DC power entry into a sealed enclosure for a single board computer

Short answer: cut the 5 V cord and pass its two cores through one M16 cable gland at the bottom of the box. The gland seals in a 16.1–16.3 mm [0.63–0.64 in] hole, and the cores land on a fused 2-pole terminal block inside. From the block, a short jumper pair feeds the power pins at the low-numbered end of your board's header, or the edge power socket when the carrier board carries one. That is one hole, one gland, one fuse, and the box keeps its rating (rated NEMA 4X / IP66 by the manufacturer; not UL listed), because the gland carries its own IP mark and the hole is cut to the datasheet diameter. Size the cord so your board sees at least 4.75 V under load, and the whole job is a bench hour.

The gland and the hole

An M16 gland fits most two-core DC cords. One maker's datasheet calls for a 16.1–16.3 mm panel hole, a 22 mm wrench on the nut and the support, and leaves a 24.5 mm hex standing proud when fitted. Give the hole 25 mm [1 in] of clear steel around it, so the wrench swings and the nut clears the fold. Match your cord's OD to the clamp range before ordering — an M16 clamp range typically spans 4.5–10 mm in the datasheet.

The M20 row has its own write-up in M20 gland hole size for metal and plastic panels. Buy the gland with an IP66 or IP68 mark: the assembly takes its weakest part's rating, so a plastic gland marked IP54 makes your 4X box an IP54 box the day it is fitted. In US installations NEC 110.28 also states that an IP code is not a substitute for an enclosure type rating, which is why the box carries the type and the gland carries its own IP mark.

Have us cut the hole before the box ships: the cutout designer places the gland hole on any face and checks the spacing, and delivery is 10 business days anywhere in the world, counted from your order — from your approval of the drawing for custom work. The fundamentals of holes, seals and windows sit in the cutouts and glands guide, and the fitting sequence is in installing a cable gland without losing the rating.

Bottom entry and the drip loop

Put the gland on the bottom face, as the lowest opening in the box. Water tracks a cord's jacket downhill, so a bottom gland meets a dry cable, while a roof gland delivers every drop on the jacket straight to the seal. Hang a 100 mm [4 in] drip loop outside — the cord dips below the gland, then rises — so water leaves the jacket before it reaches the fitting. The same logic for rigid conduit runs in the bottom entry into a floor-mounted NEMA 4 enclosure.

The cord you keep and the volts you lose

Cut the thin cord off the adapter and run 18 AWG two-core. Take a board and radio card drawing 2.5 A from the rail on a 1.5 m [5 ft] cord. Current runs out and back, so 1.5 m of two-core is a 3.0 m copper loop. Copper tables put 18 AWG at 6.4 Ω per 1000 ft [21 mΩ/m], so the loop is 3.0 m x 21 mΩ/m = 63 mΩ. The drop is I x R = 2.5 A x 0.063 Ω = 0.16 V, and about 0.02 V more goes into the short jumper at the board.

Now the cord you cut off. The 22 AWG lead many adapters ship with is 16 Ω per 1000 ft [53 mΩ/m]. The same 3.0 m loop is 0.16 Ω, and 2.5 A takes 0.39 V from it. With the adapter set to 5.10 V, your board sees 4.92 V through 18 AWG and 4.69 V through the original cord. The rail wants 4.75–5.25 V — the USB ±5 % band — so the shipped cord browns the board out and 18 AWG clears it with room. Loss scales with length and with current: at 3 m [10 ft], step up to 16 AWG [4.0 Ω per 1000 ft, 13 mΩ/m], which drops 0.20 V at the same load.

The heat those watts leave

A 2.5 A load on the 5 V rail is 12.5 W dumped into a sealed box. Stainless walls transfer k = 3.7 W/m²K, so the rise is watts ÷ (k x area). A body near 12 x 10 x 6 in [305 x 254 x 152 mm] has about 0.33 m² of surface: 12.5 ÷ (3.7 x 0.33) ≈ 10 K over ambient, which is 40 C inside on a 30 C day — the ceiling most board datasheets allow. A body near 18 x 14 x 8 in [457 x 356 x 203 mm] gives 0.66 m² and a 5 K rise, 35 C on the same day, and that is the size you order for this load. The full method is in low-power board heat in a sealed outdoor enclosure; the range runs from 6 x 4 x 4 in to 60 x 36 x 16 in [H x W x D] across the stainless wall-mount sizes.

The block, the fuse and one ground point

Land the two cores on a 2-pole terminal block within about 150 mm [6 in] of the gland: positive on the fused pole, return beside it. Ferrule the stranded ends, because a screw terminal clamped on bare strands works loose within a year of door slams. Feeding the header bypasses the fuse and reverse protection behind most boards' power jacks, so fit a 3 A blade fuse in the positive lead — 125 % of the load. With an isolated adapter, land DC 0 V on the ground bus at one point, which pins the box and the logic to one potential. When a touch display shares this enclosure, grounding the cutout instead of the GND pin is the fix that stops ghost presses, because ground current stays off the signal lines. The box's own bonding follows the grounding and bonding guide, and where the block sits against the board is laid out in the single board computer layout guide.

Header pins or the edge socket

Feed the power pins at the low-numbered end of the 40-pin header — the same pair a carrier board routes to its own regulator. Crimp a 2-pin 2.54 mm housing onto the jumper pair so the plug comes off with the board, and keep the jumper under 200 mm [8 in]. When the carrier board ships with its edge power socket loose, solder it before the board goes in the box; one pair at that socket then feeds the board and the radio card together. GPIO pins are signals, not a supply: commonly documented limits for 40-pin boards run 16 mA per pin and about 100 mA across all of them, a rounding error next to your 2.5 A rail.

The build, step by step

1. Measure the cord's OD with calipers and pick the gland row from the table; the clamp range on the gland's datasheet must cover that OD.
2. Mark the hole on the bottom face, at least 25 mm [1 in] from every fold and other opening, on the drawing you approve; send it through the cutout designer or the custom stainless cutouts page.
3. Drill or punch the hole to the datasheet diameter — a step drill stops clean in stainless — and deburr both faces flat.
4. Slide the gland's nut and seal onto the cord, strip 8 mm [5/16 in] of insulation, and ferrule both cores.
5. Fit the body through the hole, seat its gasket on the deburred face, and snug the locknut with the 22 mm wrench until the seal compresses and the cord leaves centered.
6. Land positive on the fused pole with its 3 A fuse, return on pole two, and DC 0 V on the ground bus at one point.
7. Crimp the 2.54 mm housing onto the jumper pair and plug it onto the power pins at the header's low-numbered end, or wire the edge socket.
8. Power up on the bench with the door open, before anything is mounted.

Checks with the power on

• Voltage at the block: 5.0–5.1 V.
• Voltage at the board under full load: at least 4.75 V.
• Current on the meter: about 2.5 A in the example, inside the ratings of cord and fuse.
• A firm tug on the cord moves nothing at the gland.
• After an hour at full load, inside air sits at or below 40 C — about 35 C for the 18 x 14 x 8 in box at 30 C ambient.
• Run a hose over the gland for a minute and find the inside dry.

Mistakes that cost the rating

• A 17 mm [21/32 in] drill bit because it was close: the M16 seal never compresses in an oversize hole.
• No deburr: a burr under the gasket works through it in one season of sun and washdown.
• An IP54 gland from a hardware bin: the box is an IP54 box from the day it is fitted.
• Silicone packed around a molded plug: an untested joint where a gland belongs; power cord through an enclosure knockout covers that route.
• The gland in the roof of the box: every drop on the jacket is delivered to the seal.
• No fuse on the header feed: a shorted jumper has only the adapter's current limit behind it.

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.

Questions