When to use
When specifying the rack of a network panel or a technical room, when building the bill of materials of an automation infrastructure project, when checking whether an existing rack can take an expansion, and when drawing the front elevation for the design document.
Why the U count almost never closes the first time
Whoever sizes a rack from the equipment list usually arrives at a number that is too small. The active devices — switches, converters, servers, recorders — take a fraction of the rack. The rest is infrastructure that does not appear on the IT shopping list: cable managers, patch panels, fibre organisers, power strips and the breaker panel. In the example on this page that share is 14 of the 35 occupied U — 40 %.
In an automation project the switches in this rack are the Ethernet backbone of the plant network. The field networks that hang off it are sized on their own: the Ethernet-APL network to the instruments, and the PROFIBUS DP, DeviceNet or ControlNet segments whose couplers and gateways may also take space in the rack.
Blocks that do not come apart
A switch never goes into the rack alone. Below it comes 1U of horizontal cable manager, and below the manager the patch panel where the horizontal cables end. The manager space is not waste: it is what lets the patch cords bend from the switch port to the patch panel port without forcing the bend radius. With the patch panel butted against the switch, patching is done with the cords bent right at the ports, forcing the cable exactly at the connector.
The ODF follows the same logic. The fibre arriving from outside is spliced or connected in the ODF, and the slack — which has to exist, to allow a new splice in the future — rests in a 1U organiser right below. A modular ODF chassis, with several 12-fibre modules, gets the same organiser.
Power: outlets, strips and breakers
Every powered device needs an outlet. The prudent count adds a spare — here, 20 % rounded up — because the next device always arrives before the next project. The power strips go in the middle of the rack, where the power cords of the devices above and below stay short. The DIN-rail breaker panel takes the top; the circuit that feeds it comes from a distribution board, sized in distribution panel sizing. When the rack has to ride through a power outage, the backup behind the strips is a battery bank sized for the autonomy required, and the rack feeder goes into the project cable list.
Heavy at the bottom
Servers and NVRs are the heaviest and deepest items. They go to the bottom of the rack, the 4U one first. The reason is mechanical: with the centre of gravity low, pulling a server out on its rails does not tip the rack over.
The commercial range sets the height
The required height is the occupied U plus the expansion spare. But the rack that is bought is the first of the commercial range at or above it. In the Furukawa Enterprise line the steps are 6, 9 and 12U in wall racks, 22 and 42U in closed floor racks and 36 and 45U in open racks. The steps are large: whoever needs 14U buys 22U, and the difference is expansion space too.
What this tool does not evaluate
The tool works by equipment class — 24-port switch, 2U server — and not by model. That is why it does not evaluate depth, power consumption, strip current, heat load or weight. Those depend on the model bought and must be checked in the specification: the 19” standard fixes the width and the height of the U, but a server 750 mm deep does not fit a 450 mm wall rack.
Formulas and fundamentals
U_occ = Σ U_equipment + Σ U_rule accessories + U_breakers + n_strips Rule accessories are the horizontal manager and the patch panel of every switch and the fibre slack manager of every ODF. The breaker panel takes 3U at the top and each power strip takes 1U.
U_spare = value in U | U_spare = ⌈U_occ × p / 100⌉ The spare is a design decision, not a standard requirement. Declared in U it is fixed; in percent it is rounded up and grows with the rack.
U_req = U_occ + U_spare | H = min { H ∈ range | H ≥ U_req } The required height is always rounded up within the commercial range. In the Furukawa Enterprise line the range is 6, 9, 12, 22, 36, 42 and 45U; a result of 39U becomes a 42U rack.
T = ⌈1.2 × n_powered⌉ | n_strips = max(1, ⌈T / outlets per strip⌉) One outlet per powered device (switches, converter chassis, servers, NVRs) plus a 20 % spare. Even without powered devices the rack gets at least one power strip.
Standards & methods
- EIA/ECA-310-E — Cabinets, Racks, Panels, and Associated Equipment (19" mounting width, 44.45 mm rack unit)
- ANSI/TIA-569 — telecommunications pathways and spaces (referenced in the rack specifications)
Typical reference values
| Quantity | Typical range | Note |
|---|---|---|
| Rack unit (1U) | 44.45 mm (1.75") | EIA/ECA-310-E |
| Mounting width | 19" (482.6 mm) | the hole pattern of the rails defines the standard, not the outer width of the rack |
| 12-, 24- or 48-port switch | 1U | the 12-port one is often half-width and needs a mounting kit or a shelf |
| Patch panel | 24 ports 1U · 48 ports 2U | — |
| ODF | 24F 1U · 48F 2U · 72F 3U | an ODF chassis with 12 modules of 12F takes 3U |
| Media converter chassis | 2U | — |
| Server | 2U or 4U | — |
| NVR | 1U, 1.5U or 2U | 1.5U takes 2U in the rack |
| DIN-rail breaker panel | 3U | — |
| Furukawa Enterprise range | 6, 9, 12, 22, 36, 42 and 45U | 6 to 12U wall closed; 22 and 42U floor closed; 36 and 45U floor open |
Worked example
Technical room with three switches, fibre, servers and NVR
Inputs
- Switches
- 2 × 48-port + 1 × 24-port —
- Fibre
- 1 ODF chassis 12F + 1 ODF 48F + 1 converter chassis —
- Servers and CCTV
- 2 × 2U + 1 × 4U + 1 NVR 2U —
- Accessories
- 1 shelf 1U —
- Expansion spare
- 4 U
- Outlets per strip
- 12 —
Results
- Listed equipment
- 21 U
- Accessories by rule
- 10 U
- Power (breakers + strips)
- 4 U
- Required U
- 39 U
- Selected rack
- 42 U (floor, closed)
- Outlets required
- 10 (1 strip of 12) —
The equipment adds up to 21U, but the rack needs 35U occupied: 10U of managers, patch panels and fibre slack come in by rule, and 4U go to breakers and the power strip. With 4U of spare the requirement is 39U, which does not exist as a commercial rack; the next of the range is 42U, and the 3U surplus is expansion space too. Eight powered devices need ⌈9.6⌉ = 10 outlets, which fit one 12-outlet strip.
Common mistakes
- Adding up only the equipment. Three 24-port switches add up to 3U, but their blocks — switch, manager and patch panel — add up to 9U. This is the mistake that makes the purchased rack not close at assembly.
- Butting the patch panel against the switch. Without the 1U manager between them, the cord bends right at the port, forcing the cable exactly at the connector.
- Forgetting the slack manager below the ODF. The fibre slack leaving the ODF needs a place to rest without bends; stuffed at the back of the rack, it is the first thing to break during an intervention.
- Counting power strips without a spare. A rack with eight devices and one 8-outlet strip has nowhere to plug the ninth, and the ninth always arrives.
- Putting heavy servers high up. Servers and NVRs go to the bottom: the low centre of gravity is what keeps the rack from tipping when a server is pulled out on its rails.
- Treating a 1.5U NVR as 1.5U. The half U is not usable: the device takes the next whole U.
- Confusing usable height with depth. The 19" standard fixes the width and the height of the U, not the depth: a 750 mm server does not fit a 450 mm wall rack, however many free U it has.
Frequently asked questions
How many millimetres is 1U?
44.45 mm, or 1.75 inch, per EIA/ECA-310-E. A 42U rack has about 1.87 m of usable height between the rails; the outer height of the cabinet is larger because of the base and the roof.
Is an empty 1U between the switch and the patch panel really needed?
A 1U patching space is needed, and it is usually taken by a horizontal cable manager. It is the space that lets the cords bend from the switch to the patch panel without forcing the cable bend radius. One U is enough; more than that only lengthens the cords.
Where do the power strips go?
In the middle of the rack, so that the power cords of the devices above and below stay short. When the middle cannot take the strips without breaking a switch or ODF block, they move right below the breaker panel.
How much spare space should a rack keep?
There is no normative value; it is a design decision. What matters is to declare the spare — in U or as a percentage of the occupied U — and to remember that rounding up to the commercial range usually leaves extra room.
Open or closed rack?
The open rack (two posts) makes cabling access easier, but does not protect against dust or unauthorised access. It is common in a controlled technical room; in a field panel, the closed rack is the usual choice.
Glossary
- U (rack unit)
- Standard height of 44.45 mm used to measure 19" equipment and racks.
- Patch panel
- Passive connector panel where the horizontal cables end and from where the patch cords run to the switch.
- Horizontal cable manager
- 1U accessory that organises the cords between the switch and the patch panel and preserves the bend radius.
- ODF
- Optical distribution frame — where the outside fibre is spliced or connected and distributed through optical cords.
- Front elevation
- Drawing of the rack seen from the front, U by U, showing the position of every device and accessory.