The Type number on an electrical enclosure tells an engineer or facility manager which environments the box is built to withstand. This guide explains how to read those numbers using the National Electrical Manufacturers Association (NEMA) summary of enclosure definitions, and why a NEMA Type cannot simply be swapped for an IP code. It does not replace the standard, the manufacturer’s markings or a qualified designer.

Where the definitions come from

NEMA’s summary, approved by its Enclosures Section in November 2005, quotes definitions from NEMA Standards Publication 250-2003, “Enclosures for Electrical Equipment (1000 Volts Maximum),” and refers readers to that publication for complete information. Check which edition of NEMA 250 a project specification cites.

Every Type provides a degree of protection to personnel against access to hazardous parts. NEMA stresses that each Type protects against its listed conditions only “when completely and properly installed.”

Step 1: Indoor or outdoor

For non-hazardous indoor locations, NEMA defines these Types:

  • Type 1: falling dirt.
  • Type 2: falling dirt plus dripping and light splashing water.
  • Type 5: adds settling airborne dust, lint, fibers and flyings.
  • Types 12 and 12K: circulating dust, lint, fibers and flyings, dripping and light splashing water, and oil and coolant seepage. Type 12 is built without knockouts; Type 12K has them.
  • Type 13: adds protection against spraying, splashing and seepage of oil and non-corrosive coolants.

For indoor or outdoor use:

  • Type 3: falling dirt and windblown dust; rain, sleet and snow; undamaged by ice forming on the outside.
  • Type 3R: like Type 3 but without protection against windblown dust.
  • Type 3S: like Type 3, with external mechanisms that still operate when ice laden.
  • Type 4: adds splashing and hose-directed water.
  • Type 6: hose-directed water and entry of water during occasional temporary submersion at a limited depth. Type 6P covers prolonged submersion.

Step 2: Check for corrosion

An X suffix, as in 3X, 3RX, 3SX and 4X, marks an enclosure that provides an additional level of protection against corrosion. Type 6P includes that protection without the suffix. A plain Type 4 does not.

Step 3: Check for hazardous locations

NEMA says Type 7 and 10 enclosures are designed to contain an internal explosion without causing an external hazard, Type 8 prevents combustion by using oil-immersed equipment, and Type 9 prevents ignition of combustible dust.

  • Type 7: indoor use in Class I, Division 1, Groups A, B, C or D locations as defined in NFPA 70.
  • Type 8: indoor or outdoor use in Class I, Division 1, Groups A, B, C and D.
  • Type 9: indoor use in Class II, Division 1, Groups E, F or G.
  • Type 10: built to Mine Safety and Health Administration requirements in 30 CFR Part 18.

NEMA’s table places acetylene in Group A, hydrogen in Group B, gasoline, propane and acetone in Group D, metal dust in Group E, coal dust in Group F and flour, starch and grain dust in Group G. A product suitable for one Class or Group may not suit another unless marked. Outdoor installations, or those needing extra protection, require a combination-type enclosure.

Step 4: Translate carefully to IP codes

IEC 60529, the international classification, marks enclosures with the letters IP and two numerals. According to NEMA, the first numeral covers protection of persons and against solid foreign objects, and the second covers harmful ingress of water.

NEMA says IEC 60529 does not cover mechanical damage, explosion risk, or conditions such as condensation, corrosive vapors, fungus or vermin, while NEMA 250 tests for corrosion, rust, icing, oil and coolants. The two systems therefore cannot be exactly equated, and NEMA’s conversion table works in one direction only, from NEMA Type to IP code.

NEMA’s worked example is an IP45 requirement. Types 3, 3X, 3S, 3SX, 4, 4X, 6 and 6P meet it. Types 5, 12, 12K and 13 meet the first numeral but not the water requirement. NEMA adds that Type 3 also requires outdoor corrosion, gasket aging, dust and external icing tests and allows no water penetration in its rain test, whereas the IEC rating permits water entry that does not deposit on insulation parts or reach live parts.

Installation still matters

The Occupational Safety and Health Administration’s (OSHA) wiring rule, 29 CFR 1910.305, requires cabinets, boxes, fittings and panelboard enclosures in damp or wet locations to be installed so moisture cannot enter and accumulate, with at least 6.35 mm (0.25 in.) of airspace from the wall. Nonmetallic enclosures on concrete, masonry, tile or similar surfaces are exempt from the airspace. Enclosures in wet locations must be weatherproof, and unused openings in cabinets, boxes and fittings must be effectively closed.

Common problems

  • Converting IP to NEMA. NEMA states its table cannot be used to go from an IP code to a Type, because the IEC rating does not require NEMA’s corrosion, gasket aging, dust or icing tests.
  • Reading Type numbers as a ranking. A higher number is not more protection: Type 12 is an indoor dust-and-drip enclosure without the hose-down rating of Type 4.
  • Choosing 3R for dusty sites. Type 3R does not cover windblown dust; Types 3, 3S and 4 do.
  • Expecting corrosion resistance from Type 4. That requires 4X.
  • Confusing submersion with jets. NEMA notes that under IEC 60529 an enclosure marked only with second numeral 7 or 8 is not considered suitable for water jets, numerals 5 or 6, unless it is dual coded.
  • Ignoring the install. The definitions assume a complete, properly installed enclosure, and OSHA requires unused openings to be effectively closed.