Bending radius: how tight can a cable turn?

The bending radius is the smallest radius at which you may still bend a cable, given as a factor times the outer diameter. If the datasheet says 10 × and the cable is 20 mm thick, the bend must be no tighter than a radius of 200 mm, measured to the inside of the bend. That is a circle 40 cm across.

The factor varies widely, and that tells you something about the construction. Flexible single core H05V-K is allowed 4 ×, coaxial RG 213 5 ×, rubber cable H07RN-F 6 ×, instrumentation cable RE-2X(St)Y 7.5 ×, YMvK Cca 10 ×, and the armoured VO-YMvKas and VG-YMvKas and the medium voltage cable N2XS(F)2Y 15 ×. Across the range 6 × is the most common, followed by 12 × and 4 ×. The stiffer and the more vulnerable the construction, the wider the bend.

With no datasheet at hand, IEC 60502 is often quoted as a rule of thumb: 12 × the diameter for multi-core without armour and 15 × for single-core and for armoured cable. Note that it is a rule of thumb. A manufacturer who has tested their own construction sometimes states a tighter value, such as the 10 × of our YMvK Cca. So what the sheet for that type says takes precedence.

Bending once is not the same as bending repeatedly. Many sheets therefore give two values. RG 213 is allowed 5 × the diameter when it is laid once, but 10 × as soon as it keeps moving in service. Drag chains and crane movements have their own, wider figures again. And for medium voltage, manufacturers usually keep a wider radius while pulling than in the final position, because the cable is under tension at that moment as well.

Go below it and the insulation stretches on the outside of the bend and compresses on the inside. On an armoured cable the armour can open up; on medium voltage the screen or the water barrier can give way. The awkward part is that you see nothing: the damage is inside and only surfaces at the test or later in service.

In practice, three places are critical: the entry into a cabinet or distribution board, the bend in a duct, and the last metre to the terminal strip. Work those bends out with the outer diameter of the type, not with the cross-section in mm²: a 4×25 mm² armoured is a very different cable from a 4×25 mm² unarmoured.

For the buyer: if the route has a tight bend or a cramped cabinet entry, give us the outer diameter with the enquiry. There is often a more flexible version of the same type, with a fine-stranded class 5 conductor, that does make the bend. That is a different cable, so a different price and lead time, but it saves a failed pull.