How to mount an electrical enclosure

Mount it by the feet, not through the back

The NEMA FAQ on enclosures answers this exact question, from someone who wanted to wall-mount enclosures by drilling through the back of the electrical cavity and sealing the holes with silicone. Its answer: NEMA 250 states that a number of type-rated enclosures require a mounting means external to the equipment enclosure's electrical cavity unless an intermediate bracket or foot is used, and the final installation is always subject to the approval of the authority having jurisdiction, typically the local electrical inspector.

The reason is the same rule that governs every opening: the rating of a field-installed assembly is limited by its least-rated component. A hole in the back wall sealed with a bead of silicone is not a tested component, and you cannot see or inspect it once the enclosure is on the wall. Our NEMA 4 vs 4X vs IP66 guide explains what the rating covers.

Every NEMA4 Enclosure wall-mount enclosure ships with four external wall-mount feet, and the wall-mount hole pattern is on each product's reference drawing. Mark the holes from the drawing or by holding the enclosure in place, and fix it through the feet only.

Work out the load before choosing anchors

The fasteners have to carry more than the empty enclosure. Add up:

• the enclosure and its mounting panel;
• every device on the panel and on the door (transformers and drives are heavy for their size);
• cables and conduit hanging from the entries;
• the door: an open door levers the enclosure away from the wall on the hinge side, and someone will lean on it.

Choose anchors from the anchor maker's published load data for the actual wall material and the edge distances you have, and keep within the allowable loads they state. The wall itself has to carry the load too: FEMA's guide to nonstructural earthquake damage shows a panel board dislodged because a hollow concrete block partition wall failed. For hollow walls and drywall, carry the load to the studs or structure, for example on strut channel (below) or a backboard fixed to the structure.

Mount the enclosure flat and plumb, and shim behind the feet on an uneven wall rather than pulling the body tight against it, so the body is not distorted and the door still closes evenly on its gasket.

Seismic anchorage

In earthquake country, anchorage is part of the building code. FEMA E-74 (2012 edition), summarizing ASCE/SEI 7-10 Chapter 13, describes these provisions for electrical distribution panels:

• Seismic loads come from ASCE/SEI 7 Chapter 13, and the principal objective is to prevent the component from falling.
• In Seismic Design Categories D, E and F, anchorage is required for equipment weighing over 400 lb and for items over 20 lb mounted more than 4 ft above the floor; lighter items may be exempt when the component importance factor is 1.0.
• Exempt items must still be positively anchored to the structure, and need flexible connections to their conduit where the panel and the conduit can move relative to each other.

FEMA E-74 also notes that wall-mounted panels have generally performed well in past earthquakes, in part because of their modest weight and the ductility of sheet metal cabinets, and that the main damage mode is a panel dislodging and falling. The standards have newer editions; ask the structural engineer of record or the authority having jurisdiction which edition and design category apply.

Strut channel and backboards

Metal framing strut is the usual way to mount enclosures on walls that will not take anchors where you need them, or to group several enclosures on one frame. The basic channel is an open box about 41 x 41 mm (1-5/8 in square), formed from 12 or 14 gauge sheet; British Standard BS 6946 covers this channel in the UK. Two horizontal channels anchored into the structure, with the enclosure feet bolted to them using channel nuts, also hold the enclosure off the wall, which leaves room for cleaning and air behind it.

Watch the metal combination. A stainless steel guide on contact with other metals gives the rule: in material combinations, fasteners should always be made of the more noble material, so the cathodic surface is small. Stainless steel fasteners on much larger galvanized steel components do not normally cause corrosion, but where a small galvanized or carbon steel part is surrounded by a large stainless surface, the guide warns that under corrosive conditions the less noble part may suffer accelerated corrosion. Galvanic corrosion needs an electrolyte, so the risk is highest outdoors and in washdown areas. For stainless enclosures:

• use stainless fasteners, washers and channel nuts;
• prefer stainless strut, or insulate the feet from galvanized strut with plastic bushes, in wet or coastal locations;
• for 316L enclosures in coastal or chemical areas, match the fastener grade to the enclosure; see 304 vs 316L stainless.

Pole mounting

Outdoors, enclosures often go on poles, posts or the legs of a structure. The same NEMA rule applies: the bracket attaches to the external feet or to a bracket outside the electrical cavity, never through the back wall. Two points specific to poles:

• Clamping. Band clamps and U-bolts work on round poles; check the clamp range against the pole diameter and fit stainless hardware with a stainless enclosure.
• Orientation. If you can choose, face the enclosure away from the afternoon sun and the prevailing weather, and bring cables in from below so water runs off the entries.

If your custom build needs pole-mount brackets or holes for a bracket you already have, say so in your quote request; what we can supply is confirmed on the quoted drawing. For sun, rain and ice, read the outdoor enclosure guide before choosing the site.

Floor stands and legs

Larger enclosures, and enclosures in the middle of a production floor, stand on legs or a floor stand. FEMA E-74 treats floor-mounted electrical cabinets differently from wall panels: its category covers tall, narrow floor-mounted cabinets, the principal objective is to prevent them from sliding or toppling, and the typical damage it records is overturning or sliding due to lack of anchorage or inadequate anchorage. Anchor a floor stand to the floor through its base plates, and where a tall unit is anchored to the floor while its conduit is braced to the structure above, FEMA E-74 recommends flexible connections so building movement does not tear the conduit out.

In washdown areas, legs keep the enclosure off the wet floor and leave the floor underneath open for cleaning. The washdown enclosure guide covers hygienic details.

Mounting height and door swing

Pick the height for the people who will use the enclosure, then check it against the code:

• Breakers and switches inside. NEC 240.24(A) requires overcurrent devices to be readily accessible and installed so that the center of the grip of the operating handle, in its highest position, is not more than 2.0 m (6 ft 7 in) above the floor or working platform, with listed exceptions.
• Working space. NEC 110.26(A) (2020 and 2023 editions) requires clear working space in front of equipment likely to be serviced while energized, at least 6.5 ft high or the height of the equipment, and enough room for doors to open at least 90 degrees.
• Displays and operator controls. For a screen, OSHA's computer workstation guidance is to place it so the top line is at or below eye level. See the HMI enclosure guide.

Door swing is the dimension most often forgotten. On our 24 x 20 x 8 in 304 enclosure, the reference drawing gives a door swing radius of 511.3 mm (about 20.1 in) and an overall size with feet, hinge and latch of 697.6 x 515.2 x 213.7 mm. Leave that clear on the hinge side, away from walls, columns, conduit and the next enclosure, so the door opens fully. Check the code edition your authority having jurisdiction enforces.

Frequently asked questions

Sources

1. NEMA, "NEMA FAQs: Enclosures" (Q15 weakest link, Q16 external mounting)
2. FEMA E-74, section 6.4.8.2, electrical distribution panels (ASCE/SEI 7-10 Chapter 13 provisions, damage)
3. FEMA E-74, section 6.4.7.1, control panels and floor-mounted electrical equipment (sliding, overturning, flexible connections)
4. Wikipedia, "Strut channel" (41 x 41 mm profile, sheet gauges)
5. 41 x 41 mm strut channel datasheet (BS 6946:1988)
6. Euro Inox, Stainless Steel in Contact with Other Metallic Materials (fastener rule, galvanized steel, electrolyte, plastic bushes)
7. EC&M, "What's wrong here?" (NEC 240.24(A) text)
8. EC&M, "NEC requirements for working clearances" (110.26)
9. OSHA eTool, Computer Workstations: Monitors (top line at or below eye level)
10. NFPA 70, National Electrical Code (free read-only access): 110.26, 240.24