Metal Fabrication Resources Sheet Metal Design Considerations: Maximum Amount of Bend In order to understand what the maximum amount of bend is for certain gauges of sheet metal, it’s essential to understand a few basics of bending and other terms involving metalworks. Let’s take a look at everything you need to know about this particular sheet metal design consideration and understand what the…

I’m a huge fan of Prysmain/Pirelli’s FP-200. It gets used for a lot of electrical work, especially in fire alarm installation, because it is easy to strip and terminate, can take a bit of a bashing and offers fire resilience.

However, just because it’s flexible doesn’t mean it should end up in a knot.

522.8.3 -  “The radius of every bend in a wiring system shall be such that conductors or cables do not suffer damage and terminals are not stressed.” ‘Other Mechanical Stresses’ in BS7671: 17th Edition.

This can be caused by poor installation, poor fixing, or some other problem that leads to the cable ending up out of shape.

The bend radius of fixed FP-200 Gold is listed as r=6D, where D is the diameter of the cable. So, if we’re using 1.5mm two core & earth for a fire alarm system, the diameter of the cable is 8.1mm. (Source)

This means that your 90 degree turn should complete in just under 5cm, and if you were doing a u-turn, you’d need 2r=97.2mm, just short of 10cm.

Visual inspection of the cable should make it obvious if  the turn is too harsh- the sheath of FP-200 goes white under stress, and once it has been bent, will not straighten out without leaving a kink.

So these bends are probably too tight.

Part 2 to follow soon!