Sizing a sealed enclosure for a PoE switch heat load

Short answer: the power your cameras draw rides out on the Cat 6 pairs, and only the losses stay in the box. For 13 runs at 10 W each, about 45 W stays behind — the switch's own draw plus the conversion loss of its PoE supply. A sealed stainless box sheds that at 3.7 W per square metre of wall per kelvin of rise, so a 24 x 20 x 8 in (H x W x D) enclosure sits about 11 K over the wall: 46 C on a 35 C day. Pass or fail lands on one datasheet row, the switch's maximum operating temperature. Size the box against that row, and against the watts that stay — the 400 W nameplate is the input at full budget, and most of it leaves on the wires.

The watts that stay in the box

Treat the enclosure as a bucket with a hole in the bottom. Mains power arrives, PoE leaves through the ports, and only the difference warms your air. Your cameras hang on walls and poles, so their 130 W dissipates outdoors. What heats the box is the switch's own electronics plus the conversion loss of its PoE supply section.

Two rows of the switch power table set the sum. The no-load row — about 30 W on a 24-port PoE+ switch, and this class runs 20-40 W — is the switch itself. The maximum-input row, a 400 W-class figure, already includes the full PoE budget, and most of that budget is headed down the cable to somewhere else.

Datasheets quote the PoE supply section at about 90 % efficiency, so the loss is roughly 11 % of the delivered load (1/0.90 − 1 = 0.11). With only the maximum printed, bound the heat as maximum input minus delivered PoE: 400 − 130 = 270 W for this build. That bound over-counts, because it bills you the supply losses of eleven empty ports, and it will never under-size a box.

A UPS in the same box adds its own conversion loss: online units run near 90 % efficient, so figure another tenth of whatever it carries. The outdoor PoE switch and UPS build works that case end to end.

How fast the stainless wall sheds heat

Heat leaves through the walls as Q = k × A × ΔT: watts equal transfer coefficient times area times kelvin of rise. Use k = 3.7 W/m²K for a stainless wall, the reference value this site's thermal guides carry; painted steel moves 5.5 W/m²K and plastic 3.5 W/m²K. Outdoors, stainless needs no coating, so the table below is stainless.

The table assumes a 35 C shaded ambient. 40 C inside is the default target for electronics; the 50 C and 60 C columns apply when your switch datasheet names those figures. Areas count all six faces — flat on a wall the back face sheds little, so hold it as margin rather than capacity. Direct sun pushes the skin above shade ambient and eats that margin, so mount under an eave; the hot-ambient cooling options article picks up when shade is not on offer. For any other mix of parts, the enclosure heat calculation guide carries the same method in full.

Worked example: 13 cameras at 10 W on a 35 C wall

Inputs, each with its source:

• Delivered PoE: 13 runs × 10 W per camera (camera sheet, IR on) = 130 W.
• Switch: 24-port PoE+ class, no-load 30 W and maximum input 400 W — the two power-table rows.
• PoE supply efficiency: 90 %, the figure this class of datasheet quotes.
• Ambient: 35 C on a shaded wall, the summer design point.
• Wall: k = 3.7 W/m²K, stainless.
• Candidates: 24 x 20 x 8 in (1.07 m²) and 36 x 24 x 8 in (1.73 m²), both 304 stainless.

The rule in words: heat in the box = no-load draw + 11 % of delivered PoE. Rise = heat ÷ (k × area). Inside temperature = ambient + rise, compared against the switch's rated maximum.

The arithmetic:

• Supply loss: 130 W × 0.11 = 14 W.
• Heat in the box: 30 W + 14 W = 44 W, carried as 45 W.
• Small box: 45 ÷ (3.7 × 1.07) = 45 ÷ 3.96 = 11.4 K rise, so 46 C inside.
• Large box: 45 ÷ (3.7 × 1.73) = 45 ÷ 6.4 = 7.0 K rise, so 42 C inside.

The decision turns on the switch's rated maximum. With an industrial switch rated 60 C, the 24 x 20 x 8 in box passes with 14 K in hand — order it and keep the change. With the 40 C default, both boxes miss by 2-6 K, and you fix it one of three ways: buy the industrial-rated switch (same footprint, usually the cheapest fix), step up to about 2.4 m² of wall — the 48 x 36 x 8 in box at 3.1 m² holds 39 C, which is a lot of stainless for one datasheet row — or fit a closed-loop heat exchanger sized on the same 45 W.

Run the nameplate case for confidence: delivered at full budget ≈ (400 − 30) × 0.9 = 333 W; heat = 30 + 333 × 0.11 = 67 W; in the 36 x 24 x 8 in box that is 67 ÷ 6.4 = 10 K, or 45 C inside. Even the case that scared the installation guide is a 45-50 C problem, not a 400 W problem — provided the switch inside is rated to take it.

Size your own in five steps

1. Count what you feed: powered ports in use × each camera's maximum draw with IR on. Here 13 × 10 W = 130 W.
2. Read the two power-table rows on the switch datasheet: no-load draw and maximum input. From BTU/hr, divide by 3.412.
3. Heat in the box: Q = no-load draw + 0.11 × delivered PoE. With one row only, use maximum input − delivered PoE as the bound.
4. Set your rise budget: switch rated maximum − hottest ambient at the wall. A 35 C shaded wall and the 40 C default leave 5 K.
5. Divide: area needed = Q ÷ (3.7 × rise). Match it to the table, then check the chassis fits — the 24 x 20 x 8 in box gives 19.88 in of clear inside width and 7.88 in of depth, so hold back the last inch for connectors and a service loop. The PoE switch enclosure size article walks the physical fit.

When Q lands above the column you need, stop sizing boxes: cool actively with a closed-loop unit rated for that Q, and keep the sealed rating while you do it.

Checks once it is on the wall

Prove the design on the first hot, shaded afternoon. Pull the switch's internal temperature from its management page and compare it with the row you designed on. A thermometer taped to the mounting panel reads close to the inside air; an IR gun on the door reads skin, which sits between inside and ambient.

Walk the bottom face: every gland dry, every seal seated, door gasket flat. The cutouts and glands guide covers hole sizes and spacing. Isolate the supply and prove it dead before the door comes open — a UPS-backed box stays live after the building drops.

If the window fogs at dawn, the box is cycling through dew point overnight; a small anti-condensation heater holds it clear, and the relative humidity calculation tells you how close you are.

What to order for this build

Lead with the box the numbers picked: the 24 x 20 x 8 in 304 stainless enclosure for 13 cameras and a 60 C-rated switch, or the 36 x 24 x 8 in version when the budget may fill or the chassis runs deep. Both carry a hinged gasketed door, a removable mounting panel and wall-mount feet, and the wall-mount range runs from 6 x 4 x 4 in junction boxes to 60 x 36 x 16 in. Within sight of salt water, order the same size in 316L.

Every enclosure we build is rated NEMA 4X / IP66 by the manufacturer and not UL listed; NEMA does not test or certify enclosures. Custom cutouts come from the drawing you approve: thirteen Cat 6 glands at about 1 in of bottom run each, a fiber gland and a power gland, delivered in 10 business days anywhere in the world, counted from your approval. For camera estates with a fiber trunk, the fiber and PoE camera enclosure article covers the entry layout, and the enclosure selector carries these numbers straight to a quote.

Where these installs go wrong

• The nameplate trap: designing on 400 W buys an air conditioner, its power feed and its service visits for what is really a 45 W problem.
• The no-arithmetic trap: the same box, sized by eye, drops ports at 2 pm every July — it ran cool in March because the cameras were off.
• The fan hole: a fan-filter takes the assembly down to the fan's own rating, about IP54, which is sheltered indoor duty and never a washdown wall. Keep Type 4X with closed-loop cooling.
• Full sun: the skin runs above shade ambient and spends your margin for you, so move the box under an eave or shade the wall.
• Counting ports instead of powered ports: a 24-port switch with 12 powered ports strands a camera. You need 13 powered ports and a budget near 170 W — your 130 W plus 30 % spare for camera swaps and heavier replacements.

Enclosures in this article

Questions