An equipment room, a list of gear and floor space for one open frame: the next question is 2 post vs 4 post rack. Both are open frame racks that take standard 19 inch equipment, but they carry it in very different ways. That difference decides what you can safely mount, how deep it can be, and how the rack has to be fixed to the building.
What is the difference between a 2 post and 4 post rack?
A 2 post rack holds equipment by its front ears on two uprights, so it suits shallow, light gear such as patch panels, switches and cable managers. A 4 post rack adds rear uprights, so deep or heavy equipment is supported front and back on rails. Both should be bolted to the floor; check each rack’s rated load before loading it.
2 post vs 4 post rack at a glance
Start with the load path. On a two-post frame, everything hangs from two vertical rails, so a deep unit cantilevers off them. On a four-post frame, the same unit sits between front and rear rails, or on rails that run between them.
| Factor | 2 post rack | 4 post rack |
|---|---|---|
| Support | Front ears, or centre mount | Front and rear, on rails or ears |
| Best for | Patch panels, switches, fibre panels, light shelves | Deep or heavy AV and network gear, UPS, amplifiers |
| Depth | Limited by what the ears and a shelf can carry | Set by the rear post position, often adjustable |
| Footprint | Small, access from both sides | Larger, open sides |
| Anchoring | Floor bolts, often an overhead brace too | Floor bolts at each corner |
Rack units and the 19 inch width are the same on both; our guide to what a rack unit is covers the hole pattern.
Load and depth: what each frame can carry
There is no single load figure for either type. Ratings vary by maker and model, and the rating usually assumes the load is spread evenly and the rack is anchored. Use the rated load on the rack’s data sheet, and the equipment maker’s mounting instructions, as the limits.
Depth is where the two part ways. A two-post frame suits gear that is shallow, or light enough for its ears to carry. Long, rear-heavy units need rear support, which is what four-post rack depth is for: the rear posts sit at a set or adjustable distance behind the front ones, and rails span the gap. Measure the deepest unit, add room for plugs and cable bends, and check the four-post depth range covers it.
The University of Alberta telecommunications design guidelines show the split in practice: a UPS goes at the bottom of the rack on rails in a four-post rack, or on standoff hardware in a two-post rack.
Centre mount vs flush mount on a two-post rack
Many rack-mount units come with ears that can be fixed in more than one position. Flush mounting puts the ears at the front of the chassis, so the whole unit hangs forward of the posts. Centre mounting, also called mid mount, moves the ears back so the posts sit nearer the unit’s middle and the weight is balanced on both sides.
Centre mounting is the usual way to put a deeper switch on a two-post frame, if the maker supplies the brackets. When the equipment only offers front ears and is deep, a cantilever shelf or a four-post rack is the safer choice.
Rails, shelves and accessories
Four-post racks accept sliding or fixed rails, which carry a unit along its sides and let it be pulled forward for service. Two-post racks rely on ears and cantilever shelves; a vented shelf holds non-rack gear such as a media player or a small receiver.
Fill from the bottom with the heaviest items, so the frame is not top heavy. For the loading order, ventilation and an equipment inventory, follow how to choose an AV equipment rack, and for backup power see choosing a UPS for an AV rack.
Anchoring an open frame rack and seismic bracing
An open frame rack has no side panels to stiffen it, so it relies on its fixings. The Alberta guidelines specify a two-post rack, a four-post rack and a vertical manager between them, bolted to the floor, and note that racks are often top heavy, so level floors and floor bolts are required.
In earthquake country the stakes rise. FEMA E-74, section 6.5.3.2 says communication racks should be braced, anchored or tethered, with equipment secured to the rack and cables left with slack. It warns that poorly anchored racks can slide, tip or collapse, and that cable connections can pull loose. Citing ASCE/SEI 7-10, it notes that Seismic Design Categories D to F require anchorage of equipment over 400 pounds, and of items over 20 pounds mounted more than 4 feet above the floor.
- Bolt the base into the structural floor, not just a tile or carpet.
- Tie two-post racks overhead to ladder rack or the structure where the design calls for it.
- Leave slack in cables and flexible connections to conduit.
Follow the engineer’s detail and the authority having jurisdiction. For wall-hung frames, see wall mount vs floor rack.
Cable management and clearance
Two-post racks are easy to dress: vertical managers bolt beside the posts, and a manager between two racks serves both. Four-post racks have more room for rear cable bars and power strips. Either way, the methods in network rack cable management apply.
Plan the space around the rack too. The Alberta guidelines call for at least 12 inches (300 mm) clear behind racks and on each side of two-post and four-post racks, and 40 inches (1 m) in front, measured from equipment that sticks out past the frame. They also require racks and cable trays to be grounded to ANSI/TIA-607, the NEC and local code.
When an enclosed cabinet is the better choice
Open frames assume a locked equipment room. Choose an enclosed cabinet instead when:
- The rack sits in a finished space, such as an office or a closet with other users.
- Gear needs locking against tampering.
- Dust or debris is a concern.
- Airflow must be controlled with fans and blanking panels.
Where to find racks and accessories
The AV racks category and the AV accessories category sit in the Audio and Video division, with cable managers in the wire and cable management category. For pricing and availability, ask your sales representative through the Bursma contact page or the Columbus, Ohio branch, or open a Bursma dealer account.
Frequently asked questions
Can you put a server in a 2 post rack?
Only if its maker supports two-post mounting, usually with centre-mount brackets. Deep servers normally need four-post rails.
Does a 2 post rack need to be bolted to the floor?
Yes. A two-post frame is narrow and often top heavy. University cabling standards such as Alberta’s require floor bolts, and seismic areas may also need overhead bracing.
How deep is a 4 post rack?
It depends on the model; many have adjustable rear posts. Size it to the deepest unit plus room for cables and plugs.
Is an open frame rack better than a cabinet?
For a locked equipment room, open frames give easy access and cable dressing. For finished or shared spaces, a cabinet adds security and dust protection.