Short answer: fit a thermostat-controlled anti-condensation heater sized to hold the cabinet interior about 10 °F [5-6 °C] above shop air. In a sealed 24 x 20 x 8 in stainless wall-mount box that is 16-22 W of steady heat, so a 30 W heater is the pick, stepped up to 50 W when the shop spends days near 40 °F [4 °C]. Set a heating thermostat to switch on at 65 °F [18 °C] measured inside the cabinet. Desiccant bags dry only the air already in the box, about 0.7 g [0.02 oz] of water at 60 °F and 85% RH, and one door opening undoes them; heat keeps every surface above the dew point. If a cabinet is already wet, dry it with the heater running and the door shut, and replace anything that sat under standing water.
Why a warm front fills a cold cabinet with water
The shop sat in the 30s and 40s °F [0-7 °C] all week, so the cabinet walls, the wire duct and the drive heatsinks soaked down with it. Then a warm front pushed the room to 60 °F [15.6 °C] at 85% relative humidity. Air in that state has a dew point of about 56 °F [13 °C] on the standard psychrometric tables. Every surface below 56 °F grows a water film on contact, and a wall that sat at 40 °F [4 °C] is 16 °F [9 K] below it. Big steel frames and cast iron hold their cold for hours after the air warms, which is why machines keep dripping into the afternoon. A cabinet interior has no heat source of its own, so it tracks shop temperature and the film comes back every night until something changes. The dew-point mechanics behind this are in the condensation guide, and the same shop-air-versus-cold-surface logic is in room humidity causing condensation on cold surfaces.
Symptoms and what each points to
Match what you see to a cause before ordering parts:
- Water beading inside the door and running off the gasket line after a warm front: interior surfaces below the dew point, no holding heat in the box.
- Drive, PLC or display dead on a humid morning: moisture on boards, connectors and terminals; dry the cabinet fully before the next power-up.
- A viewing window fogged at start-up that clears by mid-morning: humid interior air and cold surfaces, dried by the machine's own heat once it runs.
- White powder or rust on terminals after weeks of swings: repeated condensation cycles with nothing holding the box above the dew point.
- A puddle on the cabinet floor: condensate collecting at the low point, or water washed in; treat everything at that level as wet until dried and inspected.
Fog on the inside of a window is the cheap early warning; the cool-surface physics behind it is in when condensation forms on a cool device.
The causes, most likely first
- The swing itself. Cold cabinet, warm humid air, every surface below the dew point. Confirm it with an IR thermometer on an interior wall at the worst hour of the morning: a wall at 40-45 °F [4-7 °C] against a 56 °F [13 °C] dew point explains the whole puddle.
- No holding heat in the cabinet. The interior tracks shop air, so every swing plays out at full scale. Leave a thermometer inside overnight: if the box never sits about 10 °F [5-6 °C] above the shop, this is the cause.
- Humid air imported through openings. A door propped open, a gasket gap or an unsealed spare cutout swaps the box air for shop air on every pressure change. That imported air brings its dew point with it; the surface checks are the same ones in dew point and surface checks for make-up air.
- Wet mass already in the box. Saturated desiccant, shop rags or standing water re-evaporate into the air at every warm-up. A 50 g [1.8 oz] silica bag absorbs roughly a third of its own weight at 85% RH; past that it sits in equilibrium with the air instead of drying it.
The fix: a 30 W heater and a 65 °F thermostat
Mount the heater low on the back panel, where rising warm air sweeps the whole interior, and keep 2 in [50 mm] clear of wire duct and terminal blocks so it heats air rather than one loom. Wire it through a normally-closed heating thermostat, the contact type that closes as temperature falls, and set switch-on at 65 °F [18 °C] measured inside the cabinet. The usual 7 °F [4 K] hysteresis holds the interior in a 65-72 °F [18-22 °C] band. On a 60 °F, 85% shop day the air inside sits 9-16 °F [5-9 K] above the 56 °F [13 °C] dew point, and the surfaces follow the air. That margin is what the 10 °F rule buys. On a 90 °F [32 °C] day the thermostat stays open and the box tracks shop air, which is what you want: an interior at shop temperature cannot fall below the shop dew point.
Close the other paths at the same time. Plug unused gland holes with metal plugs rated to the box, because the lowest-rated part in a wall sets the assembly rating, and pull a feeler check on the door gasket at the corners. The heater and thermostat pair, including strip heaters in the 30-100 W range, is picked in enclosure heater and thermostat selection for cold starts, the same thermostat logic the fan version uses in thermostat control for an enclosure fan above setpoint. For a junction-box-sized cabinet the picks get smaller; small enclosure anti-condensation heater selection sizes those.
Sizing the heater for a 24 x 20 x 8 in box at a 60 °F shop
The inputs, each with its source:
- Enclosure: 24 x 20 x 8 in (H x W x D) 304 stainless wall-mount box, back flat on the wall; a size built to order in the custom stainless line.
- Exposed area, from those outside dimensions: all six faces add to 1,664 in² [1.07 m²]; minus the 480 in² back face, 1,184 in² [0.76 m²] exposed.
- Wall heat transfer for stainless: k = 3.7 W/m²K, the combined convection-plus-radiation value used for enclosure heat-loss sizing.
- Temperature lift: 10 °F [5.6 K], the anti-condensation target of holding the interior 10 °F above a 60 °F [15.6 °C] shop, sized with the machines off, because the failure happens overnight with the PLC cold.
The rule in words: heater watts equal wall heat transfer times exposed area times the temperature lift. It is the standard enclosure-heater sizing method, and its first step is the surface area exposed to open air.
The arithmetic:
P = k × A × ΔT = 3.7 W/m²K × 0.76 m² × 5.6 K = 15.8 W with the back on the wall.
Counting all six faces, the freestanding case: 3.7 W/m²K × 1.07 m² × 5.6 K = 22.2 W.
Result: 16-22 W of steady heat, so the first standard size up is a 30 W heater. Your own PLC, supply and I/O add roughly 10-20 W of losses when powered, which is margin in the overnight case rather than a reason to downsize.
The decision, by case:
| Case | Steady heat | Heater to fit |
|---|---|---|
| 60 °F shop, stainless walls | 16-22 W | 30 W |
| 40 °F [4 °C] week, stainless walls | 39 W | 50 W |
| 60 °F shop, painted steel walls | 23 W | 30 W |
| 40 °F week, painted steel walls | 58 W | 75 W |
The 40 °F rows come from the larger lift: holding a 65 °F interior over a 40 °F shop is 25 °F [13.9 K], which takes 3.7 W/m²K × 0.76 m² × 13.9 K ≈ 39 W in stainless, and painted steel at 5.5 W/m²K takes about 58 W. Drying an already-soaked box runs on the same budget. Assume 0.05 in [1.3 mm] stainless walls, about 10.7 kg [23 lb] of steel: warming it 10 °F stores 10.7 kg × 500 J/kgK × 5.6 K ≈ 30 kJ, and evaporating 50 g [1.8 oz] of condensed water takes 50 g × 2,450 J/g ≈ 125 kJ. A 30 W heater leaves about 14 W of that budget after wall losses, so the dry-out runs roughly (30 + 125) kJ ÷ 14 W ≈ 3 hours. Start it in the afternoon, door shut the whole time, and check it dry the next morning.
Recovering a cabinet that already got wet
Lock out and verify absence of voltage before the door comes open. Wipe standing water out of the floor and the wire duct, shut the door, and feed the heater from a temporary supply while the machine circuit stays locked out; the roughly 3 hours from the sizing example does the drying. Look for dried runs over the drive, the power supply and the terminal blocks, because those mark where water sat. Boards and connectors that only saw condensation film normally survive a complete dry-out. Equipment that has actually been under water is a different case. Breakers, fuses and electronics that have been submerged are replaced rather than insulation-tested back into service; that is NEMA's guidance on water-damaged equipment. On the first restart, bring the loads up one at a time and watch the supply.
Keeping the water from coming back
Leave the heater powered through the damp season and give the cabinet a cheap temperature and RH logger. The reading to expect is interior about 10 °F [5-6 °C] above shop air and RH well below the room; after the next warm front the gasket line stays dry. A control strategy built on exactly that sensor reading is in humidity sensor heater condensation control strategy. Swap desiccant bags monthly through the wet months as a buffer; they are not the control. The sealed walls that hold the heater's heat also hold the electronics' own losses, the balance explained in why your driver box runs hot inside a sealed enclosure, which is why the thermostat rather than a bigger heater is the right answer on warm days.
Keep the box sealed while you do all of this. A field-drilled drain hole belongs in a box listed for damp and wet locations, where NEC 314.15 permits drainage openings of 1/8 to 1/4 in [3-6 mm]; a hole cut into a rated Type 4X wall ends the rating, and a working heater makes the drain unnecessary because the water never forms.
When to re-specify the enclosure
Condensation control assumes a dry box to start with. Re-specify when water stands in the bottom after every washdown or storm, when the cabinet lives outdoors in the rain, or when the room gets hosed: those are ingress problems, and the fix is a gasketed Type 4X enclosure with sealed glands rather than a bigger heater. Our wall-mount boxes are manufacturer-rated NEMA 4X / IP66; not UL listed. Size the replacement with the heater on the drawing: strip heater, thermostat and 2 in [50 mm] of clearance belong on the layout before the cutouts are settled, and how to size an electrical enclosure walks the space math. A 316L box earns its premium near salt, brine and chlorides; the stainless wall-mount range filters by grade and size, and custom enclosures are built to your drawing and delivered in 10 business days from your approval of the drawing. Cabinets that live outdoors add the shoulder-season logic in when an outdoor enclosure needs an anti-condensation heater.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 316L stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 316L stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How many watts does an anti-condensation heater need in a control cabinet?
Size it as wall heat transfer times exposed area times temperature lift. For 304 stainless use 3.7 W/m²K; for painted steel use 5.5 W/m²K. A 24 x 20 x 8 in box with its back on the wall exposes about 0.76 m² [8.2 ft²]. Holding a 10 °F [5.6 K] lift needs 16 W in stainless and about 23 W painted, so a 30 W heater is the first standard size in both cases. A shop that sits near 40 °F [4 °C] makes it 39 W stainless or 58 W painted.
Can I run a cabinet heater continuously without a thermostat?
It works, but check two numbers. A 30 W heater left on continuously lifts the interior about 14 °F [8 K] above shop air and draws 0.72 kWh a day. On a 90 °F [32 °C] summer day that puts the electronics near 104 °F [40 °C], the ambient limit most drives and PLCs are rated for, so keep the thermostat in circuit and let the heater rest on hot days.
Do desiccant bags inside a control cabinet actually help?
As a supplement, yes. The air in a 24 x 20 x 8 in box holds only about 0.7 g [0.02 oz] of water at 60 °F and 85% RH, so a fresh 50 g [1.8 oz] silica bag dries it within hours. What beats the bag is the next dose of humid air: every door opening or open gland hole restarts the load, and a saturated bag stops drying. Swap bags through the damp season and let the heater hold the surfaces above the dew point.
How long before a soaked cabinet can be powered again?
Dry it with the door closed and the heater running: about 3 hours for a soaked 24 x 20 x 8 in box by the sizing example, overnight in practice. Wipe standing water from the floor and check for dried runs over drives and power supplies. Boards that only saw condensation film usually survive a complete dry-out, while equipment that has been under water is replaced rather than insulation-tested back into service. Bring loads up one at a time on the first restart.
