NEC 110.28 – Enclosure Types: The "Don't Put an Indoor Panel Outside, You Genius" Rule

The Street Translation

Alright, listen up. You know that panel you're about to install? That switchboard? That disco? It needs to wear the right clothes for where it's going. You wouldn't wear a tuxedo to dig a ditch, and you sure as hell don't put a Type 1 panel on the side of a building in Seattle.

110.28 says almost everything electrical that comes in a box—panels, switches, disconnects, motor controllers, transformers, basically anything that would make sparks if you opened it up in the rain—must be marked with an enclosure type number. Then you match that number to Table 110.28 to see where it can live without turning into a rust sculpture or a water feature.

Here's the kicker that gets people: These enclosures protect against stuff OUTSIDE the box. They don't do squat about condensation forming INSIDE, corrosion happening INSIDE, or contamination sneaking in through your conduit runs. You can have a perfect Type 4X stainless steel Cadillac of an enclosure, but if you ran EMT to it without a seal-off and now you're pumping water through the raceway like a garden hose? That's on you, not the enclosure rating.

The Alphabet Soup Decoder

The Code gives us some helpful terms:

• Raintight (Types 3, 3S, 3SX, 3X, 4, 4X, 6, 6P) – Rain hits it, rain runs off. Box stays dry.
• Rainproof (Types 3R, 3RX) – Rain can get in the knockouts, but not where it matters. The "budget outdoor" option.
• Watertight (Types 4, 4X, 6, 6P) – You could hit this with a pressure washer (well, some of them).
• Driptight (Types 2, 5, 12, 12K, 13) – Keeps drips out. Not a monsoon, just drips.
• Dusttight (Types 3, 3S, 3SX, 3X, 4, 4X, 5, 6, 6P, 12, 12K, 13) – Dust doesn't get in. Think grain elevators, saw mills.

The curveballs:

• Some Type 12 and 13 enclosures now come with a "-XH" marking meaning they can handle corrosive environments AND you can hose them down indoors. (Hello, food processing plants!)
• Some Type 4X boxes are marked "INDOOR ONLY." Yeah, you read that right. A watertight box that can't go outside. Because it might be plastic that UV rays would turn into confetti in six months.

Key Takeaways (The Stuff That Matters)

1. All major electrical equipment must be marked with an enclosure type number – panels, switchboards, disconnects, motor controllers, the works.

2. Use Table 110.28 to match the enclosure type to the location – this is your bible for "what box goes where."

3. Enclosures protect against EXTERNAL conditions only – they don't prevent internal condensation, corrosion from the inside out, or water migrating through your raceways.

4. This section doesn't apply to hazardous locations – those have their own rules (Division 1, Division 2, Zone systems—whole different ballgame).

5. Read the label, not just the number – a Type 4X marked "indoor only" or a Type 12 with "-XH" changes where you can use it.

6. Weather terms matter:

   • Raintight > Rainproof (3R is the budget version)
   • Watertight > everything else for wet locations
   • Dusttight for dirty environments

7. Your raceway sealing is YOUR problem – the fanciest enclosure in the world won't save you from a swimming pool you created by not sealing conduit entries properly.

Real-World Jobsite Scenarios

Scenario 1: The Car Wash Catastrophe

You're installing a 200A panel at a car wash. Service entrance comes in, panel feeds the bay circuits. Your supplier sends you a nice Type 1 indoor panel because that's what was on the original quote.

The Problem: Car washes are wet, soapy, and occasionally get hit with pressure washers when someone's cleaning. That Type 1 is gonna last about three months before it's a corrosion party inside.

The Solution: Minimum Type 4X (watertight, corrosion-resistant) or better yet, look for that Type 12-XH rating if it's subject to hosedown. Stainless steel or painted steel with the right gaskets. Yeah, it costs more. You know what costs even more? Replacing it in six months and explaining to the customer why you're back.

Scenario 2: The Outdoor Disconnect Dance

You're putting a disconnect on the outside of a commercial building for an RTU (rooftop unit). It's going on the north side of the building, somewhat protected by an overhang. Your apprentice grabs a Type 3R because "it's outdoor-rated, right?"

The Right Call: Type 3R is rainproof but it's not raintight. Water can get into the knockouts and around the door, though not to the live parts. For most applications, 3R is fine and saves money. BUT if this is in a coastal area with driving rain, or the overhang isn't much, or the inspector is having a bad day, you might need to step up to a Type 3 (raintight) or Type 4 (watertight).

Pro move: Take a picture of that overhang and the location. When the inspector questions it, you can show the installation isn't taking direct weather.

Scenario 3: The Food Plant Failure

You're in a food processing plant installing a motor controller for a conveyor. Everything gets hosed down daily with sanitizer and hot water. Engineer spec'd a Type 4X because "watertight."

The Gotcha: Not all Type 4X enclosures are hosedown-rated. Some are just splash-resistant. You need one marked with the "-XH" ancillary marking that specifically says it's rated for hosedown applications. The gasket material matters, the drain design matters, the mounting orientation matters.

The Save: Check NEMA 250 (now EN 10250-2024) and make sure you're getting a true hosedown-rated enclosure. Your GC will thank you when it doesn't fail inspection and the facility manager will thank you when it's still working in five years.

Scenario 4: The Condensation Confusion

You installed a beautiful Type 4X enclosure outside for a controls panel in Minnesota. Winter hits, and the customer calls: "There's water INSIDE the panel!" You drive out there thinking the gasket failed, but everything's sealed tight.

What Happened: Temperature cycling. Cold outside, heat from the equipment inside, moisture in the air condenses inside the box. The enclosure did its job—it kept outside water OUT. It never promised to prevent condensation INSIDE.

The Fix: Add a thermostatically-controlled heater inside the enclosure to keep internal temperature above the dew point. Or add drain holes with weep fittings at the bottom (check with the AHJ first). Some guys add those silica gel packs, but that's more Band-Aid than solution. This is exactly what that second paragraph of 110.28 is warning you about.

Scenario 5: The "Indoor Only" 4X Head-Scratcher

You're doing a panel swap and spec'd a Type 4X because it's going in a somewhat humid indoor location near a loading dock. Panel shows up, you start to mount it, and you notice in tiny letters: "INDOOR ONLY."

Wait, what? Yeah, some Type 4X enclosures are made from plastics or coatings that can't handle UV exposure. They're watertight and corrosion-resistant, perfect for wet indoor locations, but the sun would destroy them.

The Lesson: Always read the fine print on the label. If there's any chance it'll see sunlight—even through a window—make sure it's not marked indoor only.

What to Study (Exam Prep)

If you're studying for your journeyman or master's exam, here's what they love to hit you with:

High-Probability Exam Questions:

1. Table 110.28 matching questions – "Which enclosure type is suitable for outdoor use where subject to windblown dust and rain?" (Answer: Type 3, 3S, 3X, 4, 4X, 6, or 6P depending on other factors)

2. Definition questions – "What's the difference between raintight and rainproof?" (Raintight prevents water entry; rainproof allows some water in knockouts but not to live parts)

3. Limitation questions – "Do enclosure types protect against internal condensation?" (NOPE. That's not their job.)

4. Application questions – "A motor controller in a car wash requires what minimum enclosure type?" (Type 4X at minimum, possibly Type 12-XH for hosedown)

5. Marking requirements – "Must a panelboard be marked with an enclosure type?" (Yes, 110.28 requires it)

Make Flash Cards For:

• The watertight types: 4, 4X, 6, 6P
• The raintight types: 3, 3S, 3SX, 3X, 4, 4X, 6, 6P
• The rainproof types: 3R, 3RX (the "budget outdoor" option)
• The dusttight types: 3, 3S, 3SX, 3X, 4, 4X, 5, 6, 6P, 12, 12K, 13
• What enclosures DON'T protect against: Internal condensation, internal corrosion, contamination entering through raceways

Table 110.28 Strategy:

You need to know this table exists and roughly what the numbers mean. You don't need to memorize every single type, but you should know:

• Type 1: Indoor, general purpose (the basic box)
• Type 3R: Outdoor, rainproof, budget option (think cheap outdoor disco)
• Type 4/4X: Watertight (4X is also corrosion-resistant—think stainless)
• Type 12/12K: Industrial, indoor, driptight and dusttight (think factories)
• Type 6/6P: Submersible (think sump pump controls)

Code Reference Tricks:

• The actual Table 110.28 is your friend—it's in the book, you can use it on open-book exams
• Look for the ancillary markings: "-XH" for hosedown/corrosive indoor applications
• Remember: hazardous locations have DIFFERENT rules—this section specifically says it doesn't apply there
• When in doubt on an exam question, ask yourself: "What's the WORST condition this equipment might see?" Then pick the enclosure that handles it.

The "Gotcha" They Love:

Question: "An enclosure is marked Type 4X. Can it be installed outdoors?"

Your Brain Says: "Watertight! Corrosion-resistant! OF COURSE!"

The Correct Answer: "Only if it's NOT marked 'indoor only.'"

Read. The. Label. Every. Time.

The Foreman's Final Word

Here's the deal: 110.28 is the Code's way of making sure you don't throw just any box anywhere and hope for the best. The numbers aren't arbitrary—they're tested ratings that tell you what the enclosure can handle.

The three rules to live by:

1. Match the box to the environment – Use Table 110.28, read the enclosure marking, and think about what weather or conditions it'll face over its 30-year lifespan.

2. The enclosure is only half the battle – You still need to seal your raceway entries, slope your conduit for drainage, and think about internal heating if condensation is a risk.

3. When in doubt, go tougher – The difference in cost between a Type 3R and a Type 4X is nothing compared to a callback or a failed inspection. Buy once, cry once.

And for the love of all that's holy, stop putting indoor panels outside. I don't care if it's "under an overhang" or "the customer says it'll be fine." That's how you end up on someone's "horrible electrician" story at the supply house.

Now get out there and put the right box in the right place. Your future self will thank you.

Class dismissed. Don't forget to seal those conduit entries.