In a 600 mm control cabinet, the PLC takes the upper left, breakers and the switching power supply take the right side, terminal blocks take the bottom, and what is left for networking is often a dozen centimeters of DIN rail. When space runs tight, the first instinct is to pick the smallest unit on the market. Selecting an industrial cellular router at that point is not about the size of one box; it is about how many devices the cabinet ends up holding.
A smaller arrangement has been worked out before: a phone connected to a portable router with a data cable, the router broadcasting Wi-Fi and charging the phone. That combination is smaller than any industrial router and it does work; some installations hang it off the second WAN port of a home gateway to cover the gap when fixed broadband drops. A more direct suggestion came from the same discussion: anything that stays connected long term belongs on a cellular device with a data SIM. Both views hold up. The difference is what small means. A travel device is built small to fit in a bag; a cabinet device needs to be small to occupy less rail and still stay powered for years.
Before ordering anything, finish the cabinet layout on paper: the PLC, the I/O modules, the switching power supply, the terminal blocks, the wiring ducts. Whatever rail length and height remain is the ceiling. Clipped side by side on the rail, the router needs clearance from heat generating modules such as the power supply and the drive; mounting flush against them pushes cabinet temperature up a notch. Cable has a bend radius, so the entry direction decides how much room is needed above and below; antenna exit has to be decided in advance, since a metal enclosure eats much of the signal. One rule settles it: fix which rail segment the device occupies, what sits beside it and which side the cables come from, then read the dimensions. Three mounting methods set three floors: side by side on the rail, thickness and clearance matter; on the cabinet wall, hole positions and antenna orientation matter; inside a machine, terminal type and whether a heat dissipation face can sit against a wall matter.
Small volume does not mean a small footprint in the cabinet, and four places bring it back. Built in antennas lose signal behind a steel door, so a hole has to be drilled and the antenna led outside. Too few Ethernet ports means adding a switch, which takes another rail segment, another power feed and another patch cable. No serial port means a serial device server for the RS485 energy meter or temperature controller. A DC jack or adapter as the only power input means one more conversion stage to reach the 24 V rail. Every add-on brings another failure point and another round of acceptance testing. The comparison runs on the final device count in the cabinet, not on the size of a single unit. Once the cabinet is crowded enough to need a switch, the model with more ports saves space: the USR-G809s gives 2 x WAN/LAN combo ports, 6 x LAN ports and 2 x SFP slots, one of which is shared with a copper port, wide range DC 9-60 V with reverse polarity protection, DIN rail mounting, and external SMA-K cellular antennas. On the other side, a cabinet with only a PLC and one or two serial devices is served by a small model with three Fast Ethernet ports, one WAN port configurable as LAN; extra ports only add width.
Power: read the wide input range, not the nominal voltage. The 24 V rail inside a cabinet dips when motors start and stop, so a wide range terminal fed from the rail saves the space an adapter would take and rides through the dip. Two power inputs from separate circuits inside the cabinet keep one alive while the other is isolated for maintenance. Temperature: read the two columns separately. The datasheet figure refers to surrounding air; inside a cabinet, self heating and heat from neighboring modules stack on top, so measured values should stay 20 degrees Celsius or more below the upper limit. Mounting: land on the method the cabinet already has, rail or wall, and avoid a unit that only sits flat and then needs a bracket. The USR-G806w measures 104 x 102 x 28 mm, mounts on DIN rail or wall, takes wide range DC 9-36 V with reverse polarity protection, carries a terminal block and a DC jack as redundant power inputs, keeps Wi-Fi and cellular antennas external so they can be routed out of the cabinet, runs -20 to 70 degrees Celsius, and holds 7x24 operation on hardware and software watchdogs.
When the cabinet holds an energy meter, a temperature controller or an older PLC exposing only RS485 or RS232, the first check is whether the router carries serial terminals itself. Where it does, one unit covers the work of two, saving rail space, a power feed, a cable and one acceptance round. The USR-G816 carries RS232/RS485 terminals at 1200 to 460800 bps, adds 5G SA/NSA and dual SIM with the second slot available as eSIM, comes in a metal housing rated -35 to 75 degrees Celsius, measures 125 x 103 x 45 mm, and mounts on DIN rail or wall. For sites where no broadband can reach, or where the equipment moves, that one unit handles the uplink and the device that would otherwise hang beside it. When antennas are routed outside, feed cable length and hole position get fixed at the same time; moving either one later means opening the cabinet again.
Saving space has a second end: with parameters changed remotely, reboots issued remotely and firmware upgraded in batches, no room is needed inside the door for a laptop and no separate display for status. Three alarm classes, offline, weak signal and data limit exceeded, reach email and SMS, so an exception lands on a phone first. Before a SIM goes live, monthly usage is worth estimating from reporting interval, message size and point count, with the threshold at 70 to 80 percent of the plan, so the alert arrives before service stops rather than after. The USR-G809s has an OLED panel where a short press switches screens for status; the USR-G816 switches automatically between Ethernet, cellular and Wi-Fi, so a break in the primary link needs no one on site. At acceptance, antenna position, peak cabinet temperature and switchover time belong in the handover document, giving next year's recheck a reference.
Choosing an industrial cellular router for a small cabinet runs in one order: map the internal layout and fix the position first, count the footprint by how many devices the cabinet ends up with, then compare power, mounting and temperature. The point to stop at is where saving more starts to cost signal, power or maintenance.
