Obey the Bend, Calculating Wire Bend Radius
Written Don Schultz, trueCABLE Technical Sales Representative & Wire Expert
A long time ago when I was first running structure 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 that I still follow today.
Why should you be concerned with how much to bend a structured 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, the concept is pretty similar. A kink or too tight of a bend can and does interfere with the signaling characteristics of the cable. The hose analogy is probably 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.
Is there a hard and fast standard for how much you can bend structured Ethernet cable? Not precisely, but there is a guideline. The ANSI/TIA-568 C.2 document provides guidance around this. What’s generally not known is that unshielded and shielded Ethernet cables have different formulas.
Let’s talk about cable bend radius. Here is how that looks:
For shielded structured cable, that being F/UTP or S/FTP, the standard specifies the wire bend radius of the cable should not be tighter than 7X the Outer Diameter (OD) of the cable. It does not matter whether it’s calculated in millimeters or inches. If the Ethernet cable has a specified jacket OD published by the manufacturer, this formula is easy to apply. If the manufacturer has not provided this data, then a direct measurement of the cable jacket is recommended. A caliper or micrometer will help you in this regard. If those items are not handy, I would revert to the DVD/CD method when dealing with undocumented shielded structured cable.
THE SHIELDED FORMULA: WIRE BEND RADIUS = CABLE JACKET OD X 7
For U/UTP structured cable, that being a cable that possesses no shielding in the form of screen or foil, the bends can be nearly twice as tight. In this case, the wire bend radius should not be tighter than 4X the OD of the cable. If the manufacturer has not provided the jacket OD measurement, then time to get out a caliper or micrometer and find out! There is no handy CD/DVD reference, this time, that will help.
THE UNSHIELDED FORMULA: WIRE BEND RADIUS = CABLE JACKET OD X 4
Let's put a couple of shielded cables to the test. trueCABLE has specified the OD of their CAT6 Plenum Shielded cable as 7.00 mm. The calculation for wire bend radius is pretty straightforward… 7 x 7.00 mm yields 49 mm. A radius is half the diameter, so multiply this number by 2, then you will know the minimum diameter of a pipe you can wrap the cable around. In that case, the number would be 98 mm or approximately 10 cm. More useful, this gives you the minimum bend radius for the innermost coil in a wrapped cable.
Let’s look at the old trusty method of using the outside edge of a DVD.
This shows the minimum bending radius of the DVD/CD method. Notice how the cable jacket is not kinking?
Now, let’s take a look at the more empirical method of using the formula with an underlying CD/DVD. In this picture, the cable has been bent to the specified number confirmed by a ruler. The cable jacket is still not kinking.
Another example is trueCABLE’s CAT5e Shielded CMX Outdoor/Direct Burial cabling. The specified OD of the cable is 6.80 mm. Running this through the formula, with the wire bend radius is 7 x 6.80 mm = 47.6 mm. And the diameter of the bend is 47.6 x 2 = 95.2 mm. On the ruler, we will use 9.5 cm.
How does the DVD/CD method hold up? All good! No kinks.
Now, for the formula method. Tighter, but still no kinks.
This is what NOT to do.
The primary takeaway here is the old DVD/CD method will provide a margin of safety, but using the more cable specific formula will allow for tighter bends.
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 DVDs or CDs were harmed during the creation of this article.
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