# Rackmount cable length calculations & best practices

I'm redoing the cable management for a small company with a modest server room. It's my first time trying to organize an absolute mess that their former IT-guy left.

A huge problem that I'm finding is that very long Ethernet cords were used where short ones should have been used. I assume the IT guy was just too lazy to make a one foot cable that would fit nicely, and instead opt'ed to grab the nearest 10 foot cable. Repeat this two-dozen times and its a real mess.

As I'm going about making new cables and replacing existing ones, I'm wondering if there are any sorts of best practice methods for determining the exact cable length needed in server rooms besides obviously just using a measuring tape. I'm taking a measuring tape and trying to determine how long it is from point A to point B (routed properly and neatly).

I see so many excellent pictures of nicely wired server rooms where all the wires look very tight and neat and it looks like the cables were cut to be EXACTLY the right length in every situation. Do the pro's who do those awesome jobs really measure out each required length for every single port? Or do they have some tricks up their sleeves for quick length calculations?

• Making cables seems like a bad idea. Unless you are working for free/cheap buying patch cables will be a lot more cost effective. Nov 18, 2010 at 22:44

There is no special tool that I've found for figuring out the length you need. I used pre-made cables on a rack a few months ago and it came out ok; not too messy. I just finished up another rack and made each cable myself. I would terminate one end, plug it in and route to the proper switch port. Add about an inch, cut and terminate. The manpower is much higher but the finished product is a thing of beauty. It all depends on how your being paid and what your workload is.

When I used premade calbes I created a spreadsheet to calculate the vertical length of the run by subtracting the differences in elevation (in U's) and multiplying by 1.75". I then added 3' for the combined horizontal distance and rounded up to the next standard length (3', 5', 7', 10' etc...) Of course this only worked for cables terminating in the same rack.

• I really like your approach: Terminating one end of the cable, leave the Cat5 box at your source port then routing/pulling the cable to its destination and then going back to the box and finding the length you need and then cutting and terminating it. Sounds much better than using a tape measure and then making the cable. Nov 19, 2010 at 4:13
• One possibility would be to have a long piece of cable, with an TJ-45 connector at one end, having marked distances for each "standard" cable length you have. This would allow you to find the distance needed with something having the exact properties of a cable, routed "properly" and would probably lead to a good intuition for the required lengths after few iterations. Nov 25, 2010 at 12:26

Best wishes for the cleanup project. A wise move - but one management rarely will understand...

1. If you are using more than 1 rack - USE PATCH PANELS - a punch down tool and patch panels allow for you to very simply run cable at any length you desire - very clean and neat. AND when complete - you can than close up everything and just place in short patch cables. One reason I love this approach is because it allow for you to never have to replace anything but the patch cable ever again. When using more than 1 rack We setup 1 rack as a mini-MEET-ME rack or cabinet. It makes a huge difference and is very clean, allows all switches to be in one location - and works for both Fiber as well as Copper/Ethernet.

2. Don't forget about the sides of the rack as well.

4. Label - Label - Label - It is important to understand where everything runs - to and from. Generally larger datacenters will create a block scheme with numbers running one way and letters on the opposite wall. This helps you figure out where the wires are going - in a large facility. You can do much the same here - simply keep whatever your doing the same across the board.

5. Use a system such as RackTables to help you manage everything long term. It is opensource and costs nothing but time.

6. Use Cable Straps or Velcro - Zip ties are bad business - if they are to tight you can actually kill the speed / transmission rates of the wire

7. Use a good color scheme: Orange = Fiber, Blue = switch to switch, Yellow = Cross-over, etc etc et

8. One last note - ALWAYS TEST ALL CABLES - a good cable tester will help you always.

• I really like that method of removing the slack from cables that are all the same length. I must remember that for my next rack... Mar 27, 2012 at 21:55
• Special kudos for RackTables, I know Denis since maybe 2004 or so (he used to live in Donbass back then) and we used RT to help keep our small free software hosting when it was still young... Apr 28, 2018 at 13:19

I like to cut them to exact lengths, with a little practice you can run a new line every ~2 minutes. If things are color-coded, neat and cut to length, chances are the next guy who walks in won't want to mess up your handy-work. As a bonus, they won't have to tug on a bunch of cables to figure out where something runs, (hopefully) they might even try to follow your lead for future installations :)

• i like the RTFM sticker Nov 19, 2010 at 0:01
• Are you saying that you make your own cables or that you have them custom made? Nov 19, 2010 at 2:05
• I make them myself. Nov 19, 2010 at 5:11
• On the contrary, allways let some additional length to your fibers. Nov 22, 2010 at 14:39

I second Zoredache's comment, making cables leaves a huge margin of error that will be far more difficult and time consuming to do than dealing with a messy server room.

Crimping patch cables, even if you have your technique down pat, I have never seen take quicker than approximately 90 seconds. Combine that by 100 and you can pop down to your local wholesaler and pick up 100 patch leads with time to spare.

Now to your question, there are fairly standard lengths (10cm, 30cm, 50cm, 1m). You need to find the closest match (that's longer, obviously), and cable tie the bunches together. Given a rack is 19" wide, it's generally less than 19" of "slack" in each cable compared to the longest distance, so hiding that much length to make it appear tidy is usually just as letting the cable sag behind the server by a few cm.

Don't forget that if your server is on sliding rails, you need to give the cables enough slack that you can slide the server out, which is often up to 80cm.

What you're seeing in these beautiful server rooms is the front of the racks. Generally I always mount my switches at the back of the rack, and they never, ever, look as neat as those rooms, nomatter how hard I try or effort I put it.

Far more important than making it look nice is a decent labling scheme. There are are good questions about that on here if you do a bit of a search around.

• NEVER USE CABLE TIES - instead use something like velcro or an actual cable strap. Nov 19, 2010 at 5:12
• let me clarify ... zip ties are bad business... they can cause major transmission issues if they are done to tight... If you do mount your switch in the back of the rack - keep airflow in mind. Hot aisle vs cool aisle... Nov 19, 2010 at 5:15
• Velco stripes > Zipties Nov 20, 2010 at 4:43

Yes some pros - or their cabling contractors - measure every cable to make it 'just perfect' but most people just round to the nearest metre or half-metre - and they just do it the way you would, with a tape-measure or similar.

Don't forget the 100m per run rule either.

I’ve been involved with many rack installations and always under a VERY tight time restraint - which always leads to no time to dress racks properly. As I said to one audit person who made a derogatory remark on a install we were doing - " you want pretty, then pretty costs more" - which is true, it takes a LOT of time to dress racks and when on site, time is money.

There are 3 ways to dress racks:

1. each cable cut to length - very time consuming to cut , lay, terminate and test and fraught with termination issues.
2. best guess - problem is you will spend a lot of time hiding excess cables
3. apply some calculations based on source, destination points and cable routing between racks.

I’m currently working on an install where in excel I identify each cables source and destination noting the rack number, U number, Front/Rear connecting, Left/ Right connecting for each end then apply formula's to calculate the length ( e.g same rack but going from Left to Right, Front to Back, or different racks Front to Front, Left to Right etc etc) - its taken a long time as each cable has to be identified - im also at the same time giving each cable a unique ID label and colour based on cable type.

Then apply some sensiblisation to come up with industry standard lengths. From this I compose a table which gives me the numbers of the different lengths and colour required. I then purchase pre made cables ( and hopefully have cable labels applied by the factory) - this will save a huge amount of time on site - its still a work in progress but I’m slowly getting the different formulas to give sensible results.

One thing Im doing too which will save a lot of time , is routing cables via the top of the racks using cable trays made to fit directly onto the top of the rack – these look like an ‘H’ to allow cables to transit across the rack top or go from front to rear, left to right – there are holes in the ‘H’ to allow cables to drop down into the rack.

• Hi Tony, I like your answer, would you happen to have a generalized spreadsheet template that you'd be willing to share? If not it sounds straight forward enough to produce. I spent some of my own time making a small length calculator but maybe yours is more robust. Anyway thanks for your answer. Mar 23, 2015 at 18:39

Take a look at the LazyTape tool to keep your cables clean. I use it to help me cut custom cables to the right length or find the length of the pre-made patch cable I should use. It saves me a lot of cabling time and having to buy excess cable inventory.