wires identified in paragraph 5.3).
5.5 Refer to STD-P338A for information regarding installation of low-point breather drains at bottom of terminal boxes to prevent accumulation of condensation in terminal boxes not installed in climate‑controlled atmospheres (for example, outdoors, water treatment areas).
- APPROVED MANUFACTURERS
6.1 Terminal Boxes, Hoffman Engineering Company or equal.
6.2 Terminals blocks and associated hardware, Allen Bradley Company or equal.
- AIR PRODUCTS COMMONLY USED NEMA CLASSIFICATIONS
STANDARD (NEMA) ENCLOSURE TYPES
| Type | Description | Application |
| NEMA-1 | General Purpose | Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection against falling dirt. |
| NEMA-4 | Watertight and dust-tight | Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water and hose-directed water, and that will be undamaged by the external formation of ice on the enclosure. |
| NEMA-4X | Watertight, dust-tight and corrosion-resistant | Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, hose-directed water, and corrosion, and that will be undamaged by the external formation of ice on the enclosure. |
| NEMA-12 | Industrial use, dust-tight and drip-tight | Enclosures constructed (without knockouts) for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers and flyings; and against light splashing of liquids. |
- TERMINAL BOX ASSEMBLY
8.1 Terminal boxes should be purchased as directed on the contract drawings.
8.2 All terminal boxes shall contain a minimum of 20% spare terminal blocks for future growth.
8.3 Terminals shall be mounted on DIN rails with adequate spacing between the box sides and terminals to allow for field wiring to be installed without the use of tools (other than a screwdriver) or without the need to force wires into the terminals on top of the wire bundles. Adequate spacing shall also be maintained between the rows and the top and bottom of the enclosure.
8.4 Adequate spacing between terminal rows shall also be maintained to allow room for field wiring terminations. Cable bundles terminating on the terminal strips shall not come into contact with the terminals or rails. Any spare conductors shall be coiled neatly at the bottom of the terminal box and shall be identified as spare or as noted on the contract drawings.
8.5 Terminal rows shall be mounted vertically unless specified differently on the contract drawings.