Talk:19-inch rack/Archive 1

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I'm interested in the history of it -- how did telcom racks begin to be used for computers? Who pioneered that, and when did it first happen?

I'm interested in this too. I've seen military rack mounted equipment (both 19 and 23 inch) from the 1950s and so know the standard has existed since at least then. Has the 23" rack business died? I used to see them all the time and one nice thing was you could get a normal keyboard to fit on a shelf. Marc Kupper (talk) (contribs) 20:52, 1 September 2007 (UTC)
There's a bit of history on, but I don't know what his sources are. (talk) 12:44, 30 January 2008 (UTC)

Old Discussion

  • Uhm, all this "incorrect" business makes for a bad entry. How about merging it together? - Jombu
Thank you for your suggestion! When you feel an article needs improvement, please feel free to make whatever changes you feel are needed. Wikipedia is a wiki, so anyone can edit almost any article by simply following the Edit this page link at the top. You don't even need to log in! (Although there are some reasons why you might like to…) The Wikipedia community encourages you to be bold. Don't worry too much about making honest mistakes—they're likely to be found and corrected quickly. If you're not sure how editing works, check out how to edit a page, or use the sandbox to try out your editing skills. New contributors are always welcome. Dan | Talk 15:14, 31 Oct 2004 (UTC)
Is that a reasonable compromise? Alter it if you feel necessary, but I'm not sure about Wiki policy on accuracy. Perhaps the first value should also be an exact one? Estel 14:44, Nov 10, 2004 (UTC)

- Could I request historical content? Who started the 19 inch rack standard, and roughly when did it first appear? Was it a military thing? UK or US or other? Wartime, peacetime? Cheers. Ian.


This article does a good job of describing the height unit U. Does anybody know anything about the thickness unit HP? -- (unsigned)

"HP" stands for horizontal pitch. 1HP = 0.2 inches (5.08mm). This is the height of a single card (for example ATCA) when it is mounted vertically (making its height become a measurement on the horizonal axis). A 19 inch rack holds a max of 84 HP, but most subracks use some of the space for controller boards, power FRUs, etc. -- RevRagnarok 13:23, 29 April 2006 (UTC)
Thank you, I was unsigned above by accident. Jimaginator (talk) 17:42, 8 September 2008 (UTC)


I still cannot understand how tall does 1U has?

  • Each "U" or (unit is equivalent to 1.75 Inches).

Hence 3U =5.25 inches, counting from the bottom inside cutout of the rack.

Hope this helps.
Regards Scotty

The URL has changed [1] and in case it changes again you are looking for
December 2004
January 1998
Chapter 9 it titled "ENCLOSED ELECTRICAL/ELECTRONIC EQUIPMENT" and I did not see much that would be a useful addition to the Wikipedia article. It doesn't even mention the unit spacing, etc. for racks. It is a useful document should someone be charged with task of installing a new rack, getting the electrical power to it, etc. Marc Kupper (talk) (contribs) 21:04, 1 September 2007 (UTC)

metric conversions

The metric quantities in the article should be *exact* conversions from the inches. These are specifications, not measurements, so do not round them off. Exact conversion: 1 inch == 25.4 mm. 14:18, 7 October 2005 (UTC)

This is very important when buying a rack. Remember in height a unit= 1.75 inches what ever that converts to; Good job, since I'm lost on metric stuff Scott 14:40, 7 October 2005 (UTC)

Actually it would be interesting to see a copy of the DIN standard and use the metric numbers from that. Rees11 (talk) 08:47, 5 December 2007 (UTC)

British Standard BS 5954-2:1985 (IEC 297-2:1982) quotes a hole centre to centre dimension of 465.1mm (18.311 in) just to add to the confusion! —Preceding unsigned comment added by (talk) 15:52, 27 January 2009 (UTC)

SC48D = IEC Subcommite

According to this pdf:, SC48D is an IEC Subcommitie?

Better pictures please

I don't see how this article could possibly need three different pictures of "generic rack setups", and still not have any good illustrations of the actual physical dimensions. There's a lot of numbers in there to keep track of, and some visual aid couldn't hurt. Even a closeup of two or three rack-mounted servers (or any equipment) would be useful.

(It'd be different if the pictures illustrated different types of equipment well, like "telephone switching equipment, circa 1962"; "a modern server room - 2U servers on the left, ethernet switches, etc, on the right"; "musical equipment - sound modules and audio effects processors". Now it all seems more like "old stuff in a rack"; "server stuff in a rack"; "stuff in a rack at location X" -- there doesn't seem to be any thought to it, just whatever was available at the moment. And yes, the bland descriptions contribute to that feeling.)

I'd suggest keeping the first (black and white) picture, with its generic description, and getting rid of the other two. It fits the mental image best, somehow.

-- magetoo 12:38, 4 December 2005 (UTC)

Bolt Sizes

"Originally the mounting holes were tapped to receive a particular type of threaded bolt. Racks with plain square holes are now common. Square-holed racks allow boltless mounting, and can be adapted for use with bolts by the use of cage nuts. A cage nut consists of a spring steel cage, designed to clip onto a square mounting hole, within which is a captive nut."

Although the statement above is correct, many of the common 19" racks still sold are tapped to utilize three standard bolt sizes. It may be wise to include the three standard bolt sizes (12-24, 10-32, & M6-Metric) as they are still pertinent in the industry.

Go for it! Be Bold! -- RevRagnarok 12:03, 23 May 2006 (UTC)

Rack rails

What's the difference between fixed rails and telco rails? Edward 16:47, 4 June 2006 (UTC)


What is the 19" RETNA standard?

There's no such thing - perhaps you meant RETMA (Radio-Electronics-Television Manufacturers' Association) which is now called the Electronic Industries Alliance (EIA). If you want to sound like an old timer (pre 1957) then call them RETMA racks. That brings back a memory of gunmetal gray racks with the rounded corners. :-) Marc Kupper (talk) (contribs) 03:30, 2 September 2007 (UTC)

Two-hole rack unit?

Some racks don't have the middle hole in each RU drilled (results in two close holes, then a big gap, two close holes, etc etc). Is this a different/older standard or the manufacturer being lazy? 21:51, 30 October 2006 (UTC)

In the telecommunications industry, it is very rare to see the third hole. This is mainly because the third hole is used for a half of an RU, which some equipment sometimes require, but no telecommunications equipment that I've ever come across needs. Typically, an RU is recognized by the space in between one pair of holes (one on top of the other) and the next pair. It's usually measured by the distance between the centre of the bottom hole of the first pair, and the centre of the top hole of the second pair. -- 15:59, 20 March 2007 (UTC)

Hewlett Packard and Cisco used to produce a fair bit of rackmount data switching equipment that measured 1.5U in height - although thankfully this nonsense appears to have ceased in newer product lines, perhaps they finally realised what a pain it is to efficiently mix in one rack with equipment of integral-value RU height. (talk) 19:03, 3 March 2009 (UTC)
EIA-310 defines two kinds of hole spacings: Universal Spacing, which has the 3rd hole, and Wide Spacing, which does not. jhawkinson (talk) 01:02, 2 June 2009 (UTC)


Does anyone know where the unit of 19" for telecommunications equipment comes from ? I am curious why this size and not 20 or 18 inch ???

The outside dimensions were about 20" and they probably were that size so the racks could be rolled or moved through doorways on ships, trains, and airplanes. In the 1950s & 1960s nearly all electronic equipment was rack mounted and the racks were on rollers. The original "portable computer" was, no-doubt, a series of such racks. :-) Marc Kupper (talk) (contribs) 03:55, 2 September 2007 (UTC)

Merging Rack unit into this article

The Rack unit article is not a stand-alone article; it should really be a section of this article. Any opposition? --EEPiccolo 21:38, 26 February 2007 (UTC)

There are two common "widths" of Racks. 17 inch and 19 inch. Rack Unit is a form of measurement common to all racks, regardless of width (where width is generally defined by the spacing between the left and right post's mounting holes.) If it was merged, I would recommend making the article non-specific as to the rack width. Else-wise, there should be a separate article for each rack width and a general RU article. -- 15:54, 20 March 2007 (UTC)

The Rack unit article only mentions 19" racks. If 17" racks exist, they should be mentioned in that article. I do not believe 17" racks exist. Perhaps you are thinking of the inside measurement of 19" racks? However, it does appear that 23" racks exist. If this turns out to be true, 23" racks should be mentioned in the Rack Unit article, and the merge tags can be removed. EEPiccolo 19:34, 5 April 2007 (UTC)

23" Racks

There are no references in this article, so I can't check out what is stated about railroad relays. But is it possible that everything this article describes could also apply to 23-inch racks, in addition to 19-inch racks?

If not, what are the uses of 23-inch racks? EEPiccolo 20:09, 5 April 2007 (UTC)

Yes, the 23" racks have the same hole spacing as 19" racks. The 23" racks used to be quite common and you could get radios, radars, test equipment, etc. in the 23" size. I suspect as we moved away from vacuum tubes to ICs there's less and less need for the 23" size. A place I worked at a while back had some 23" racks and I found them handy as you could put standard size "desktop" computers and keyboards in them (on shelves, not slide rails). I personally don't know of equipment being made today that can only fit in the 23" size but I'm sure it exists as people continue to make the 23" racks. Marc Kupper (talk) (contribs) 03:45, 2 September 2007 (UTC)

23-inch is very much the standard in US telecom

I went the first four or five years of my telecom career without ever *seeing* a 19-inch rack, because everything in the business is 23-inch. Sure, some of the fuse panels have reversible mounting ears that'll allow mounting in 19-inch datacomm racks too, but we regarded those as a curiousity. I'll update the main page to be more inclusive of this. —Preceding unsigned comment added by (talk) 12:03, 19 September 2007 (UTC)

Incomplete merge from Cabinet (computer)

That article had several informative points that didn't get fully merged into this article. For instance, the Cabinet article mentioned electromagnetic shielding and acoustical damping. Perhaps it's' because "cabinet" has a clearly defined sense of enclosure while "rack" has not. Here's the text from tha last independent version of Cabinet (computer):

Wikipedia servers installed in a server cabinet. Hosted in Tampa, Florida as of September 2005

The cabinet of a computer is a tall movable closet used to house multiple computers and computer equipment. The form of the modern cabinet is standardized by the Electronic Industries Alliance, so that equipment can be placed in any manufacturer's cabinet. The primary design criteria are:

  1. Access to equipment - various kinds of locks and latches restrict access.
  2. Airflow - Cabinets are designed to be placed side-to-side, so airflow is vertical, with vents and mounting brackets for fans.
  3. Mounting Brackets - Mounting brackets have mounting holes at standard spacings and are a standard distance apart, to allow a variety of equipment to be installed in several configurations.
  4. Grounding - The mounting brackets are conductive, acting as grounding strips for the cabinet and equipment, allowing the whole cabinet to be connected to the building ground.
  5. Cable Access - The bottom of the cabinet is usually open, allowing external cables to drop through a raised floor.

Acoustic noise reduction is built into some cabinets; Electromagnetic shielding is built into others.

The most common type of modern cabinet is known as the "EIA standard" or "19-inch rackmount cabinet", where 19-inch refers to the approximate internal width of the cabinet, from mounting bracket to mounting bracket. "Rackmount" computer equipment is standardized to this width, with mounting holes conforming to the mounting bracket standard. The computer height is measured in U`s, where 1U is 1.75 inches. The standard height of a rack unit is also referred to as a "Rack Space," or "R.U." Rack equipment or the rack cabinet holding the equipment, can be various rack height sizes, starting at 1U (1.75") to 10U (17.50") and up. Racks are often 42 R.U. high, though some much larger and smaller are available. Small cabinets of 4 to 8 R.U. are often used in the music industry.

Cabinets and relay racks typically have 3 mounting holes for every 1RU. These holes are 0.5 inches, 0.5 inches, and 0.75 inches apart along the height of the rack. These holes are typically square (rapid rack) holes which equipment clips into or threaded round holes in either 10-32, 12-24, or M6 threading.

Mayb some of this can be incorporated here. Otherwise the merge will not be complete. Binksternet (talk) 04:12, 12 January 2008 (UTC)