An outdoor panel needs a Type 3R, 4 or 4X box, a sealed cooling unit sized with the sun load added, and a heater for cold nights. Sun on a gray box can add about 14 W per square foot of surface. That one number often moves a panel up a cooling size. The steps below give the math and the part numbers.
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Step 1: Pick the box
| Box | Rating | Example part | Use it when |
|---|---|---|---|
| Painted steel | Type 4 | Hammond 1418N4E12 (24 x 20 x 12 in, gray) | Rain, sleet, snow and hose water. Most pump, lift station and site panels |
| 316 stainless steel | Type 4X | Hammond 1418N4S16B6 (16 x 12 x 6 in) | Salt air, road salt spray, chemical plants and wastewater |
| Fiberglass | Type 3R, 4, 4X, 6P, 12 | Stahlin CL707W (Classic series) | Corrosive outdoor sites. UL 746C (f1) for UV, rated -76 to +274°F |
Ratings from the Hammond and Stahlin part data on each product page. Under NEMA 250, Type 3R covers rain, sleet and snow. Type 4 adds hose-directed water and windblown dust. Type 4X adds a corrosion test. More in the Hammond and Stahlin collections. See NEMA 4 vs 4X vs 12.
Step 2: Add the sun to the heat load
Sun heats the outside of the box. The inside then has more heat to get rid of. nVent Hoffman publishes these numbers for a gray box:
| Item | Value |
|---|---|
| Peak sun over most of the US | About 97 W/ft² |
| Peak sun in the high, dry Southwest | About 111 W/ft² |
| Sun load to hold an ANSI 61 gray box at room temperature | About 14 W/ft² of surface |
| Sun load if the box may run 20°F above room | About 6 W/ft² of surface |
| Rise from sun alone, ANSI 61 gray, no load inside | About 40°F |
| Light from the ground and walls nearby | Can add up to 30% |
Source: nVent Hoffman technical note Spec-00488, solar heat gain section (chart of solar load by color and temperature rise). White and light finishes take less; black takes more. Color data is also an input in Pfannenberg Sizing Software (PSS).
Worked example: 24 x 20 x 12 in pump panel in full sun
Hammond 1418N4E12 box, gray. 200 W of drives and power supplies inside. Hottest day 100°F. Keep the inside at 95°F. All six sides add up to 14 ft².
| Step | Numbers | BTU/hr |
|---|---|---|
| Heat from parts | 200 W x 3.413 | 683 |
| Heat in through the walls | 1.25 x 14 ft² x 5°F (100 minus 95) | 88 |
| Sun | 14 W/ft² x 14 ft² = 196 W, x 3.413 | 669 |
| Total | About 1,440 | |
| Pick | Next size up, Type 3R/4, 115 V | Pfannenberg DTS 3061, 13382344351 (2,000 to 3,000 BTU/hr) |
Wall formula from the Hammond air conditioner sizing guide: BTU/hr = (watts x 3.413) + (1.25 x area ft² x ΔT°F). Sun value from the nVent chart above. Without the sun the total is about 770 BTU/hr, and the smaller DTS 3031, 13383144351 (900 to 1,300 BTU/hr) could be the pick. Check the unit performance curve at your hottest day. Example values only. Or run your own numbers in the heat load calculator.
Step 3: Cooling that stays sealed
A Filterfan pulls outside air, rain and dust into the box, so it does not fit a Type 4 or 4X panel. Use a closed loop unit with the same rating as the box.
| Part | Rating | Outdoor data |
|---|---|---|
| Pfannenberg DTS 3061, 13382344351 (115 V) or 13382341351 (230 V) | NEMA 3R/4 | Ambient 32 to 131°F. Coated coils and pipes on the outside air side. Set point 50 to 104°F, factory set 95°F |
| Pfannenberg DTS 3031, 13383144351 (115 V) | NEMA 3R/4 | 900 to 1,300 BTU/hr for small boxes |
| All enclosure air conditioners | Type 12, 3R/4 and 4/4X | Side, top and flush mount |
Data from the Pfannenberg part data on each product page. If the outside air is always at least 10°C (18°F) below the panel target, a sealed heat exchanger can do the job. See air conditioner vs heat exchanger.
Step 4: Heat for cold nights
A Michigan winter can run below 0°F. Drives and displays have a low limit, and a cold box sweats when it warms up. A heater with a thermostat holds the inside above the low limit.
| Step | Numbers | Result |
|---|---|---|
| Temperature gap | Outside -10°F, inside 41°F: 51°F = 28.3 K | |
| Heat loss | 28.3 K x 0.51 (painted steel) x 14 ft² | 202 W |
| Outdoor box | Hammond says double it | 405 W |
| Minus parts left on | Power supplies and PLC: 50 W | 355 W |
| Pick | Next size up at 115 V | Hammond FLHTF400A115 or Pfannenberg FLH-TF 400, 17040015407, 400 W |
Formula and k values from Hammond heating products data: heater watts = (ΔT x k x A) minus the watts from parts in the box, doubled for outdoor boxes. k: painted steel 0.51, stainless 0.344, plastic 0.325 W/ft²K. Example values only. See all enclosure heaters.
Step 5: Seal the entries and stop the surge
| Part | Data |
|---|---|
| AGRO Progress MS 1000.20, M20 nickel-plated brass | IP 68 (10 bar) and IP 69, -40 to +100°C. Clamps 8 to 15 mm cable |
| Citel DAC104US-480D | Type 1 SPD, 480 V 3PH delta, In 20 kA, Imax 100 kA, SCCR 200 kA, -40 to +85°C, DIN rail |
| Citel surge protection | Type 1 DIN rail and flange mount models, 120 V to 600 V |
Data from the AGRO and Citel part data on each product page. A UL 1449 Type 1 SPD can go on the line side of the service disconnect. A Type 2 cannot.
Common questions
Is Type 3R enough for an outdoor panel?
Type 3R covers rain, sleet and snow from above. It does not cover hose water or blowing dust. If the panel gets hosed, sits in a dusty yard or near a road with salt spray, use Type 4 or 4X.
How much does the sun add to my cooling load?
For a gray box, nVent Hoffman gives about 14 W per square foot of surface to hold the box at room temperature. A 14 ft² box picks up about 196 W, or 669 BTU/hr. A white or light box picks up less.
Can I put a Filterfan on an outdoor panel?
Not on a Type 4 or 4X panel. The fan pulls outside air, rain and dust through the box. Use a sealed NEMA 3R/4 cooling unit like the Pfannenberg DTS 3061.
Why is there water in my sealed outdoor panel?
Most often a gland, fan or window does not match the box rating. A sealed box can also pull in damp air as it cools at night. Check every entry first, then add a heater with a thermostat to keep the inside above the dew point.
Does the panel need a heater and a cooling unit?
Often, yes, in Michigan. The cooling unit runs in July and the heater runs in January. The DTS 3061 lists 32 to 131°F ambient, so the heater covers the cold side.
Get the outdoor panel quoted
Send the box size, the rating, the voltage, the heat load or drive horsepower, and the hottest and coldest days at the site. We send back the enclosure, cooling unit, heater, glands and surge protector as one quote with lead times.
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Related: Enclosure cooling guide · Washdown and corrosive areas · Water and wastewater · Hoffman DAH heater cross · Thermal management · All cross references
NEMA is a registered trademark of the National Electrical Manufacturers Association. Hoffman and nVent are trademarks of nVent Services GmbH or its affiliates. Applied Control Inc. is not affiliated with nVent. This page is for selection only; check the current manufacturer datasheet before you specify.