Short answer: a triac solid state relay holding a 400 mA solenoid valve sheds about 1 W, and that watt lifts a small sealed stainless enclosure about 1 °C [2 °F] above ambient. You need no heat sink, no fan and no vent. Against a 70 °F [21 °C] room, the inside of a 10 x 8 x 6 in wall-mount box settles near 72 °F [22 °C] and stays there all year. The heat comes from the relay's on-state voltage drop, about 2 V across the output plus a fraction of a watt at the input, and the walls shed it on their own. Size the enclosure for the terminals, the DIN rail and the spare space around the relay; the relay itself adds nothing to the sizing.
Where the relay's heat actually goes
When the relay closes, the load current crosses a semiconductor instead of contacts, and the device holds a volt or two across itself. Relay datasheets list this as the output on-state voltage drop. Transistor outputs sit near 1 to 1.3 V; triac outputs are usually quoted at 1.5 V maximum at rated current, so 2 V is a safe working figure with margin in it. At the 0.4 A holding current, the output heat is 2 V × 0.4 A = 0.8 W. At 0.4 A on a 25 A relay, the real drop sits at the low end of the datasheet curve, so 2 V errs on the warm side.
The input side adds less. The control LED in a typical SSR draws 10 to 20 mA (every SSR datasheet lists its own input current), which is 0.05 W at a 5 V logic input and about 0.5 W at 24 V. Call the whole relay a watt and your sizing stays on the safe side of honest.
The valve's 0.9 A inrush looks worse than it is. It lasts only the tens of milliseconds the plunger takes to seat; even at a generous 50 ms, the heat is 2 V × 0.9 A × 0.05 s = 0.09 J, the same heat as 0.09 seconds of the holding load. Size the heat for the holding current and ignore the inrush.
Write the scaling rule on the panel drawing: at a 2 V drop, every ampere of continuous load makes 2 W. A 5 A load means 10 W; a 10 A load means 20 W.
How a sealed box gets rid of a watt
A sealed enclosure has no fan, no filter and no hole, so the heat leaves one way: through the wall, by convection on both faces plus radiation from the outer skin. The effective coefficients this site's sealed-box guides use are 3.7 W/m²K for stainless, 5.5 W/m²K for painted steel and 3.5 W/m²K for plastic. The rule in words: rise in °C equals the watts divided by (3.7 × wall area in m²) for a stainless box.
That hands you a reject rate. A 10 x 8 x 6 in box carries about 0.242 m² of wall, so it rejects 3.7 × 0.242 ≈ 0.9 W for every 1 °C of rise, about 0.5 W per °F. Grow the box and the wall area grows with it, so the reject rate climbs in step. To run the same path backwards from a known load, see estimate the thermal resistance from the heat load. To find where the watts in a mixed panel come from, add up the component losses first.
One watt in a 10 x 8 x 6 in box at 70 °F
The case: a 24 VAC water solenoid valve with a 0.9 A inrush and a 400 mA holding coil, switched by a 25 A triac-output SSR in a sealed 10 x 8 x 6 in [254 x 203 x 152 mm] stainless wall-mount box, in a room that holds 70 °F [21 °C] year-round.
The inputs:
- Holding current 0.4 A, continuous (the valve's holding coil; continuous is the worst case).
- On-state drop 2 V (worst-case triac figure; your relay's datasheet states its own).
- Input power 0.2 W (typical 15 mA at a 12 V DC control voltage — an example assumption; substitute the input spec of the relay you bought).
- Wall area 0.242 m² (six faces of the box, computed below).
- Wall coefficient 3.7 W/m²K (stainless, convection plus radiation, the figure the sealed-box guides use).
- Ambient 70 °F [21 °C], held year-round.
The heat: 2 V × 0.4 A = 0.8 W at the output, plus 0.2 W at the input, for 1.0 W total.
The area: 2 × (0.254 m × 0.203 m) + 2 × (0.254 m × 0.152 m) + 2 × (0.203 m × 0.152 m) = 0.103 + 0.077 + 0.062 = 0.242 m².
The rise: 1.0 W ÷ (3.7 W/m²K × 0.242 m²) = 1.0 ÷ 0.90 = 1.1 °C, and 1.1 × 1.8 = 2 °F.
The result: the interior settles near 72 °F [22 °C] against the 70 °F room, valve holding all day.
The decision: switch the valve, close the box and walk away. The around-70 °F target holds while the room holds 70 °F, and the box tracks the room plus 2 °F, so a 90 °F summer day puts the interior near 92 °F. The threshold in this box is the 0.9 W per °C reject rate: keep the total heat inside under 5 W and the rise stays near 6 °C [10 °F]. Past 17 W the interior reaches 40 °C [104 °F], the ambient most drives and control electronics carry as their rating, and cooling stops being optional.
What the rating covers and what it leaves out
The rating on our wall-mount boxes — manufacturer-rated NEMA 4X-type / IP66, not UL listed — speaks to water, dust and corrosion: hose jets and splashing, airborne dust, and the salt-spray exposure the 4X adds. Temperature sits outside the test. Nothing in the rating promises a cool interior, and nothing about this heat load threatens the rating either; at this wattage the two questions never meet.
Keep the box sealed and it stays rated. Every breather, louver, drain or fan cutout carries its own ingress rating, and that lower figure becomes the rating of the whole assembly. The fastest way to lose NEMA 4X / IP66 is to drill a ventilation hole the heat load never asked for. If moisture rather than heat is the worry in this panel, the sealed-box-versus-vent comparison for outdoor boxes carries the numbers on which design stays drier.
An internal circulation fan does not change the arithmetic. Fans inside a sealed box only stir the air; the heat still exits through the walls, and at a single watt there is nothing worth stirring. The internal-fan walkthrough traces where the heat actually ends up.
Plastic, painted steel or stainless
At this wattage the material answers itself. Plug the same 1 W and the same 0.242 m² of wall into each coefficient:
| Wall material | Effective coefficient | Rise from the example load in the 10 x 8 x 6 in box |
|---|---|---|
| 304 or 316L stainless | 3.7 W/m²K | 1.1 °C [2 °F] |
| Painted steel | 5.5 W/m²K | 0.75 °C [1.4 °F] |
| Plastic | 3.5 W/m²K | 1.2 °C [2.1 °F] |
Stainless and plastic land within 5% of each other, and painted steel sheds slightly more because it radiates better. Choose the wall for what hits its outside: washdown, chlorides and coastal salt point to 316L stainless, UV and mechanical knocks to painted steel, dry indoor duty to plastic. A plastic box running this valve warms by a degree you will never feel with your hand on it.
When the load current makes a heat sink real
The relay in this panel idles at 1.6% of its 25 A rating, which is why no heat sink appears anywhere in the arithmetic. Load current is what moves the story, because output heat scales at 2 W per ampere:
| Continuous load at a 2 V drop | Heat, output + 0.2 W input | Rise in the 10 x 8 x 6 in stainless box |
|---|---|---|
| 0.4 A | 1.0 W | 1.1 °C [2 °F] |
| 1 A | 2.2 W | 2.5 °C [4 °F] |
| 2 A | 4.2 W | 4.7 °C [8 °F] |
| 5 A | 10.2 W | 11 °C [20 °F] |
| 10 A | 20.2 W | 23 °C [41 °F] |
Between 8 and 9 A the interior of this box passes 40 °C [104 °F] against a 21 °C room. A 25 A relay run at full output makes 50 W and belongs on the heatsink its datasheet assumes; that is a mounting decision on the relay, not a box-size decision.
Relay makers publish derating curves for exactly this question: the maximum load current the relay carries against ambient temperature, on the mounting the curve assumes. Above roughly 5 A of continuous load, read the curve at your ambient and mount to match it. In a sealed box the wall is your heat sink: bolt the relay base straight to the back plate or the enclosure wall and the steel carries the flux out. Running the box as a heat sink by conduction and heat sink thermal resistance in sealed enclosures put numbers on that path. The PoE switch heat-load sizing example runs the same sum at 30 W. Past about 10 W in a box this size, the hot-ambient cooling options line up a bigger box, a conduction plate and active cooling, and what a 75 W cooler can hold at 25 °C sizes the cooler route.
What to put on the order
Size the enclosure for the wiring, not for the watt: SSR input terminals, the valve's conductors, bending space at the terminals, and 20 to 30% spare rail for the next device. The enclosure sizing guide walks the fill and bending-space checks. Land the wires with the supply locked out and absence of voltage proven; after that, the relay asks nothing of the box but its terminals.
Order the box sealed and cut to your drawing. Our wall-mount stainless line spans 6 x 4 x 4 in to 60 x 36 x 16 in outside, every cutout machined to the drawing and manufacturer-rated NEMA 4X-type / IP66, not UL listed. Delivery is 10 business days, counted from the order; for custom work, from your approval of the drawing. The stainless size finder filters the line by size and grade, and the enclosure selector narrows by environment, listing and inside dimensions.
The decision by case: relays, solenoid valves and other loads under about 2 W — seal any sensibly sized box and forget the heat. Loads of 2 to 10 W — run watts ÷ (3.7 × wall area) with every component's losses included. Loads past 10 W in a box this size — conduction plate, bigger box or a cooler, in that order of cost.
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 |
|---|---|---|---|
| 10 × 8 × 6 in | 304 stainless | 10 x 8 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
How much heat does a solid state relay make at a 400 mA load?
About 1 W in total. A triac output drops roughly 2 V when it is on, so a 400 mA holding load makes 0.8 W, and the input LED adds 0.05 to 0.5 W depending on the control voltage. In a 10 x 8 x 6 in sealed stainless box that wattage raises the interior about 1 °C [2 °F] above ambient, so the relay runs bare, with no heat sink.
Can a sealed enclosure hold 70 °F inside?
Only when the room is at or below 70 °F, because a sealed box has to run warmer than ambient to push its heat out through the walls. At this small load the gap is modest: a 10 x 8 x 6 in stainless box running the example relay sits about 2 °F above the room, so a 70 °F room gives an interior near 72 °F. The box tracks the room plus that 2 °F all year.
Does a plastic enclosure run hotter than stainless at this wattage?
Barely. Plastic walls move about 3.5 W/m²K against 3.7 W/m²K for stainless, so the same 1 W in the same 10 x 8 x 6 in box gives a 1.2 °C rise instead of 1.1 °C, about 5% apart. At loads this small, pick the material for the chemicals, washdown and sun around the panel, not for the relay's watt.
At what load current does an SSR need a heat sink?
Follow the watts: a triac output makes about 2 W per ampere of continuous load. The example 10 x 8 x 6 in stainless box rejects about 0.9 W per °C, so 10 W raises the interior 11 °C and 17 W reaches 40 °C inside against a 21 °C room. Above about 5 A, read the relay maker's derating curve at your ambient and bolt the relay base to the enclosure wall or a plate so the steel carries the heat out.
Can I add a vent or a fan instead of a heat sink?
At this load you need neither. A breather, louver or fan cutout carries its own ingress rating, and that lower rating becomes the assembly's rating, so a vent on a manufacturer-rated NEMA 4X-type / IP66 box ends the claim. An internal fan only stirs air; the heat still leaves through the walls. Sealed, the 0.242 m² stainless wall set sheds this load with a 1.1 °C [2 °F] rise.
