Short answer: seal the box and condition it from the inside. Put both starters in one 316L stainless wall-mount enclosure, manufacturer-rated NEMA 4X / IP66 and not UL listed, with a 150-250 W heater holding +5 C (41 F) through winter and a closed-loop air conditioner carrying the summer load — about 1,000 BTU/h for the 24 x 20 x 8 in example below. Leave the ventilation fan and its damper off the drawing: every vent hole is the weakest penetration and sets the rating of the whole assembly. Enclosed soft starter datasheets commonly give -5 to +40 C (23 to 104 F) as the operating range, so the whole job is holding one sealed box inside that band through a northern year that runs from -20 C (-4 F) nights to 35 C (95 F) afternoons.
What a northern year does to the box
Your box takes four attacks at once. Rain and freezing rain work at the door gasket and every penetration. Snow buries the bottom wall unless you hang the box above drift depth. July sun pushes heat inward through the walls; the worked example below assumes 150 W of peak solar gain on a box this size, and the solar heat load article shows how to compute the number for your orientation. The starter's own datasheet range, -5 to +40 C (23 to 104 F), sits inside a site swing twice that wide, so the enclosure's job is to shrink the swing.
Condensation follows the swing. Warm humid July air drops water on anything below its dew point, which is the job you give the air conditioner. Winter air is nearly dry: air at -10 C (14 F) holds at most about 2.4 g of water per m³, against 17 g/m³ at +20 C (68 F). Sealed walls, a heater in January and a closed loop in July beat any fan-and-damper scheme on both counts.
Sealed NEMA 4X with nothing that opens it
Put the Type on the drawing. NEMA 250 defines Type 4 by rain, splashing and hose-down tests; Type 4X adds 200 hours of salt spray next to an AISI 304 test specimen (NEMA 250, Table 5-1D). An IP66 mark alone will not carry a US specification: NEC 110.28 states an IP code cannot stand in for the enclosure Type rating. Put the Type on the spec sheet and let the IP code travel with it for international reviewers; the outdoor enclosure guide maps which Type answers which weather.
Every hole in the wall has to hold the same rating. A louver, drain or fan vent rated lower drags the whole assembly down to its own number. That is why the motorized damper loses before its first winter: it adds a moving gasket and a lower-rated opening to solve one crossover season the heater already handles. The breather-vent article shows which penetrations survive a 4X rating and which end it. The sealed layout needs no air path. The heater sits inside. The air conditioner is a closed-loop unit that bolts through a gasketed cutout, keeps its condenser loop separate from the box air, and leaves the rating intact — closed-loop cooling explains the split.
304 or 316L for the walls
Pick 316L whenever anything salty reaches the box: de-icing salt splash along a road, coastal air, chloride washdown. The molybdenum in 316L lifts pitting resistance, and pitting starts exactly where salt films sit — 304 pits at room temperature under chloride deposits in a gasket crevice. Both grades share the same stress-corrosion threshold near 60 C (140 F), so shade the box and keep salt out of the crevices instead of leaning on the grade alone.
Inland sites clear of de-icing salt can run 304 and keep the difference. Whichever grade you order, rinse the exterior occasionally; wet-dry cycling stains 316L too, as the wet-dry cycling article shows. The 316L wall-mount line runs from 6 x 4 x 4 in to 60 x 36 x 16 in, so the size you land on exists in both grades.
Size the box around the starter group
Take the example bill of materials — swap in your own parts, the method holds: one 40 A soft starter on a 30 hp, 460 V motor, its three-pole breaker, two distribution blocks and a bank of interposing relays. That group fits a 24 x 20 x 8 in 316L wall-mount box. Keep about 20% of the backplate empty, one starter-width row, so the next revision drops in without a second box. The 8 in depth carries the wire bending space at terminals that NEC 312.6 sets for cabinets and cutout boxes.
A starter and breaker alone drop to a 20 x 16 x 8 in box, in 304 or 316L. Both systems in one cabinet moves you to a 24 x 24 x 10 in 316L box, with one disconnect per power drop. Mount it so every operating handle stays no higher than 2.0 m (6 ft 7 in) above the floor under NEC 240.24(A). Leave NEC 110.26(A) working space in front of the door — at least 3 ft (900 mm) deep for 0-150 V to ground in Condition 1. Your 460Y/277 V feed sits at 277 V to ground, so plan on more depth, with room for the door to swing 90 degrees.
Winter: hold +5 C with a heater
Set the heater thermostat to close at +3 C (38 F) and reopen at +8 C (46 F). That holds the interior near +5 C, 8 C above the -5 C (23 F) datasheet floor even at turn-on, and 24 K below the cooling band. Wire the heater through the normally closed contact so it makes on a temperature fall; the heater and thermostat selection article walks the contact choices.
Now size the heater. Stainless walls pass heat at k = 3.7 W/m²K, the figure the enclosure heat-calculation guide works to, and the 24 x 20 x 8 in box presents about 1.07 m² of surface. At the -20 C (-4 F) design low against a +5 C interior:
3.7 W/m²K x 1.07 m² x 25 K = 99 W of wall loss in still air.
Wind on an exposed wall adds roughly 40% — a rule of thumb for open sites, and a sheltered corner can keep the still-air number — taking the loss to about 140 W. A 150 W heater holds the box with the door closed; at that loss the steady interior works out near +7 C and the thermostat simply cycles. Step up to 250 W for the two-starter box or for fast recovery after a mid-winter service visit. Insulating the inside with 20 mm of closed-cell foam at about 0.04 W/mK drops the wall to 1 / (1/3.7 + 0.5) = 1.3 W/m²K, and the same design day costs 1.3 x 1.07 x 25 = 35 W. The heater then idles most of the winter and your margin doubles.
The mid-winter service call is easier than it sounds. Air that leaks in during a short door opening has a dew point near -10 C, some 15 K below the +5 C walls, so it sheds almost no moisture inside. Lock out both disconnects and confirm no voltage at the lugs before you open the door, keep the heater on through the visit, wipe any frost off the door lip, and close it.
Summer: size the closed-loop unit to the load
Start at the starter's datasheet. The example assumes 100 W of loss at full load with the bypass contacts closed — put your unit's figure in, because the loss scales with current and jumps whenever the SCRs carry the start current instead of the bypass. If your duty starts the motor dozens of times an hour, price the loss at that duty; the drive-enclosure article works the same logic for a drive. Add 20 W for the relays and terminals, add the 150 W solar term, then subtract what the walls shed on their own at the 40 C ceiling with 35 C outside:
3.7 W/m²K x 1.07 m² x 5 K = 20 W of natural dissipation.
The closed-loop unit then removes:
100 W + 20 W + 150 W - 20 W = 250 W = 853 BTU/h (250 x 3.412).
Buy the next size up: a 1,000 BTU/h (about 300 W) closed-loop air conditioner, a 17% margin, through the side-wall cutout with its condenser airflow clear of the wall. Give it a second thermostat that starts cooling at 38 C (100 F) and stops at 32 C (90 F), so the box tops out under the starter's 40 C ceiling; the 24 K gap to the heater band means the two never run against each other. Face the box north or shade it and the 150 W solar term nearly vanishes — the cheapest cooling capacity you will ever buy.
What to put on the order
Order the sealed box and its cutouts from us; the heater, both thermostats and the closed-loop unit come from your electrical distributor. Send the cutout drawing with gland positions and the AC template, and it comes back for sign-off before cutting. Delivery runs 10 business days from that approval.
| Item | Spec for the example build |
|---|---|
| Enclosure | 24 x 20 x 8 in wall-mount, 316L stainless, hinged gasketed door, removable mounting panel, wall feet; manufacturer-rated NEMA 4X-type / IP66, not UL listed |
| Cutouts | Bottom wall: two power glands sized to cable OD; side wall: AC opening to the unit template; rated plugs in every unused hole |
| Glands | NEMA 4X/IP66-rated pattern, clamping range matched to each cable, drip loops below the entry |
| Conditioning | 150 W heater and thermostat; 1,000 BTU/h closed-loop unit and thermostat (sizes by case below) |
| Case | Box | Heater | Closed-loop cooling |
|---|---|---|---|
| One starter group (the example) | 24 x 20 x 8 in, 316L | 150 W, on +3 C / off +8 C | 1,000 BTU/h, on 38 C / off 32 C |
| Both systems in one box | 24 x 24 x 10 in, 316L | 250 W | 1,500 BTU/h |
| Inland, no de-icing salt | Same sizes in 304 | Same | Same |
That is the decision. Sealed 316L walls, a heater for the cold half of the year, a closed loop for the hot half, and the starters sit inside their -5 to +40 C range in every month of the calendar.
Enclosures in this article
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 24 × 20 × 8 in | 316L stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 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 |
| 24 × 24 × 10 in | 316L stainless | 24 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Can both systems share one enclosure?
Yes, when you size it on the combined load. Two starters at 100 W each, 40 W of relays, 150 W of July sun and minus 25 W of wall dissipation in a 24 x 24 x 10 in box gives 365 W, or 1,245 BTU/h, so a 1,500 BTU/h closed-loop unit carries it. Winter loss runs about 126 W in still air and 176 W with wind, so the 250 W heater holds +5 C. Keep one disconnect per power drop.
What contact does the heater thermostat need?
A small one. A 150 W resistive heater draws 1.25 A at 120 V and 0.6 A at 240 V; the 250 W unit draws 2.1 A at 120 V. Any panel thermostat contact carries that. Wire the heater through the normally closed contact so it makes on a temperature fall, and put the air conditioner on its own thermostat.
Do vents or a fan keep the NEMA 4X rating?
They end it. A vent is the weakest penetration and the whole assembly drops to the vent's own rating; a fan-and-louver also pulls dust and humid summer air straight into the box you are drying. Keep the walls solid: heater inside, closed-loop air conditioner through a gasketed cutout, and the manufacturer-rated NEMA 4X / IP66 rating stands (ours are not UL listed).
How much heat does a soft starter give off?
The example assumes 100 W at full load with the bypass closed on a 40 A starter; your datasheet's loss figure replaces that, since the loss scales with current. Every start pushes full current through the SCRs, so a jog duty or long-ramp cycle holds the average well above the running number. Size the cooling on the duty, not the nameplate.
Will opening the door mid-winter cause condensation?
Almost none lands inside. Air at -10 C (14 F) holds at most about 2.4 g of water per m³, so what leaks in during a short opening has a dew point near -10 C, some 15 K below the +5 C walls, and water condenses only on surfaces below the dew point. Keep the heater on, wipe frost off the door lip, and close it.
