Cable Waste Math: How to Estimate Your Box Count

Written by Don Schultz, Senior Technical Marketing Specialist, Fluke Networks Certified Copper/Fiber CCTT, BICSI TECH, INSTC, INSTF Certified

Have you ever had an Ethernet cable installation where one -- or even more worse ALL -- of your runs were just one foot too short? Have you found yourself with many extra boxes or spools of Ethernet cable after an installation? Avoiding these common mistakes means knowing how to properly measure and calculate your drop lengths while taking factors into account you may not have been aware of.

The following video and blog will guide you step by step through this process, saving you time, waste, and a lot of hair pulling.

Video coming soon

Please take the time to watch this video and read the blog because…

The most expensive Ethernet cable in the world, on a per foot basis, is the one that is one foot too short!

Step 1 - Measure Your Cable Runs

A question trueCABLE is often asked is “How much ethernet cable do I need for my installation?” Well, the first step to getting to the correct amount of cable for your installation is to measure, to the best of your ability, each and every drop involved on the horizontal plain from telecommunications closet (head end) to remote outlet. Do not worry about extra slack at either end yet, nor padding for measurement mistakes. These items are often overlooked (and often the source of mistakes) and will come later.

Tip: Keep track of the number of turns the cable makes along the entire drop length. This is important and we will discuss why in the “Do the Math” section.

Existing Pathway Method

If you installed the cable tray, ladder rack, and other mounted cable support hardware you likely have a good starting point for measurements.

Building Plans Method

Utilize the planned or as-built “T” (Technical) drawings. The “T” (Technical) drawings show actual pre-planned drop routes and cable pathways, accounting for any obstacles. The nice thing about the Technical drawings is you have the full horizontal calculation for each and every drop from telecommunications rooms to the outlet. If the distance is not on the drawing, there is a scale and you can calibrate a measuring pen to trace them out to get extremely accurate estimates.

Pacing Method

The simplest method, requiring no tools or plans, is to pace-estimate the drop length. First, decide if you wish to use normal or long steps to do this. Once you settle on how you wish to pace off, measure the distance from toe tip to toe tip at the maximum distance of your stride. Having the stride distance calculated, now pace the path the cable will run from telecommunications room to outlet.

Measuring Wheel Method

Measuring wheels can be purchased for relatively little money but do a great job of helping you estimate drop length. Set your measuring wheel for meters or feet depending upon which units you are measuring, reset the counter to zero, and use the measuring wheel to trace the route of the cable drop. Start measuring from the telecommunications room and proceed to each outlet separately. Don’t forget to reset the wheel meter to zero between drops!

Ceiling Tile Method

If your drops are located in a space with ceiling tiles, this is a great way to come up with an estimate for drop length. Ceiling tiles come in known standard sizes, typically 2x2 or 2x4. Count ceiling tiles from the telecom room to the outlets.

Laser Distance Measure Method

Finally, there is the laser measure. This method is most accurate for straight long distance runs, but requires a lot of stopping and re-measuring as you can only get one distance at a time. Measuring wheels, by contrast, don’t need to be set up for a new measurement at every turn, making the measuring wheel a more practical instrument.

Tip: On complex jobs, combining methods may be necessary.

Do not skip a site survey, even if you have Technical drawings available. It is better to plan around issues you know about prior to pulling cabling!

Step 2 - Do the Math (Your Box Count Formula)

This section is where we take your raw horizontal measurements and add some additional footage to come up with an actual drop length you will pull to each outlet--and by extension the amount of cable you will need to order.

The measurements you took do not yet account for vertical drops or rises of your cable, service slack, turns, and potential measurement mistakes.

Account for Vertical Drops and Rises

Every cable run has a horizontal distance you estimated AND potentially multiple vertical components you will also need to account for. Typically, vertical drops and rises occur at:

• Down the inside of a wall from cable support hardware to outlet
• Obstacles

Add the ceiling height (or floor-to-wall-plate distance) to every run. Don't forget to check for obstructions that may cause vertical direction changes!

Add Service Slack at Both Ends

Service slack is extra cable intentionally left at both ends of the cable drop.

• Service slack is left for termination at the outlet plus extra cable that should be coiled up in the wall cavity or outlet box to allow for future upgrades or changes.
• More service slack is left in the telecommunications room or backboard/enclosure area to accommodate for initial termination, adds and changes to terminations, plus equipment movement in the room or at the backboard (or both).

Service slack shown stored in “figure 8” pattern above residential enclosure

Service slack in telecommunications room, stored on ladder rack

In all cases, add two feet at the end of each drop for termination at the outlet side. For service slack at the head end of the installation (where your patch panels and switches are) there are two different calculations that must be made depending upon the environment.

• Residential: Retain enough service slack at the backboard/enclosure area to reach the farthest corner of the backboard. Typically this is five to 10 feet. It is not recommended to have less than five feet.
• Commercial/Enterprise: Service slack should be enough to reach the farthest corner of the telecommunications room + floor to ceiling from where the cable enters/exits the room. This is typically 24 feet. For example, if the room is 16 feet corner to corner and eight feet floor to ceiling you would add 24 feet to every drop.
• Service slack at the head end should be stored in a “figure 8” pattern to prevent cable to cable cross-talk and Radio Frequency Interference. Additionally, store the service slack on the backboard/enclosure in residential situations and on the ladder rack in larger telecommunications rooms. Do not accidentally violate the cable manufacturer’s stated bend radius limits while creating your “figure 8” pattern.

Do not use service slack to account for potential measuring mistakes! Measurement uncertainty is another calculation we cover below.

Add 1 Foot per Turn

Every 90-degree bend in the cable route adds routing length. Add approximately one foot for each turn. On a job with lots of corners, this adds up fast. To be sure, one foot is going to be over-estimating but it is better to be safe than sorry and come up short!

Add a Waste and Mistake Buffer (7-10%)

No estimate is perfect. Add a minimum of 7-10% to your total. Increase this buffer as uncertainty goes up: new building type, first time in a space, or complex routing. It's far cheaper to have a half-used box left over than to stop a job and re-order.

Calculate Your Total Footage

This is where the calculation fun begins! So, you have a number of figures that you have measured and collected for each and every drop, right?

Here is an example quick table to help you (residential example):

Keep in mind that you could end up with a measurement that exceeds the maximum permitted length for Ethernet cable. There are hard limits and limits imposed due to environmental factors. If you are not familiar with the rules of the road regarding maximum lengths, please see Calculating Ethernet Cable Overall Channel Length for Success and also see Temperature's Effect on Ethernet Cable Length.

It may be necessary to reduce the service slack at the head end of the installation to keep cable lengths within boundaries. This is not an ideal solution. Check into PoE powered switch extenders as a possible solution. Another option is fiber optical cable, which goes much further.

Divide by Box Size - Then Round Up

In this simple residential/small office case you have settled on riser rated unshielded Cat6A cable as you wish to support 10 Gbps in the future and have no plenum rated areas. This cable is sold in 1000 foot lengths only, and on spools. The final answer is you will need exactly one spool, and you will have 595 feet left. Regardless of the number of boxes or spools you calculate, always round up so you don't come up short!

Need Help Selecting the Correct Ethernet Cable?

Please see Facts About Ethernet Cable Jacket Ratings to select the correct jacket type for your Ethernet cable. Also take a look at How to Choose the Right Ethernet Cable.

We have a large selection of Ethernet cable to choose from.

Pro Tips to Avoid Running Short

• Measure twice, order once
• When in doubt, go up a box/spool - you can always use leftover cable on the next job
• Label your measurements per run on a floor plan sketch before calculating
• Account for cable routed around obstacles (HVAC, structural beams, electrical conduit)
• If plans change mid-job, re-calculate before pulling more cable

Conclusion

So there you have it! The entire process is likely what you expected, which is to measure and then add it all up to get the amount of cable you need to order. Of course if it was quite that simple then I would not have written this blog. There are a number of seemingly innocuous steps that if not accounted for will add up quickly and cause significant over or under estimation. Take the structured approach to this effort and save significant time and money on the job!

HAPPY NETWORKING!

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