Panel space goes to parts doing one job each, to heat and to wiring room. Space comes back where one part does two jobs, sheds less heat or moves out of the main panel.

Panel inches go three places: parts that each do one job, heat that needs clearance and cooling, and wiring that needs room to land. The inches come back from parts that combine jobs, parts that shed less heat, and hardware that sits in the product or at the wall and off the main rail. Settle the layout before the enclosure is ordered.
A panel that will not close goes up a size, and the machine frame, the door swing and the floor plan move with it.
Pack a panel tight and the heat has to be planned. Figure roughly one to one and a half watts per amp through a solid state relay, and leave the heat sink its room.
Every cable through the wall takes a gland and the spacing around it, and a pre-terminated cable loses its connector on the way in.
The data port and the entry are part numbers when they are decided early and change orders when they are decided late. A meter added late, with no rail left to sit on, is a change order too.
One part per decision, each with one fact from its own sheet. The brand name opens that product line.

The supply sets how much rail and how much clearance the DC bus takes.
The printed fact. Custom AC/DC supplies from 50 W to 60 kW. More watts per inch, less heat to dump.

Switching in a slim DIN-rail package, with versions that arrive on their own heat sink.
The printed fact. 22.5 mm DIN-rail celpac and 45 mm okpac packages, single to four phase, with ready-to-use versions with heatsinks.

The motor branch from one catalog, with a coil that runs cool in a tight panel.
The printed fact. Contactors from compact through heavy industrial frames with a 3 W sealed coil that runs cool, and UL 489 miniature circuit breakers beside them.

The display and the computer are one fanless unit in the door.
The printed fact. Panel PCs from 10.1 to 21.5 inches, IP65 front, fanless, Windows or Linux.

The network switch is designed into the product, so it takes no rail in the panel.
The printed fact. Board-level switching 55 by 55 by 10 millimeters, rated -40 to +85 C, designed into the enclosure instead of bolted on.

One device on the rail where the panel would otherwise carry several.
The printed fact. BFM-II: up to 54 single-phase or 18 three-phase circuits from one DIN-rail unit. PRO: a Class 0.2S revenue meter and a Class A power quality analyzer in one device.

The enclosure wall stops being a row of glands, and the cables keep their connectors.
The printed fact. The cable goes through the frame with its connector on and the frame holds IP66. UL Recognized, one frame in place of a row of glands, seals swapped from outside.

The splits come off the feed on the rail itself.
The printed fact. Finger-safe power distribution blocks for the splits from the feed, with screw, spring cage, push-in and stud connections on one rail system.
Premium PSUCelducIMO AutomationGETTBrainboxesSATECConta-Clip
Every product line on one page
Also designing for:
Look at the three places the inches go: parts that each do one job, heat that needs clearance, and wiring room. Combine jobs in one device where its sheet supports it. A SATEC PRO, for example, is a Class 0.2S revenue meter and a Class A power quality analyzer in one. Pick parts that shed less heat. Pass pre-terminated cables through one split frame in place of a row of glands.
It can. Celduc celpac relays are 22.5 mm wide on the DIN rail, and there are ready-to-use versions with heatsinks. The heat still has to go somewhere: figure roughly one to one and a half watts dissipated per amp conducted. If there is no thermal budget left in the panel, an electromechanical contactor may be the better part, and we carry those too.
With a split cable entry. The cable goes through the Conta-Clip KDS frame with its connector on, the wave-cut seal closes around it, and the frame holds IP66. It is UL Recognized, and the seals are swapped from outside.
Yes. Brainboxes board-level switching is 55 by 55 by 10 millimeters, rated -40 to +85 C, and designed into the enclosure instead of bolted on.
The enclosure size you are trying to hold, the panel layout, the one-line and the cable schedule. From that an application engineer walks the design path with you and names where the inches come back.
After 30-plus years in the industrial hardware space we know these products, on our line card and off it. Bring us your most stringent or most mundane requirement. If one of our product lines fits, we will show you where. If it does not, we know someone who will. We would be happy to help.
Every product line on two pages, in the order a machine gets designed.