How do you estimate (mathematically) cable lengths from room to room?

Question

Other than calling an electrician in, I need your help in resolving this.I am trying to pull CAT5e cable myself with the help of relatives.
Need to know the rules in measuring the length of cable required from a basement to the 2nd floor.
I can't recall what the mathematical estimation is to figure this out.
Ex., let's say you have Room A with dimension L (A) x W (A), and Room B with dimensions L(B) x W (B), and distance between each floor is 10 feet.
I want to be able to move the cable around anywhere in the perimeter of ONLY Room B. Room A is fixed, where cable will start the run.
How much cable will be needed to run from Room A to Room B (anywhere within)?I would like to have a 25% growth for cable plus any loop arounds - like going around vent systems, crossing over other electrical wires.
I think there was a formula (which i can't remember) where you take the perimeter of the room, multiply it by 25%, then add the # of feet per each floor for the vertical runs. Can u pls help?

Answer 1

I pull cable all the time for customers and we always over estimate since its easier to cut back than splice a new cable.

As a professional installer for this type of wiring, I would recommend just buying an entire 1000 foot pull box.

I know alot of you may disagree and say its a waste of wire, but the main reason why I like pull boxes is because the cable comes off the roll easier and doesn't have any knots or un-necessary twists which can effect performance.

Whenever I buy cable by the foot, It takes way more time because it takes 2 people because one person has to make sure the wire doesn't get tangled up.

If you absolutly want to measure, just measure the distance on all the walls and add like 20-30 feet and you should be fine.

But estimating correctly is more of a skill you accuire with experence than just a set percentage. Me and my father get into arguments over this all the time, last time we got into it we hooked up an automatic generator. He Figured we needed 90 foot sections of #6 Copper cable, and when he sent me to the store I bought 120 feet to be safe.

He told me I bought too much cable and I was actually 3 feet over, so If I would have listened to him we would be under by about 17 feet!.

Heres a perfect example of my wiring Job to prove I know what im talking about here. I ran 23,000 feet of Cat6 and RG6 in my new home...
http://www.dishretailer.com/newhouse.htm

Answer 2

How you buy is not the same as knowing the minmum you need, which is the starting point. That may show you that a 500 ft roll is 30 feet short and help you buy more economically.

Trace the path of the cable. Measure across floors, up walls where appropriate. etc- and remember that there will be obstacles and framing that extend the dimensions and/or cause detours. I would add one foot to each dimension across a room, and two feet to each vertical rise floor to floor or to an outlet. The total would be a minimum, and I would still add a good buffer if you plan to buy a cut length.

What you buy (roll size or cut) may be decided by knowing how much it will take to do the job.

Answer 3

Cable Length Calculation Formula

Answer 4

I only had to run 4 or 5 all less than 100 feet in length. So I bought some 50, 75 and 100 foot Cat 5e cables of several different colors that already had the RJ-45 connectors installed on them. You only need slightly larger holes drilled to feed the RJ-45 through, and you can always replace a connector later if need be. The colored wire is nice because you immediately know the function you assigned to it at the router, PC or other network peripheral. You can guess within 25 feet whether you need the 50,75 or 100 foot length and simply coil up any extra at both ends of the run. Start each run half-way and feed the wire in both directions at once.

http://www.cnaweb.com

Answer 5

You can just add the distances together, then add 25%.

For example, up 2 floors at 10 feet a floor is 20 feet.
To move it anywhere in a room, take the diagonal measurement (length squared + width squared = diagonal squared. then get the square root of that number.

Don't forget to accommodate for the gaps between floors and walls.

The best way to do this is to get a spool of cable, run it from one point to another, then cut it when you have your desired length.

Answer 6

What are you planning to use the cable runs for? Phone, or data? Very important to know. If it's just phone, pull one run from the basement to the attic, then splice off that run and "drop" down to the bedrooms. If it's data, then you need a "home run" for each outlet, NO SPLICING. Also, you can only cross power lines perpendicularly, you can not run the data lines along side the power lines, and if you plan on using cold air returns, or air ducts to get from point A to point B, you need to buy the more expensive Plenum Rated Cat 5e. Feel free to email me with any additional questions.

Answer 7

Are you trying to save money by purchasing only the cable you need? Your best bet (honestly) is to buy a 500' role and just pull the cable lengths that you need and then install the terminals. A 500' spool shouldn't cost more than $30 and the female ends (RJ45) are only $2-3 each and they will come with a small punch tool, just follow the color codes and it should work just fine.

Answer 8

First of all you really don't want to "loop" Cat 5. Meaning you don't want to go from room to room to room if you can help it. Your best bet is to run a dedicated line from each room to the basement to a control panel. Then run a line outside to the phone companies hook up. As for knowing how much you need don't waste your time with all that headscratching math it is cheaper to go to Lowe's or Menard's or Home Depot, preferably Lowe's and buy a box of Cat 5 wire. It will be blue.

Answer 9

Terminate the cable in a socket in the room.Then extend the cable lenght as required to the item.Untidy surplus cable bundled up in a room is messy and unfinished looking and in my experience never works out long term.No formula is required,estimate the run then get working.

Answer 10

way to scientific......buy 1000 foot roll if short splice it, if long try to return it or keep it as price of job. This cable you are talking about is not a super high cost item. Skip the pencil time and get busy crawling around on the floor.