Obey the Bend, Calculating Wire Bend Radius
Written by Don Schultz, trueCABLE Technical Sales Representative & Fluke Networks Certified Technician
A long time ago when I was first running Ethernet cabling, I was told by an older installer that the best way to determine how much you could bend a cable was with a DVD/CD. He said the cable should not curve tighter than the outer edge of the disk. I thought this was a great rule of thumb! This was wise advice then, but a bit outdated today. Further, this was in the context of RG6 coaxial cable, a different animal than what we are talking about here.
Why should you be concerned with how much to bend an Ethernet cable (or any cable for that matter)? Think of your cable as a garden hose. Water should flow through it freely. Now, put a nice hard kink in that hose and the water stops. When discussing Ethernet cable, this is not a bad way of thinking about it. A kink, or too tight of a bend, can and does interfere with the signaling characteristics of the cable. The hose analogy is an extreme example as the water stops altogether. In the case of Ethernet cable the speeds at which devices connect may be reduced, or there may be consistent packet errors. Or even worse…intermittent and hard to track down packet errors.
Are there rules for how much you can bend Ethernet cable? Yes. According to ANSI/TIA-568-0.E, a manufacturer’s guidance around maximum bend radius trumps any generic guidelines. In absence of manufacturer stated rules, the generic guidance is four times (4X) the cable jacket diameter for U/UTP Ethernet cable or eight times (8X) for F/UTP (and SF/FTP) solid copper structure cable.
These rules change for stranded copper Ethernet patch cabling, however. Patch cables are held to the 4X rule, whether shielded or not. If you are confused about the difference between stranded and solid copper conductor cabling, see Solid vs Stranded Ethernet Cable.
This is what a bend radius looks like:
For trueCABLE U/UTP solid copper unshielded cable, we follow the ANSI/TIA guidelines. The inside radius of bends should be no tighter than 4X the outer diameter (OD) of the cable. For trueCABLE F/UTP solid copper shielded cable the inside radius of the cable should not be tighter than 7X the OD of the cable. See the table below for actual measurements, giving a far more useful dimension, the bend diameter.
This is what a bend diameter looks like:
If you are dealing with cable that is not trueCABLE brand, and that manufacturer provided no guidance around bend radius limitations, there is a nice little formula and it does not matter whether it is calculated in millimeters or inches. If the Ethernet cable has a specified OD published by the manufacturer, this formula is easy to apply.
When putting this into practice, visualizing a bend radius is tough. Rather than rely upon bend radius to be your guide, rely on bend diameter. Bend diameter is simply calculated as:
The Better Formula:
The result is far easier to visualize. Diameter is the width of the inside of a circle. For an example of how this looks in practice, here are some pictures that may help.
The difference a shield makes. Cat5e Shielded Riser on the left. Cat5e Unshielded Riser on the right.
Cat6A Unshielded Riser on top, and Cat5e Unshielded Riser on the bottom.
Cat5e Unshielded Riser in the center. The big black cable is Cat6 Direct Burial Shielded!
For scale. Cat5e Unshielded Riser & Cat6 Direct Burial Shielded with a coffee cup.
Just in case anyone was wondering, this is what NOT to do….
I am sure many out there will provide stories about how much tighter they can bend a cable and get away with it. Sometimes, a sharper turn is simply unavoidable. My only advice is to obey the rules of thumb, but use common sense and don’t bend a cable at a right angle! Performance testing is the best way to confirm your cabling works as expected.
No coffee cups were harmed during the creation of this article. HAPPY NETWORKING!
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